Unveiling the Phase Formations in the Sr–Zn–Eu³⁺ Orthophosphate System: Crystallographic Analysis and Photoluminescent Properties

Abstract

This study investigates phase formation in the Sr–Zn–Eu³⁺ orthophosphate system, focusing on double- and triple-phosphates. The isomorphisms and phase formation in Sr_3–1.5xEu_1+x(PO₄)₃, Sr_9–1.5xZn_1.5Eu_x(PO₄)₇, Sr_9.5–1.5xZnEu_x(PO₄)₇, Sr_3–xZn_xEu(PO₄)₃, and Sr_3–xZn_x(PO₄)₂ series were studied using powder X-ray diffraction and Rietveld refinement. A ternary phase diagram was constructed, identifying concentration limits for pure phases and multi-phase regions as well as areas of stabilization of strontiowhitlockite-, palmierite-, eulytite-, and strontiohurlbutite-type phases. The combinatorial complexity of Sr-based phosphates is discussed. The β-Sr₃(PO₄)₂ isostructural to whitlockite was found to exhibit the highest isomorphic capacity for Eu³⁺ cations, which is advantageous for its application as a red-emitting phosphor. Photoluminescence properties were studied, and analyzed based on structural data. Photoluminescence studies confirmed intense red-emission dominated by the ⁵D₀ → ⁷F₂ transition of Eu³⁺, with the β-Sr₃(PO₄)₂-based phosphor showing the highest emission intensity.

1. Introduction

Interest in new oxide hosts for doping with rare earth elements (REE) is quite high and can be explained by the expansion of areas of application of luminescent materials, for example, in dosimetry, forensics, verification, etc. [1]. Previously, the potential of different strontium phosphate phases doped with REE [2] as photoluminescent (PL) materials has been shown. The reasons for their intense PL properties are due to the volumetric coordination polyhedra formed by Sr²⁺ ions, which are well-matched with REE ions without concentration quenching effects. The incorporation of ions with small radius (Zn²⁺, Mg²⁺, Cu²⁺, etc.) that significantly differ in size from Sr²⁺ allows the formation of various crystallographic environments and influences the PL properties of REE ions.

Some double Sr-phosphates doped with ions with a small radius M²⁺, where M²⁺ can be Zn²⁺, Mg²⁺, Cu²⁺, etc., are known. Sr²⁺ ions always form large polyhedra with CN (coordination number) equal to 8 or 9 [3], while M²⁺ forms the most characteristic coordination polyhedra. The Cu₃(PO₄)₂–Sr₃(PO₄)₂ system is shown to form several Sr–Cu double phosphates [4], where Cu atoms have tetrahedral [5] or octahedral [4] coordination.

The substitution relations in the Sr²⁺–Zn²⁺–Eu³⁺ orthophosphate system can be presented by the ternary phase diagram (or Roozeboom’s triangle) (Figure 1). The limit points in the Sr²⁺–Zn²⁺–Eu³⁺ system are phosphates with the palmierite Sr₃(PO₄)₂ [6], hopeite Zn₃(PO₄)₂ [7], and monazite EuPO₄ [8] structures. The projections of the crystal structures of these compounds are shown in Figure 1.

1.1. Monazite EuPO₄

The phosphates with the general formula LnPO₄, where Ln is a rare earth element from La to Gd (except Pr), have monazite crystal structure with monoclinic syngony [9] (space group (SG) P2₁/n). The phosphates with small lanthanides are crystalized in xenotime structure with cubic syngony (SG I4₁/amd) [10]. So, the EuPO₄ (ICSD_201840) forms a monazite crystal structure (SG P2₁/n), the unit cell parameters are a = 6.639 Å, b = 6.823 Å, c = 6.318 Å, β = 104°, Z = 4. There is a single crystallographic site for the Eu atom with a disordered nine-fold coordination [11,12] (Figure 2), linked by PO₄ tetrahedral in 3D framework (Figure 2). The Eu-surrounding structure is characterized by two different Eu-O distances, leading to disordering of polyhedra. The distance Eu–Eu between two polyhedra changes from 4.01 Å to 4.24 Å. EuPO₄ shows PL properties associated with Eu³⁺ emission.

1.2. Hopeite α-Zn₃(PO₄)₂

The α-Zn₃(PO₄)₂ (ICSD_27554) is crystallized in SG C2/c [8]. There are two disordered Zn sites, which are connected by PO₄ tetrahedra into a 3D framework (Figure 3). The unit cell parameters are a = 8.140 Å, b = 5.630 Å, c = 15.040 Å, β = 105.13°, V = 665.36 ų, Z = 4. According to the characteristic small ionic radii for Zn²⁺ ions, the cationic sites have tetrahedral coordination. The distances Zn–O are d_min = 1.929 Å and d_max = 2.031 Å in the Zn1 site, and d_min = 1.864 Å and d_max = 2.017 Å in the Zn2 site. The distance Zn1–Zn2 is 3.38 Å (Figure 3) between Zn in tetrahedra.

For the Zn₃(PO₄)₂–EuPO₄ system, there are currently no intermediate individual phases. However, EuPO₄ and Zn₃(PO₄)₂ structures themselves have low isomorphic capacity due to the mismatch in the Eu³⁺ and Zn²⁺ ions’ sizes and coordination environment. Thus, in [13,14] the doping of Zn₃(PO₄)₂ by Eu³⁺ ions shows a very low limit of substitution for single-phase formation not exceeding 3 mol%.

1.3. Palmierite α-Sr₃(PO₄)₂

The strontium phosphate (Sr₃(PO₄)₂) can be stabilized in two phases: α- and β-phases, which exhibit structural differences. Low-temperature modification, α-Sr₃(PO₄)₂ (ICSD_150869), is related to the palmierite-type structure–M₃(TO₄)₂ where M = Ba²⁺, Sr²⁺, Ca²⁺, Pb²⁺, and T = V⁵⁺, As⁵⁺, P⁵⁺ [15] (SG R$\overline{3}$m, unit cell parameters a = 5.3901(8) Å, c = 19.785(5) Å, V = 497.81(19) ų, Z = 3) [16].

There are two different crystal sites for Sr atoms in the palmierite-type structure (Figure 3) characterized by different CN: Sr1 and Sr2. The Sr1 site shows 12-fold symmetrical coordination with two different Sr–O distances. The first interatomic distance is d_Sr1–O2 = 2.57 Å, which forms the symmetrical octahedral surrounding of Sr²⁺ (Figure 4). The second one is d_Sr1–O1 = 3.11 Å, which forms 12-fold coordination for Sr²⁺ in the Sr1 site. The Sr2 site has CN = 10, and three different Sr–O distances: d_Sr2–O2 = 2.75 Å, d_Sr2–O2 = 2.69 Å, and d_Sr2–O1 = 2.43 Å (Figure 4).

Phosphorus atoms form PO₄ tetrahedra that link cationic sites. The structure can be described as a sequence of polyhedra composed of PO₄–Sr2O₁₀–Sr1O₁₂–Sr2O₁₀–PO₄ forming the column along the c-axis (Figure 4) [17]. The interatomic distance between the Sr1 and Sr2 sites is 4.09 Å, while the distance between cationic sites in the nearest columns is 5.39 Å (Figure 4).

The α-Sr₃(PO₄)₂ is stable at room temperature, while the high-temperature structure, namely β-Sr₃(PO₄)₂, is significantly different and is isostructural with mineral strontiowhitlockite Sr₉Mg(PO₄)₆(PO₃OH) (PDF-2 No 48-1855) [18]. However, the β-Sr₃(PO₄)₂ structure can be stabilized at room temperature by introducing small radius cations, such as Mg²⁺, Zn²⁺ [19], Fe²⁺ [20].

1.4. Strontiowhitlockite β-Sr₃(PO₄)₂

As mentioned above, the incorporation of Zn²⁺ ions into the palmierite-type α-Sr₃(PO₄)₂ phosphate forms an octahedral coordination that is characteristic of relatively small Zn²⁺ ions. Consequently, Zn-substituted Sr₃(PO₄)₂ exhibits a strontiowhitlockite structure. The relations between Sr₃(PO₄)₂–Zn₃(PO₄)₂ at different temperatures was studied in [19]. It was shown that at 12–15% mol.% of Zn₃(PO₄)₂ (or 0.35 ≤ x ≤ 0.45 in Sr_3–xZn_x(PO₄)₂), solid solutions with the β-Sr₃(PO₄)₂-type structure can be stabilized as a single phase. The described novel phase was (Sr_0.85Zn_0.15)₃(PO₄)₂ (PDF-2 No 14-207, Z = 21) in [21]. This formula, according to the crystallographic structure, can be represented as Sr_8.925Zn_1.575(PO₄)₇ (Z = 6). It should be noted that in the case of the Sr₃(PO₄)₂–Mg₃(PO₄)₂, the range of solid solutions with the β-Sr₃(PO₄)₂-type structure is considerably wider, ranging between 10–35 mol.% of Mg₃(PO₄)₂ [19]. Later, it was shown that the β-Sr₃(PO₄)₂ structure is stable at room temperature in strontiowhitlockite mineral Sr₉Mg(PO₄)₆(PO₃OH) [19]. To date, some REE-activated β-Sr₃(PO₄)₂-type phosphors have been investigated: (Sr_0.85Zn_0.15)₃(PO₄)₂:Eu³⁺ [21], (Sr_0.86Mg_0.14)₃(PO₄)₂:Eu²⁺, Mn²⁺ [22], Sr₈ZnIn_(1–m)Lu_m(PO₄)₇:Eu²⁺/Mn²⁺ [23], Sr₈ZnIn_0.6Sc_0.4(PO₄)₇:Eu²⁺/Mn²⁺ [24].

The phosphates (Sr_0.85Zn_0.15)₃(PO₄)₂ [19] and Sr₈ZnEu(PO₄)₇ (CCDC No 2424984) [2,25] with the β-Sr₃(PO₄)₂-type structure are significantly more complex (Figure 5a) compared to α-Sr₃(PO₄)₂. It is worth noting that the SG of the mineral Sr₉Mg(PO₄)₆(PO₃OH) was determined as polar R3c [18], similar to its calcium analogue, whitlockite Ca₉Mg(PO₄)₆(PO₃OH) [26,27]. However, later studies on synthetic analogues of strontiowhitlockite using the second harmonic generation method revealed the absence of nonlinear optical activity and confirmed a nonpolar structure corresponding to the SG R$\overline{3}$m [25,28]. Due to its extremely low abundance, similar studies on the natural mineral have not been conducted since its discovery.

In contrast to calcium-based whitlockite [29], which can incorporate many ions of different sizes and charges [30], the strontiowhitlockite has limited isomorphic capacity as was shown in [19]. It should be noted that the heterovalent substitutions can also be realized in a β-Sr₃(PO₄)₂ structure according to the substitution scheme 3Sr²⁺ → 2R³⁺ + □, where □ is a vacancy, with the formation of Sr₉R³⁺(PO₄)₇ phosphates, where R³⁺ is an ion with small radius, such as Sc³⁺, Ga³⁺, Cr³⁺, Fe³⁺, In³⁺ [31], Lu³⁺ [32], or Y³⁺ [33]. In Sr₉R³⁺(PO₄)₇, R–La³⁺ was crystalized in a palmierite structure, as was shown in [34]. However, co-doping 4Sr²⁺ → 2R³⁺ + M²⁺ can lead to the β-Sr₃(PO₄)₂ structure. Structural analysis of the triple phosphate Sr₈ZnR(PO₄)₇ R = Eu [25], R = La [35] revealed two nonequivalent positions of Sr atoms—Sr1 and Sr2 (Figure 5b)—jointly occupied by strontium and europium atoms, while the Zn atoms form octahedral polyhedra located on the 3-fold c axis. The SG was refined as centrosymmetric R$\overline{3}$m based on the second harmonic generation tests. The unit cell parameters are a = b =10.607(5) Å, c = 19.658(8) Å, Z = 3. The shortest interatomic distance between the cationic sites is d_Sr1–Sr2 = 3.92 Å (Figure 5c), while the largest distance of 4.56 Å is observed between Sr1 sites in the nearest columns.

1.5. Eulytite Sr₃Eu(PO₄)₃

The existence of a single intermediate phase with the eulytite structure Sr₃Eu(PO₄)₃ (ICSD_195181, SG I$\overline{4}$3d, Z = 4) is known in the system Sr₃(PO₄)₂–EuPO₄ [36]. The unit cell parameters are a = 10.120(2) Å, V = 1036.5(5) ų. The structure contains one crystallographic Sr|Eu site (Figure 6), which is jointly occupied by Sr and Eu atoms in a 3:1 ratio. The interatomic distance between the nearest sites is 3.98 Å, and the largest distance in the nearest columns is 6.50 Å (Figure 6). The oxygen environment of these sites is a twelve-vertex polyhedra (Figure 6) with two different Sr|Eu–O distances: 2.517 and 2.630 Å. Two types of PO₄ tetrahedra link the structure into a 3D framework (Figure 6). It is worth noting that in the Sr₃(PO₄)₂–CePO₄ phosphate series, the formation of the strontiowhitlockite phase was observed at a molar fraction of CePO₄ less than 5% and a temperature above 1400 °C [37]. Similar studies have not been carried out for the Sr₃(PO₄)₂–EuPO₄ series, but it is known that the impurity of Eu³⁺ in Sr₃(PO₄)₂ up to 3 mol.% preserves the palmierite-type structure [38].

1.6. Strontiohurlbutite SrZn₂(PO₄)₂

In addition to the strontiowhitlockite phase (Sr_0.85Zn_0.15)₃(PO₄)₂ [19], isostructural with tricalcium phosphate β-Ca₃(PO₄)₂ [39], one more individual compound is known in the Sr₃(PO₄)₂–Zn₃(PO₄)₂ series (Figure 1). This is the double phosphate SrZn₂(PO₄)₂ with the hurlbutite structure [40]. However, the existence of single-phase intermediate phosphates and their compositional limits has not been established. SrZn₂(PO₄)₂ (ICSD_68881) is isostructural with the mineral hurlbutite CaBe₂(PO₄)₂ [40], and its strontium analogue strontiohurlbutite, which is crystallized in the form SG P2₁/c. The unit cell parameters are a = 8.323(4) Å, b = 9.510(4) Å, c = 9.032(4) Å, β = 92.3(3)°, V = 714.33(6) ų, Z = 4. The Sr atoms occupy one crystallographic site, surrounded by a seven-vertex oxygen polyhedron, which represents a slightly distorted monocapped trigonal prism. This is presented with two different Sr–O distances. The Zn atoms form tetrahedra (Figure 7), similar to orthophosphate α-Zn₃(PO₄)₂. The PO₄ tetrahedra connect the cationic sites in a 3D framework. The distance Sr–Sr between nearest polyhedra is 5.533 Å, while the second nearest Sr-Sr distance is 6.801 Å. The SrZn₂(PO₄)₂ host can be doped with small amount of Eu³⁺ and Bi³⁺ ions not exceeding 3 mol.%, as was shown in [41]. These atoms are located only in Sr sites of the structure. So, the hurlbutite structure is characterized by a low isomorphic capacity.

In summary, all described structures are formed by metal atoms’ coordination polyhedra linked into a 3D framework through PO₄ tetrahedra. These connected framework structures offer several advantages for REE ions’ luminescence properties. Such 3D framework prevents defects and oxygen vacancies, while the rigid framework shields luminescence activators from oxidation and thermal degradation. Framework structures of phosphates also allow for easy variation of activator composition and coordination environment. This enables more precise control of emission wavelength, reduces concentration quenching, and increases quantum yield. Therefore, this work aims to discover new phases in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ system and characterize them using powder X-ray diffraction methods and quantitative phase analysis. The structures of new phosphates were refined by the Rietveld method. The PL properties of the synthesized series were studied and compared among the series. The PL properties are discussed based on the crystallographic criteria.

2. Results and Discussion

2.1. Substitution Sr²⁺ → Zn²⁺

The PXRD patterns for the series of phosphates with Sr²⁺ → Zn²⁺ substitution in the Sr_3–xZn_x(PO₄)₂ series are presented in Figure 8a. The Zn²⁺ concentration varied from x = 0 to x = 2 (

Table 1. The main Crystallographic data for Sr₉Zn_1.5(PO₄)₇ phosphate.

Sample	Sr9Zn1.5(PO4)7
SG	R$\overline{3}$m
Lattice parameters: a, Å	10.595(3)
c, Å	19.738(6)
Unit cell volume, Å3	1918.9(1)
Calculated density, g/cm3	4.0277
Data collection
Diffractometer	RIGAKU Ultima III
Radiation/Wavelength (k, Å)	CuKα1+α2/1.54186 Å
2θ range (o)	5–65
Step scan (2θ)	0.02
Refinement
Background function	15 Legendre polynoms
R and Rw for Bragg reflections, %	9.01/10.42
RP, RwP, Rexp, %	7.57/11.23/4.37
Goodness of fit (ChiQ)	2.58
Max./min. residual density, e/Å3	1.68/−1.48
CSD number	2,494,889

). At x = 0, the structure corresponds to palmierite α-Sr₃(PO₄)₂-type phase. The amount x = 0.43, which is equal to the formula Sr₉Zn_1.5(PO₄)₇ (Z = 6), is crystallized at β-Sr₃(PO₄)₂ structure as (Sr_0.85Zn_0.15)₃(PO₄)₂ [19]. The range 0.5 ≤ x ≤ 1.5 demonstrates formation of both the β-Sr₃(PO₄)₂ and the strontiohurlbutite structures. So, Zn²⁺ stabilizes the β-Sr₃(PO₄)₂ type structure. This phase is mostly found in the range 0.5 < x ≤ 1.5 (Figure 8a). The amount x = 2 was crystalized in the strontiohurlbutite structure SrZn₂(PO₄)₂ [40] (Figure 8b). Deviations from the stoichiometric compositions of Sr₃(PO₄)₂ or SrZn₂(PO₄)₂ results in a two-phase region except for the range 0.35 ≤ x ≤ 0.45, where pure strontiowhitlockite (β-Sr₃(PO₄)₂) is stabilized. Quantitative phase analysis shows that at x = 1.5, the ratio between SrZn₂(PO₄)₂ and β-Sr₃(PO₄)₂ was 38.6/61.4%. At x = 1, the ratio was 23.1/76.9%. For x = 0.5, the ratio of α-Sr₃(PO₄)₂ to β-Sr₃(PO₄)₂ was 90.2/9.8%.

2.1.1. Rietveld Refinement

The crystal structure refinement of Sr₉Zn_1.5(PO₄)₇ (Z = 6) or Sr_2.57Zn_0.43(PO₄)₂ (Z = 21) was carried out by the Rietveld method using Jana2006 software [42]. The centrosymmetric crystal structure model of the phosphate Sr₉Ni_1.5(PO₄)₇ [43] was used as the initial model for further refinement. Pseudo-Voigt functions were used for fitting the reflection profiles. The background was described by a Legendre polynomial function.

At the first step of the refinement, Zn atoms were placed into the M4 and M5 sites (like Ni atoms in the initial Sr₉Ni_1.5(PO₄)₇ structure), and the coordinates and atomic displacement parameters (U_iso) were refined. High residual electron density was detected near the Sr1 site. Thus, this position was split into Sr1_a and Sr1_b sites. Due to the large values of U_iso for the O21 and O24 sites, they were split into two positions. The P–O distances were restricted at 1.54 Å ± 7%.

In the beginning, the U_iso for all O atoms were fixed as the same value and refined together. The same approach was performed for Sr1, Sr3, Zn4, Zn5 and P1, P2 sites. During one of the last cycles of refinement, negative values were obtained, which makes no physical sense. That is why the U_iso value for the above sites was fixed on the final cycle of refinement.

In the final stage, the coordinates of all atoms were refined, as well as the occupancies (a_i) of the Sr1_a, Sr1_b, Sr3_a, Sr3_b, O21_a, O21_b, O24_a, O24_b sites, and U_iso for Zn4, Zn5, P2, and O11 sites. The final results are presented in Table 1 and Figure 9.

Table 2. Fractional atomic coordinates, site symmetry, isotropic displacement atomic parameters (U_iso∙100) and site occupation for Sr₉Zn_1.5(PO₄)₇ sample from PXRD data.

Atom	Occupation, ai	x	y	z	Uiso∙100
Sr1a	0.87(1)	0.1885(4)	−0.1885(4)	0.5362(2)	0.14
Sr1b	0.13(1)	0.235(3)	−0.235(3)	0.5504(15)	0.14
Sr3a	0.401(3)	−0.5208(6)	0.5208(6)	0.0093(5)	0.49
Sr3b	0.099(3)	0.389(1)	0.611(1)	−0.017(2)	0.49
Zn4	0.0833	0.174(6)	0.087(3)	0.388(3)	3
Zn5	1	0	0	0	1.3(4)
P1	1	−2/3	2/3	1/6	0.01
P2	1	0.4886(6)	−0.4886(6)	0.3979(8)	2.8(4)
O11	1	−0.542(2)	0.811(2)	0.138(1)	0.1
O21a	0.62(5)	0.463(2)	−0.463(2)	0.319(1)	0.1
O21b	0.38(5)	0.454(6)	−0.561(7)	0.323(2)	0.1
O22	0.87(1)	0.5758(8)	−0.5758(8)	0.407(1)	0.1
O24a	0.13(1)	0.455(6)	−0.455(6)	0.471(3)	0.1
O24b	0.87(1)	0.1885(4)	−0.1885(4)	0.5362(2)	0.1

lists the fractional atomic coordinates, site symmetry, occupancy, and isotropic atomic displacement parameters.

Table 3. Selected interatomic distances (Å) in Sr₉Zn_1.5(PO₄)₇ from Rietveld Refinement data.

Bonds		Distance, Ắ	Bonds		Distance, Ắ
Sr1a	O21b	2.45(4)	Sr3b	O21a × 2	2.16(3)
	O22	2.41(1)		O22 × 2	2.58(3)
	O11	2.48(3)		O21b × 2	2.76(9)
	O11	2.55(3)	Zn4	O11	1.91(6)
	O21a	2.53(2)		O11	1.91(6)
	O24a	2.58(1)		O21b	1.91(9)
	O24a	2.58(1)		O21b	1.91(9)
	O24b	2.79(6)		O11	2.60(5)
Sr3a	O22	2.46(1)		O11	2.60(7)
	O22	2.57(1)		O11	2.84(6)
	O22	2.57(1)		O11	2.84(6)
	O21b	2.55(5)	Zn5	O24a × 6	2.21(2)
	O11	2.93(3)	P1	O11 × 4	1.54(3)
	O11	2.93(3)	P2	O24a	1.61(1)
	O21b	2.85(5)		O21b	1.62(5)
	O21b	2.85(5)		O22 × 2	1.70(1)

include selected interatomic distances.

2.1.2. Combinatorial Complexity Calculations

The combinatorial complexity $I_{\mathrm{G}}^{\mathrm{K}\mathrm{K}}$ was calculated for some β-Sr₃(PO₄)₂- and β-Ca₃(PO₄)₂-type phosphates to analyze the influence of substitutions on the crystal structure. As a rule, combinatorial complexity of the crystal structure provides a negative contribution to the configurational entropy of a crystalline solid [44]. The value of combinatorial complexity of a crystal structure sensitive to partial occupancies was introduced by Kaußler and Kieslich [45]. $I_{\mathrm{G}}^{\mathrm{K}\mathrm{K}}$ depends on its ordinary Krivovichev complexity [46] $I_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}}$:

$$I_{\mathrm{G}}^{\mathrm{K}\mathrm{K}}={I_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}}+I}_{\mathrm{m}\mathrm{i}\mathrm{x}},$$

where I_mix ≥ 0 is a contribution of mixing to the overall crystal–chemical complexity. As was shown by Krivovichev et al. [47], the overall crystal–chemical complexity ${(I}_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}})\prime$ should be calculated as follows:

$${(I}_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}})\prime ={2I_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}}-I}_{\mathrm{G}}^{\mathrm{K}\mathrm{K}}.$$

The calculated values are given in

Table 4. Combinatorial complexity values for some Ca- and Sr-substitutes phosphates with the whitlockite-type structure.

Sample	Ref.	Complexity, bit/atom
		${\mathit{I}}_{\mathbf{G}}^{\mathbf{s}\mathbf{t}\mathbf{r}}$	${\mathit{I}}_{\mathbf{G}}^{\mathbf{K}\mathbf{K}}$	Imix	${(\mathit{I}}_{\mathbf{G}}^{\mathbf{s}\mathbf{t}\mathbf{r}})\mathbf{\prime }$
Ca9Zn1.5(PO4)7		4.076	4.098	0.022	4.054
Sr9Zn1.5(PO4)7	This work	3.267	4.203	0.936	2.331
Sr9.3Ni1.2(PO4)7		3.016	3.887	0.871	2.145
Sr8ZnEu(PO4)7		3.016	3.445	0.429	2.588

. The value of ${(I}_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}})\prime$ accounts for the influence of chemical substitutions and vacancies on the overall structural complexity and can therefore be considered as a degree of atomic order that takes into account not only structural architecture, but also its chemical nature. The presence of chemical substitutions (including substitutions by vacancies) results in a decrease of the atomic order and thus in a decrease of ${(I}_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}})\prime$. Moreover, unlike $I_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}}$, in some cases it is possible that ${(I}_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}})\prime$ < 0.

2.1.3. DRS Study

The UV-Vis diffuse reflectance spectra of the synthesized Sr₉Zn_1.5(PO₄)₇ phosphate reveal a complex absorption profile, indicative of multiple electronic transitions. For a detailed comparative analysis of the absorption edge, the spectra were examined in the 200–450 nm range and normalized to the maximum intensity within the 250–270 nm interval (Figure 10a).

A notable feature is the pronounced absorption tail extending to 400 nm, indicative of defect states or structural disorder. The spectrum was deconvoluted into three components: two Lorentzian peaks (Peak 1, 2) and one Gaussian (Peak 3), as summarized in Table S1 of the Supporting Information.

Peak 2 represents the fundamental absorption edge, while the Gaussian Peak 3 corresponds to defect-related transitions. The substantial contribution of Peak 3 (22%) confirms significant density of in-gap states. Peak 1 corresponds to a high-energy electronic transition. The optical band gap (E_g) was determined from the DRS data using the Tauc method (Figure 10b). The absorption coefficient was derived from the Kubelka–Munk function, F(R_∞). Empirical analysis revealed that the best linear fit in the Tauc plot was achieved for the direct allowed transition model (exponent n = 2). This indicates that the dominant optical transition in Sr₉Zn_1.5(PO₄)₇ is direct, likely induced by the modification of the electronic structure upon zinc doping.

The analysis of the (F(R_∞)·hν)² vs. hν plot revealed two distinct linear regions, allowing for the determination of two optical band gaps: E_g1 = 4.92 eV, E_g2 = 3.80 eV. The presence of two E_g values suggests a complex band structure and the possibility of several direct optical transitions in the material.

2.1.4. Raman Spectroscopy Study

The Raman spectrum of the Sr₉Zn_1.5(PO₄)₇ phosphate was analyzed to gain insight into its local structure and the vibrational behavior of phosphate anions. The spectrum exhibits a complex profile characteristic of phosphates with isolated PO₄ tetrahedra situated in multiple non-equivalent crystallographic sites. For a detailed analysis, the spectrum was deconvoluted into individual Lorentzian components across three characteristic regions (Figure 11), reflecting the different types of vibrational modes. Lorentzian profiles were used for the fitting of the internal phosphate modes (ν₁, ν₂, ν₄), while the low-intensity bands in the lattice mode region were estimated without fitting.

In Region 1 (100–350 cm⁻¹, Figure 11a), the observed low-intensity bands (peaks 19–23) are attributed to external lattice modes, which involve translational and vibrational motions of the PO₄ units coupled with vibrations of the Sr/Zn cationic framework.

Region 2 (350–650 cm⁻¹, Figure 11b) contains the internal bending vibrations of the PO₄ tetrahedra. The deconvolution of the ν₄ mode (peaks 7–14) revealed multiple components, with the most intense bands at 642.6 cm⁻¹ (peak 8) and 650.7 cm⁻¹ (peak 7). The ν₂ mode (peaks 15–18) is characterized by a broad envelope with a maximum at 443.9 cm⁻¹ (peak 17). The significant splitting and broadening observed in this region are direct evidence of the low local symmetry and distortion of the PO₄ tetrahedra. This finding is in excellent agreement with the structural disorder and splitting of oxygen positions revealed by the Rietveld refinement.

The high-frequency Region 3 (900–1200 cm⁻¹, Figure 11c) corresponds to the P–O stretching vibrations. The deconvolution of the ν₁ mode resolved two dominant components at 988.6 cm⁻¹ (peak 5) and 984.3 cm⁻¹ (peak 6). The ν₃ mode is represented by a complex contour, with the main fitted component at 1026.7 cm⁻¹ (peak 3). Additional high-frequency components at 1150 cm⁻¹ (peak 1) and 1103.8 cm⁻¹ (peak 2) were also identified (Figure 11c, black box). The observed splitting of the ν₁ and ν₃ modes is a characteristic feature of whitlockite-type structures. Similar to β-Ca₃(PO₄)₂ (β-TCP) [48], the presence of multiple components in the ν₁ region directly reflects the existence of several non-equivalent PO₄ tetrahedra with distinct P–O bond lengths. The broadening of these Raman bands in Sr₉Zn_1.5(PO₄)₇, compared to the sharper peaks in well-ordered phosphates like hydroxyapatite [49], indicates a significant structural disorder. This disorder arises from the random distribution of Sr²⁺, Zn²⁺, and vacancies over multiple cationic sites, analogous to the half-occupancy of the Ca(4) site in pure β-TCP, which also leads to broad spectral features [50]. The observed peak broadening is therefore a direct spectroscopic signature of the high combinatorial complexity calculated for this compound (Section 2.1.2).

A notable correlation is observed between the full width at half maximum (FWHM) of the ν₁ mode components and the overall fit quality (χ²) for Region 3. The broader bands of the ν₁ components (peaks 5 and 6), indicative of a greater distribution of P–O bond lengths and angles, correlate with a higher χ² value for the multi-peak fit. This phenomenon can be understood by considering the extreme sensitivity of the ν₁ mode to the local cationic environment. In substituted whitlockites, the nature of the substituting cation significantly influences the ν₁ band positions [50]. In our case, the random distribution of Sr/Zn/vacancies creates a continuous range of local environments, leading to inhomogeneous broadening. This broadening, in turn, causes severe overlap of the ν₃ components, making their reliable deconvolution more challenging and increasing the χ² value. Thus, the correlation between FWHM(ν₁) and χ² is not an artifact but a direct spectroscopic consequence of the chemical and positional disorder within the Sr₉Zn_1.5(PO₄)₇ structure.

The Raman data thus provide independent confirmation of the structural conclusions drawn from the Rietveld refinement. The observed peak splitting and broadening corroborate the model of a complex crystal structure with distorted phosphate tetrahedra occupying several non-equivalent sites. Furthermore, the Raman spectrum of Sr₉Zn_1.5(PO₄)₇ is distinctly different from that of the palmierite-type α-Sr₃(PO₄)₂, which exhibits a simpler spectral profile [51]. The complex and broadened features observed here are unequivocally characteristic of a whitlockite-type structure with a high degree of cationic substitution and disorder [52], firmly supporting the phase identification made by PXRD. The correlation between vibrational peak broadening and the combinatorial structural complexity, as quantified in Section 2.1.2, underscores the profound influence of cationic disorder on the local anion environment in the Sr₉Zn_1.5(PO₄)₇ host.

2.2. Substitution Sr²⁺ → Eu³⁺

The Sr²⁺ → Eu³⁺ substitution revealed severely limited isomorphic capacity in phases with the palmierite structure type (α-Sr₃(PO₄)₂ [6]). As was previously demonstrated in [38], Sr²⁺ → Eu³⁺ substitution in Sr₃(PO₄)₂ can be realized up to 0.3 mol.% of Eu³⁺ ions [53], even when a coupled charge compensation was used. Increasing the Eu³⁺ concentration in Sr₃(PO₄)₂ results in the formation of an impurity phase with eulytite-type structure [38].

In the phosphate series with eulytite-type structure with general formula Sr_3–1.5xEu_1+x(PO₄)₃ where x varies from 0 to 2.0, all intermediate substances are not single-phased, except for the initial solid solution of double phosphate Sr₃Eu(PO₄)₃ (x = 0) (Figure 12a) and the end-member EuPO₄ (x = 2). The identified impurity phase for intermediate substances is monazite EuPO₄ (Figure 12b), consistent with classical phase diagrams showing a two-phase region between Sr₃Eu(PO₄)₃ and EuPO₄. Quantitative phase analysis reveals that as Eu³⁺ concentration increases, the EuPO₄ amount rises from 8% at x = 0.2 to 23% at x = 0.5. At x = 1.7, the monazite phase becomes dominant, reaching 79% at x = 1.7.

2.3. Co-Substitution Sr²⁺ → Zn²⁺, Eu³⁺

2.3.1. Sr_9–xZn_xEu(PO₄)₇

The series of Sr_9–xZn_xEu(PO₄)₇ phosphates was previously studied by us in [54]. Stabilization of phases in Sr_9–xZn_xEu(PO₄)₇ solid solution with the β-Sr₃(PO₄)₂ at room temperature structural type is observed only for the stoichiometric phosphate Sr₈ZnEu(PO₄)₇ [2,54]. This formula corresponds to formation of the octahedral site fully occupied by Zn atoms. This site was named M5, like in the base β-Ca₃(PO₄)₂ structure. The second harmonic test revealed that the Sr₈ZnEu(PO₄)₇ sample has a centrosymmetric structure (SG R$\overline{3}$m). Changes in zinc concentration destabilize the structure through incomplete occupation of the octahedral site. This destabilization arises because the large ionic radii of Sr²⁺ or Eu³⁺ (

Table 5. Ionic radius difference percentages (D_r) between host cations and doped R³⁺ ions.

Doped R3+ Ion	Radius, Å/CN	Dr, %
		Sr2+ 1.26 Å/8	Sr2+ 1.18 Å/6	Ca2+ 1.12 Å/8	Ca2+ 1.00 Å/6	Zn2+ 0.74 Å/6
Ga3+ [58]	0.62 Å/6	–	47.5	–	38	16.2
Lu3+ [59]	0.98 Å/8	22.2	–	12.5
	0.86 Å/6	–	27.1	–	14	16.2
Y3+ [33]	1.02 Å/8	19	–	8.9
	0.90 Å/6	–	23.7	–	10	21.6
Eu3+ This work	1.07 Å/8	15.1	–	4.5		–
	0.95 Å/6	–	19.5	–	5.0	28

) are incompatible with the octahedral coordination environment. This differs from calcium analogs, where the concentration series Ca_9–xZn_xEu(PO₄)₇ exists across the entire range—both the phosphates Ca₉Eu(PO₄)₇ (with calcium cations occupying the M5 position) [55] and Ca₈ZnEu(PO₄)₇ [56] are stable. In contrast, the phosphate Sr₉Eu(PO₄)₇ cannot be stabilized as a single phase, which is explained by the difference in ionic radii (Table 5). Table 5 also summarizes data on radius difference percentage (D_r) [57] in Sr₉R³⁺(PO₄)₇ phosphates, where R³⁺–Ga³⁺, Lu³⁺, Y³⁺.

2.3.2. Sr_9.5–1.5xZnEu_x(PO₄)₇

PXRD patterns for the series with heterovalent double substitution Sr²⁺ → Zn²⁺, Eu³⁺ with the general formula Sr_9.5–1.5xZnEu_x(PO₄)₇ are shown in Figure 13a. At 0.25 ≤ x ≤ 1.0 values the PXRD patterns are identical. All reflections correspond to the phosphate (Sr_0.85Zn_0.15)₃(PO₄)₂ (PDF-2 No 14-207). However, the composition x = 0 (Sr_9.5Zn(PO₄)₇) is different. A mixture of α- and β-Sr₃(PO₄)₂ was observed (Figure 13b) at amounts of 18 and 82%, respectively.

An amount of Zn²⁺ ions in the β-Sr₃(PO₄)₂ host less than 15 mol.% leads to instability in this structure. There are crystal–chemical rules for β-Sr₃(PO₄)₂ crystallization: requirements are the formation of an octahedral M5 site (Figure 5) occupied by Zn atoms and the occupation of a position with variable occupancy M4 on the c axis with a strongly distorted octahedral environment. As was mentioned above [19], the β-Sr₃(PO₄)₂ phase remains stable at room temperature only within a narrow range of x from 0.35 to 0.45 in Sr_3–xZn_x(PO₄)₂ or Zn²⁺ content at 12–15 mol.%. In series with heterovalent co-substitution Sr²⁺ → Zn²⁺,Eu³⁺–Sr_9–xZn_xEu(PO₄)₇ [54] and Sr_9.5–1.5xZnEu_x(PO₄)₇, the stabilization of the β-Sr₃(PO₄)₂ phase occurs at a lower Zn²⁺ concentration of 10 mol.%. This happens because the M4 site axis becomes vacant according to the substitution scheme Sr²⁺ → Eu³⁺ + □ (where □ represents a vacancy). Similar behaviors in phase formation were observed in Sr_9–xMg_xEu(PO₄)₇, Sr_9–xMn_xTb(PO₄)₇ [54], and Sr_9–xMn_xEu(PO₄)₇ [60].

2.3.3. Sr_9–1.5xZn_1.5Eu_x(PO₄)₇

PXRD patterns for another series with heterovalent substitution with the general formula Sr_9–1.5xZn_1.5Eu_x(PO₄)₇ are shown in Figure 14a. Unlike the previous phosphate series Sr_9.5–1.5xZnEu_x(PO₄)₇, the sample with x = 0—nominal formula Sr₉Zn_1.5(PO₄)₇—is single-phased (Figure 9 and Figure 14a). At 0 ≤ x ≤ 0.75, the observed reflections correspond to the β-Sr₃(PO₄)₂ phase, with no reflections from secondary phase impurities. An unbroken series of solid solutions is thus formed. For the sample with x = 1.0—nominal formula Sr_7.5Zn_1.5Eu(PO₄)₇—reflections of the eulytite Sr₃Eu(PO₄)₃ phase are observed (Figure 14b). Quantitative phase analysis shows that the main β-Sr₃(PO₄)₂ phase and the impurity Sr₃Eu(PO₄)₃ phase are present in a 95:5 ratio.

2.3.4. Sr_3–xZn_xEu(PO₄)₃

A series of Sr_3–xZn_xEu(PO₄)₃ (0 ≤ x ≤ 2) phosphates with heterovalent Sr²⁺ → Zn²⁺, Eu³⁺ co-substitution was studied, with eulytite phase Sr₃Eu(PO₄)₃ as the initial structure at x = 0. Contrary to expectations, regions of β-Sr₃(PO₄)₂-type structure stabilization were identified instead of phases based on the eulytite structure. In the composition range 0.25 ≤ x ≤ 0.5, a single phase corresponding to β-Sr₃(PO₄)₂ phase crystallized. Increasing Zn²⁺ concentration led to the appearance of reflexes on the PXRD patterns corresponding to the monazite phase (x = 1.0 and 2.0). This indicates that zinc ions displace europium ions from structural positions, and the phases of double Sr-Zn phosphates are more thermodynamically stable than Sr-Eu phosphates, as no impurities of the Zn₃(PO₄)₂ phase are observed. The co-substitution Sr²⁺ → Zn²⁺, Eu³⁺ in the phosphates successfully expanded the concentration limits for stabilizing the β-Sr₃(PO₄)₂-type phase.

2.3.5. The SHG Test for Co-Substitution Sr²⁺ → Zn²⁺, Eu³⁺ Phosphates

The structure of β-Sr₃(PO₄)₂-type compounds was calculated using different models with different SG. In [61], the SG R$\overline{3}$m was used for the refinement of the base Sr₉Mg_1.5(PO₄)₇ and doped Sr₉Mg_1.5(PO₄)₇:Ce³⁺, A⁺ (A⁺ = Li⁺, Na⁺, K⁺) structures. In Sr₈ZnIn_(1−m)Lu_m(PO₄)₇:Eu²⁺/Mn²⁺ [23], the model with SG I2/a was used related to the Sr₉In(PO₄)₇ initial phosphate. At the same time in [62], the polar R3c SG was used for the Sr₉LiY_0.667(PO₄)₇ structure.

The SG for the synthesized β-Sr₃(PO₄)₂-type series was confirmed by SHG measurements. For the Sr_9.5–1.5xZnEu_x(PO₄)₇ and Sr_9–1.5xZn_1.5Eu_x(PO₄)₇ series, SHG signals were completely absent across the entire single-phase range of x. This indicates that the samples are centrosymmetric, confirming that the SG R$\overline{3}$m should be used. In the two-phase sample with nominal formula Sr_9.5Zn(PO₄)₇, the SHG value was approximately zero. The absence of signal in this sample is likely because the second phase is palmierite α-Sr₃(PO₄)₂ (PDF-2 № 80-1614), which also crystallizes in SG R$\overline{3}$m. In contrast, the presence of a signal for the sample with formula Sr_7.5Zn_1.5Eu(PO₄)₇ may be due to a eulytite-type phase impurity with non-centrosymmetric structure (SG I4$\overline{3}$d). The SHG signal in this sample is ~1.0 unit relative to the quartz reference. This small optical nonlinearity value is attributed to the small quantity of impurity, at 5%.

2.4. Eu³⁺ Doping of the Described Hosts

To compare the PL intensity of Eu³⁺-doped phases in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ system, samples with α-(palmierite)-, β-Sr₃(PO₄)₂ (strontiowhitlockite)-, SrZn₂(PO₄)₂ (strontiohurlbutite)-, and Sr₃La(PO₄)₃ (eulytite)-type structures were doped at 0.3 mol.% of Eu³⁺ ions. The corresponding formulas were calculated to be Sr_2.985Eu_0.01(PO₄)₂, Sr_0.985Zn₂Eu_0.01(PO₄)₂, Sr_2.535Zn_0.45Eu_0.01(PO₄)₂ and Sr₃La_0.99Eu_0.01(PO₄)₃. The PXRD patterns of the synthesized phases are shown in Figure 15. For all samples, the number and position of reflections exactly correspond to the respective structural types: Sr₃(PO₄)₂ (PDF-2 № 80-1614), SrZn₂(PO₄)₂ (PDF-2 № 80-1062), (Sr_0.85Zn_0.15)₃(PO₄)₂ (PDF-2 № 14-207), and Sr₃Eu(PO₄)₃ (PDF-2 № 48-410). No reflections corresponding to secondary phases or initial reagents were observed (Figure 15). Thus, doping these structures with Eu³⁺ ions up to 0.3 mol% does not prevent phase formation.

2.5. PL Study

2.5.1. PL Study for Co-Substituted Sr²⁺ → Zn²⁺, Eu³⁺ Phosphates

The PLE spectra monitored at λ_em = 614 nm for the Sr_9.5–1.5xZn_1.5Eu_x(PO₄)₇ series are shown in Figure 16. The number and positions of the observed bands corresponding to the 4f–4f transitions of Eu³⁺ ions for the samples with x = 0.25–1.0 remained unchanged regardless of the cation ratio in the host. The broad band from 250 to 300 nm can be attributed to the charge-transfer band (CTB), and a series of narrow bands in the 300–500 nm range can be attributed to the intracenter f–f transitions of Eu³⁺. The CTB corresponds to electrons excited from the 2p delocalized orbital of the O²⁻ ion to the incomplete 4f orbital of the Eu³⁺ ion [63]. Linear dependence on x is not observed for series. In single-phased samples x = 0.25–0.75, it may be attributed to different occupations of sites by Eu³⁺ in the strontiowhitlockite host. As was shown in [57], the intensity of CTB depends on the polyhedra DI and bond length for calcium whitlockite, where Eu³⁺ ions occupy sites in a statistically random manner. The decrease in CTB intensity for x = 1.0 may be due to impurity phases isostructural to Sr₃Eu(PO₄)₃. The bands located at 320, 361, 376, 382, 395, 416, and 465 nm correspond to the ⁷F₀ → ⁵H₃, ⁵D₄, ⁵G_J, ⁵L₇, ⁵L₆, ⁵D₃, and ⁵D₂ transitions of Eu³⁺ ions. The PLE spectra for Sr_9.5–1.5xZnEu_x(PO₄)₇ series demonstrate the same behavior and are shown in Figure S2 of the Supporting Information.

The PL spectra of Sr_9–1.5xZn_1.5Eu_x(PO₄)₇ series at 395 nm excitation are shown in Figure 17a (Figure S3 of the Supporting Information shows the PL spectra for Sr_9.5–1.5xZnEu_x(PO₄)₇). The characteristic emission bands for Eu³⁺ ions are observed in the spectra. The narrow lines at 578, 594, 617, 650, and 700 nm correspond to ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) (Figure 17a). The most intense band is observed at 615 nm attributed to the hypersensitive ⁵D₀ → ⁷F₂ electric-dipole transition. This fact indicates the absence of inversion symmetry in the Eu³⁺ ion surroundings in the Sr_9–1.5xZn_1.5Eu_x(PO₄)₇ samples. The same trend is observed in Sr_9.5–1.5xZnEu_x(PO₄)₇ series (Figure S3). The intensity of ⁵D₀ → ⁷F₂ transition shows a maximum at x = 0.75 in accordance with PLE spectra (Figure 17a). Transitions corresponding to the 4f5d–4f transitions of Eu²⁺ are not observed in the PL spectra (Figure 17a). The PL spectra confirm the absence of partial self-reduction Eu³⁺ → Eu²⁺ in strontiowhitlockite compared to Eu³⁺-doped palmierite-type Sr₃(PO₄)₂:Eu³⁺ [38] or Ba₃(PO₄)₂:Eu³⁺ [16] phosphors. In this research, the samples were obtained in air. Commonly, for Eu²⁺ stabilization in strontiowhitlockite [64,65,66], thermal treatment in a reducing atmosphere is required. Additionally, in the series Sr_9–xMn_xEu(PO₄)₇ [60] the Eu²⁺ state has been detected; however, in the current orthophosphates no reducing agents are present. Thus, Eu is stabilized at the 3+ state in the strontiowhitlockite structure.

The normalized integral intensity is shown in the inset in Figure 17b. The PL intensity changes non-linearly depending on Eu³⁺ content in the host, which may indicate its non-uniform distribution in the cationic sublattice. However, the most efficient intensity was observed for samples with the highest Eu³⁺ concentration, thus, no concentration quenching was observed. The comparison of the samples in the Sr_9–1.5xZn_1.5Eu_x(PO₄)₇ and Sr_9.5–1.5xZnEu_x(PO₄)₇ series at x = 1 reveals that increased Zn²⁺ concentration in the sample has a positive effect on the PL properties, as previously was observed in some Ca-based whitlockite-type phosphors [56]. The values of calculated asymmetry ratio R/O, calculated by the equation R/O = I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁), are listed in

Table 6. Calculated asymmetry ratio R/O, CIE coordinates (X;Y) and decay times (τ) for Eu³⁺ emission in synthesized Sr-based phosphate hosts.

	Sr9–1.5xZn1.5Eux(PO4)7
x	0.25	0.5	0.75	1.00
R/O	3.68	3.52	3.72	3.44
CIE (X;Y)	0.653; 0.351	0.649; 0.348	0.653; 0.349	0.649; 0.350
	Sr9.5–1.5xZnEux(PO4)7
x	0.25	0.5	0.75	1.00
R/O	3.65	3.51	3.58	3.63
CIE (X;Y)	0.650; 0.349	0.652; 0.348	0.651; 0.349	0.650; 0.349
	Sr3(PO4)2:Eu3+	SrZn2(PO4)2:Eu3+	Sr3La(PO4)3:Eu3+	(Sr0.85Zn0.15)3(PO4)2:Eu3+
crystallographic site symmetry	C1 + C3v	C4v	C3	C1
R/O	1.75	0.7	1.8	3.8
CIE (X;Y)	0.632; 0.366	0.623; 0.376	0.638; 0.361	0.651; 0.348

. There are no significant differences in Eu³⁺ surrounding for Sr_9.5–1.5xZnEu_x(PO₄)₇, based on R/O data, while some deviations in Eu³⁺ surrounding are observed for Sr_(9–1.5x)Zn_1.5Eu_x(PO₄)₇. Additionally, all phosphates demonstrate closed values for CIE coordinates (X;Y) (Table 6). Calculated CIE coordinates (X;Y) are located at red region and closed to red standard.

2.5.2. PL Study of Eu³⁺-Doped Hosts

To compare the PL properties of different phases in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ system, the synthesized phases were doped by Eu³⁺ ions at 0.3 mol.% (Table 7 in Section 3.1. Synthesis). The PL spectra of these phases are shown in Figure 18. All PL spectra were recorded at room temperature using an excitation wavelength of λ_ex = 395 nm. The spectra consist of intracenter 4f–4f transitions of Eu³⁺ ions and can be attributed to transitions from the ⁵D₀ excited state to terms of the ground state ⁷F_J (J = 0–4).

The emission spectra of Eu³⁺ doped phosphate (Figure 18a) consist of narrow transition bands located at 578 nm (⁵D₀ → ⁷F₀), 586 nm (⁵D₀ → ⁷F₁), 615 nm (⁵D₀ → ⁷F₂), 654 nm (⁵D₀ → ⁷F₃), and 696 nm (⁵D₀ → ⁷F₄). A difference in intensity of transitions is observed for different hosts. The electro-dipole (ED) transition ⁵D₀ → ⁷F₂ exceeds the magneto-dipole (MD) transition ⁵D₀ → ⁷F₁ (Figure 18a) in α-Sr₃(PO₄)₂:Eu³⁺, (Sr_0.85Zn_0.15)₃(PO₄)₂:Eu³⁺, and Sr₃La(PO₄)₃:Eu³⁺, while the inverse relationship was detected for SrZn₂(PO₄)₂.

The comparison of the integral intensity of the doped samples measures in the same conditions is shown in Figure 18e. At equal Eu³⁺ concentration in the host, the most intensive properties were observed for the (Sr_0.85Zn_0.15)₃(PO₄)₂:Eu³⁺ sample with the β-Sr₃(PO₄)₂ structure.

The asymmetry ratio (R/O) values are shown in Table 6. The lowest R/O is observed for SrZn₂(PO₄)₂; due to formation of a symmetrical Eu³⁺ surrounding, the polyhedra are characterized by the C_4v point group (Table 6). The α-Sr₃(PO₄)₂ and the Sr₃La(PO₄)₃ hosts are characterized by close values of 1.75 and 1.8, respectively, with domination of ED transition. So, the Eu³⁺ ions occupy sites with lower symmetry in α-Sr₃(PO₄)₂ and Sr₃La(PO₄)₃ than in SrZn₂(PO₄)₂. This dominance of the ED transition in Eu³⁺ ions relates to distortion of the local coordination environment within centrosymmetric SG α-Sr₃(PO₄)₂ and the Sr₃La(PO₄)₃ hosts. Moreover, both α-Sr₃(PO₄)₂ (Figure 4) and Sr₃La(PO₄)₃ (Figure 6) are characterized by higher distortion in Eu-O distances compared to SrZn₂(PO₄)₂ (Figure 7). This distortion arises from the charge imbalance introduced by the heterovalent substitution 3Sr²⁺ → 2Eu³⁺ + □. This mismatch creates defects in the host and distorts the cationic polyhedra, resulting in the dominance of the hypersensitive ⁵D₀ → ⁷F₂ transition. Similar behavior was observed in palmierite-type phosphors [16,67]. Conversely, charge compensation in the host leads to higher intensity of MD transition [38]. Increasing R/O value in the series SrZn₂(PO₄)₂ → Sr₃(PO₄)₂ → Sr₃La(PO₄)₃ is well correlated to a decrease in total integral intensity. The strontiowhitlockite exhibits the largest R/O value and largest integral intensity. The highest distortion of Eu³⁺ is observed—that is, all Eu³⁺ polyhedra can be characterized by C₁ symmetry—while lattice symmetry is described by SG R$\overline{3}$m. In summary, both reduced crystallographic site symmetry of Eu³⁺ and increased lattice symmetry led to the dominance of the ⁵D₀ → ⁷F₂ transition and the increase in total integral intensity. Additionally, the CIE coordinates of strontiowhitlockite are shifted more to the red region than other hosts (Table 6 and Figure 18f). This confirms that strontiowhitlockite is an ideal host for the realization of Eu³⁺ emissions as red phosphors.

The PLE spectra monitored at λ_em = 614 nm for various series are shown in Figure 18g. The number and positions of the narrow bands correspond to the 4f–4f transitions of Eu³⁺ ions. The broad band from 250 to 300 nm can be attributed to the CTB. The intensity of the CTB region is strongly dependent on Eu-O polarity and bond length. As was described in the Section 1, the highest bond length is observed for (Sr_0.85Zn_0.15)₃(PO₄)₂ in closed oxygen environment, probably leading to the highest intensity in CTB.

2.6. Phase Diagram in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ System

The phase analysis for all synthesized series in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ system can be presented in a ternary plot (Figure 19). The ternary plot allows us to obtain the following results: phases with the eulytite and hurlbutite structures do not have isomorphic capacity, which is apparently due to significant differences in the characteristic coordination environment of the Sr²⁺ and Zn²⁺ ions, caused by a significant size mismatch of these ions. Previously [19], it was assumed that the β-Sr₃(PO₄)₂ structure exists in a very limited range of Zn concentrations at room temperature. However, our results demonstrate that the stability region of the β-Sr₃(PO₄)₂ structure is significantly expanded upon heterovalent substitutions that facilitate vacancy formation. Thus, solid solutions with the β-Sr₃(PO₄)₂ structure were obtained in series with Mg²⁺ [54] and Mn²⁺ [60], and a combination of Zn²⁺ and Mn²⁺ [35].

3. Materials and Methods

3.1. Synthesis

In the present work, a series of phosphates with substitutions Sr²⁺ → Zn²⁺, Sr²⁺ → Eu³⁺, and co-substitution Sr²⁺ → Zn²⁺, Eu³⁺ along with some individual compositions were synthesized and studied. The data on the objects of research are summarized in Table 7. The phosphates were prepared from stoichiometric amounts of the initial reagents SrCO₃ (99.9%), ZnO (99.99%), NH₄H₂PO₄ (99.9%), and Eu₂O₃ (99.99%) by high-temperature solid-state reactions in muffle furnaces by heating slowly to avoid the rapid release of gaseous substances from the reaction zone and possible losses. The phosphates were heated to 1100 °C for 48 h, with preheating stages and intermediate homogenization of the reaction intermediates. The completeness of the synthesis was judged by the identity of the last two diffraction patterns.

Table 7. The synthesized series of solid solutions in the Sr–Zn–Eu phosphate system.

Substitution Type	General Formula of the Series	Range of x
Sr2+ → Zn2+	Sr3–xZnx(PO4)2	0 ≤ x ≤ 2.0
Sr2+ → Eu3+	Sr3–1.5xEu1+x(PO4)3	0 ≤ x ≤ 2.0
Sr2+ → Zn2+, Eu3+	Sr3–xZnxEu(PO4)3	0 ≤ x ≤ 2.0
	Sr9–xZnxEu(PO4)7 [54]	0 ≤ x ≤ 1.5
	Sr9.5–1.5xZnEux(PO4)7	0 ≤ x ≤ 1.0
	Sr9–1.5xZn1.5Eux(PO4)7	0 ≤ x ≤ 1.0
Individual compositions	Sr0.985Zn2Eu0.01(PO4)2; Sr2.985Eu0.01(PO4)2; Sr2.535Zn0.45Eu0.01(PO4)2 Sr3La0.99Eu0.01(PO4)3	x = 0.01

Table 7. The synthesized series of solid solutions in the Sr–Zn–Eu phosphate system.

Substitution Type	General Formula of the Series	Range of x
Sr2+ → Zn2+	Sr3–xZnx(PO4)2	0 ≤ x ≤ 2.0
Sr2+ → Eu3+	Sr3–1.5xEu1+x(PO4)3	0 ≤ x ≤ 2.0
Sr2+ → Zn2+, Eu3+	Sr3–xZnxEu(PO4)3	0 ≤ x ≤ 2.0
	Sr9–xZnxEu(PO4)7 [54]	0 ≤ x ≤ 1.5
	Sr9.5–1.5xZnEux(PO4)7	0 ≤ x ≤ 1.0
	Sr9–1.5xZn1.5Eux(PO4)7	0 ≤ x ≤ 1.0
Individual compositions	Sr0.985Zn2Eu0.01(PO4)2; Sr2.985Eu0.01(PO4)2; Sr2.535Zn0.45Eu0.01(PO4)2 Sr3La0.99Eu0.01(PO4)3	x = 0.01

3.2. Methods of Investigation

3.2.1. Powder X-Ray Diffraction (PXRD) Study

Powder X-ray diffraction (PXRD) was performed using a Rigaku SmartLab SE: 3 kW powder diffractometer with sealed X-ray tube, D/teX Ultra 250 silicon strip detector, vertical type θ–2θ geometry, and a HyPix-400 (2D HPAD) detector (Rigaku, Tokyo, Japan). Before conducting the diffraction analysis, the tube was adjusted to a voltage of 40 kV and a current of 15 mA. PXRD data were collected at room temperature in the 2θ range between 5° and 80° with a step interval of 0.02° and cuvette rotation speed of 30 rpm.

PXRD phase identification for the synthesized phases was performed using the Crystallographica Search-March program (version 2.0.3.1) and the Profile Diffraction Data PDF-2 and Cambridge Crystallographic Data Centre (CCDC) databases. The quantitative phase analysis was performed by March! (version 3.15) based on the Profile Diffraction Data PDF-4.

The Rietveld method was applied using the JANA2006 [42] software (version 20/02/2023). Background refinement was performed using a fifteen-order polynomial. Peak profiles were fitted using a modified Pseudo-Voigt function. At the first stage the unit cell parameters were refined by LeBail method. The obtained unit cell parameters were used as starting parameters in the Rietveld method. After the last refinement procedure, good agreement was found between the experimental and calculated patterns.

Illustrations of the crystal structures were created with DIAMOND [68] software (version 4.6.8).

3.2.2. Second Harmonic Generation Study

Second harmonic generation (SHG) was used to determine whether the sample has a center of symmetry. This confirmed the space group selected for structure refinement. The SHG signal was measured using a Q-switched YAG:Nd laser at λ_ω = 1064 nm in reflection mode.

3.2.3. Diffuse Reflectance Spectroscopy

Diffuse reflectance spectroscopy (DRS) was acquired using a Shimadzu UV-2600 Plus spectrophotometer (Shimadzu Corporation, Kyoto, Japan) equipped with an integrating sphere ISR-2600 (Shimadzu Corporation, Kyoto, Japan). The measurements were performed in the 200–1400 nm range with a scanning speed of 200 nm/min, a data interval of 1 nm, and a slit width of 2 nm. The integrating sphere was calibrated prior to measurements using a certified BaSO₄ plate as a 100% reflectance standard. Solid dispersion samples were gently ground to a fine powder and packed uniformly into a powder sample holder to ensure a flat surface for analysis.

3.2.4. Raman Spectroscopy

Raman spectra were collected using a Renishaw InVia Qontor confocal Raman microscope (Renishaw plc, Wotton-under-Edge, UK). Measurements were performed using 532 nm laser excitation with a 50× objective. The laser power at the sample surface was maintained at 75 mW to ensure a strong signal while avoiding sample degradation. Spectra were acquired in the extended range of 50–1200 cm⁻¹, covering both lattice modes and internal vibrations of the phosphate groups, using a 1200 lines/mm grating, which provided a spectral resolution of approximately 2 cm⁻¹. For each measurement, 25 accumulations with an exposure time of 10 s per accumulation were recorded to optimize the signal-to-noise ratio. To ensure data reproducibility and account for potential sample heterogeneity, spectra were collected from at least five different spots on the surface of each powdered sample. Wavelength calibration was verified prior to measurements using a silicon standard (520.7 cm⁻¹ band).

3.2.5. Combinatorial Complexity

The combinatorial complexity $I_{\mathrm{G}}^{\mathrm{K}\mathrm{K}}$ was calculated using crystIT ver. 0.2.1 [45] software, whilst the values of ordinary Krivovichev complexity $I_{\mathrm{G}}^{\mathrm{s}\mathrm{t}\mathrm{r}}$ were calculated using ToposPro ver. 5.5.4.1 [69]. The cif files for Ca₉Zn_1.5(PO₄)₇, Sr₈ZnEu(PO₄)₇ [25], Sr_9.3Ni_1.2(PO₄)₇ [43] and as-refined Sr₉Zn_1.5(PO₄)₇ were used for calculations.

3.2.6. Photoluminescence Study

A Cary Eclipse (Agilent Technologies) fluorescence spectrometer equipped with a 75 kW xenon light source (pulse length τ = 2 μs, pulse frequency ν = 80 Hz, wavelength resolution 0.5 nm; PMT Hamamatsu R928) was used to record photoluminescence emission (PL), excitation (PLE) spectra, as well as decay curves. All measurements were performed at room temperature and corrected for the sensitivity of the spectrometer.

4. Conclusions

This work comprehensively investigated phase formation in the Sr₃(PO₄)₂–Zn₃(PO₄)₂–EuPO₄ phosphate system. A series of novel phosphates were synthesized via solid-state reactions and characterized using X-ray diffraction and quantitative phase analysis. It was demonstrated that continuous solid solutions do not form in the Sr_3–xZn_x(PO₄)₂ and Sr_3–1.5xEu_1–x(PO₄)₃ series. Instead, new concentration regions for the stabilization of the strontiowhitlockite (β-Sr₃(PO₄)₂) phase were discovered. The stabilization of this phase in double phosphates requires a minimum of 15 mol.% of Zn²⁺ ions for their complete localization in two crystallographic positions along the c-axis. Conversely, in series with heterovalent co-substitution Sr²⁺ → (Eu³⁺, Zn²⁺), such as Sr_3–xZn_xEu(PO₄)₃ and Sr_9.5–1.5xZnEu_x(PO₄)₇, the formation of vacancies enables the crystallization of solid solutions with the strontiowhitlockite structure at a lower Zn²⁺ content of 10 mol.%. In these structures, the voluminous crystallographic sites are jointly occupied by Sr and Eu atoms, which creates new opportunities for tailoring the luminescent properties of Eu³⁺ ions. Photoluminescence studies confirmed that the strontiowhitlockite-based host provides the most efficient red emission, making it a promising candidate for luminescent applications.