Abnormal Eu³⁺ → Eu²⁺ Reduction in Ca_9−xMn_xEu(PO₄)₇ Phosphors: Structure and Luminescent Properties

Abstract

β-Ca₃(PO₄)₂-type phosphors Ca_9−xMn_xEu(PO₄)₇ have been synthesized by high-temperature solid-phase reactions. The crystal structure of Ca₈MnEu(PO₄)₇ was characterized by synchrotron X-ray diffraction. The phase transitions, magnetic and photoluminescence (PL) properties were studied. The abnormal reduction Eu³⁺ → Eu²⁺ in air was observed in Ca_9−xMn_xEu(PO₄)₇ according to PL spectra study and confirmed by X-ray photoelectron spectroscopy (XPS). Eu³⁺ shows partial reduction and coexistence of Eu²⁺/³⁺ states. It reflects in combination of a broad band from the Eu²⁺ 4f⁶5d¹ → 4f⁷ transition and a series of sharp lines attributed to ⁵D₀ → ⁷F_J transitions of Eu³⁺. Eu²⁺/Eu³⁺ ions are redistributed among two crystal sites, M1 and M3, while Mn²⁺ fully occupies octahedral site M5 in Ca₈MnEu(PO₄)₇. The main emission band was attributed to the ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ at 395 nm excitation. The abnormal quenching of Eu³⁺ emission was observed in Ca_9−xMn_xEu(PO₄)₇ phosphors with doping of the host by Mn²⁺ ions. The phenomena of abnormal reduction and quenching were discussed in detail.

1. Introduction

The global search for obtainable phosphors emitting in the red region of the spectrum continues to the present, due to the requirements for creating high-quality light from modern LED illuminators. The aims of red phosphors are to improve the color rendering (CRI) and lower the resulting corelated color temperature (CCT) of the LED package. The main requirements for such phosphors are:

(1)

A broad excitation band which can be matched well with the light from the LED chip (usually, at 450–470 nm from the InGaN chip). A number of Eu³⁺-doped inorganic red phosphors have been developed with the narrow emission due to the electric dipole ⁵D₀ → ⁷F₂ transition located at 610–630 nm. This transition is dominant in most hosts due to the noncentrosymmetric environment. However, such luminescent materials mismatch the excitation wavelengths from the LED chip since the main excitation band of the Eu³⁺ ion is located obviously at 392–396 nm and has a narrow character;

(2)

A narrow emission band in the red region (full width at half-maximum (FWHM) should not exceed 20 nm to reduce radiative losses in the near-IR range) [1]. Moreover, the barycenter of the emission band must not lie beyond 650 nm to minimize wasted emission [2];

(3)

A high stability in the environment and high luminous efficiency of radiation (LER). LER values increase with the narrowing of the emission line from red phosphor [1]. At the moment, the luminous efficiency from red phosphors can reach values of 54 lm/W (lumens per watt) [3].

Red-emitting phosphors that mostly meet the technological requirements can be created, for example, based on nitrides [4] or oxynitrides, such as (Ba,Sr)₂Si₅N₈:Eu²⁺ [5], (Ca,Sr)SiAlN₃:Eu²⁺ [6] or Sr[Li₂Al₂O₂N₂]:Eu²⁺ [7]. For obtaining such compositions, metal nitrides Si₃N₄ and AlN are usually used as raw materials which are air-sensitive and require a combination of high temperatures (up to 1700 °C) with a reducing atmosphere [8].

Phosphors based on hexafluorometallates, such as K₂SiF₆:Mn⁴⁺ [9], for instance, satisfy the requirement to reduce the consumption of rare-earth elements [10] and emit an extremely narrow photoluminescence band (used to increase the brightness of displays) due to intra-configuration 3d-3d transitions of Mn⁴⁺ ions. However, high-cost Si sources and not environment-friendly hydrofluoric acid are used in the synthesis. Moreover, they are very sensitive to moisture in the air and have long decay times, which limit their application.

According to the above, phosphate-based phosphors are important luminescent materials due to their excellent stability and available synthesis conditions. At the same time, the incorporation of Mn²⁺ ions into the β-Ca₃(PO₄)₂-type (β-TCP) structure phosphates makes it possible to both stabilize the crystal structure [11] and to obtain photoluminescent properties from Mn²⁺ ions [12]. The stabilization of the lattice occurs due to the reduction of geometric stress in the octahedral site during Ca²⁺ → Mn²⁺ substitution since the ionic radii of Mn²⁺ (r_VI = 0.83 Å) are less than Ca²⁺ (r_VI = 1.00 Å). Mn²⁺-doped phosphates show a broad emission band at 600–750 nm, peaked at 650 nm [12,13,14], which corresponds to the red region of the visible spectrum. Mn²⁺ ions, as an activator, show a wide emission band from the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition in the PL spectrum [13]. This emission strongly depends on the crystal field and can shift from green to red color. In an octahedral environment with a strong crystal field, Mn²⁺ ions usually generate red emission. If Mn²⁺ ions are located in a tetrahedral environment with a weak crystal field, green emission could be observed [15,16]. A serious advantage of Mn²⁺ doping into the β-TCP structure is a strong absorption of excitation at 450–480 nm, which is matching with the InGaN blue chip [13].

Co-doping strategy using Eu²⁺/Mn²⁺ [17] or Ce³⁺/Mn²⁺ [18] ions can improve the emission intensity due to the energy transfer processes in comparison to single-doped β-TCP-type hosts. For the CIE color adjustment and white light production, the combinations of rare-earth ions with Mn²⁺ at different concentrations can be used, such as Ce³⁺/Tb³⁺/Mn²⁺ or Eu²⁺/Tb³⁺/Mn²⁺. Moreover, there is a possibility to obtain Mn²⁺ to Eu³⁺ energy transfer for enhancement of red Eu³⁺ emission [15,19,20] while Eu²⁺ to Mn²⁺ energy transfer is commonly observed [21]. Some data on Mn²⁺ and RE/Mn²⁺ (RE—rare-earth element) doped β-TCP phosphors are summarized in

Table 1. The characteristics of PL spectra and some features of Mn²⁺ or RE/Mn²⁺-doped β-TCP-type phosphors.

Host	RE/Mn2+ Combination	PL Spectra (Band Location/Peak of the Band)	Features	Ref.
Ca9Mn1−xNa(PO4)7:xM2+, M = Zn, Mn	Mn2+ and Eu2+/Mn2+	600–750 nm/655 nm	The co-doping of Zn2+/Mg2+ and Mn2+ broke the intrinsic structural confinement of Mn2+ and improved its red emission.	[13]
Sr19(Mg1−xMnx)2(PO4)14: yEu2+	Eu2+/Mn2+	550–750 nm/610 nm	The emission bands were attributed to Eu2+ and Mn2+ in different sites.	[17]
Ca8MgGd(PO4)7:Eu2+/Mn2+	Eu2+/Mn2+	600–750 nm/650 nm λex = 365 nm	Efficient Eu2+ → Mn2+ energy transfer was observed, Eu2+ emission intensity decreased.	[22]
Ca9MMn(PO4)7 (M = Li, Na, K)	Mn2+	580–720 nm/645 nm	The excitation by β-source 90Sr-90Y. Mn2+ occupies M5 site, M+ ions (Li, Na, K) are located in M4 site.	[23]
Ca8.82−zGa(PO4)7:0.18Ce3+, zMn2+	Ce3+/Mn2+	Two broad emission bands: 350–450 nm/380 nm (Ce3+ emission) 600–700 nm/625 nm (Mn2+ emission)	The concentration quenching was observed above 9 mol. % Mn2+; the decreasing of quantum yield from 62.3% to 67 % with increasing Mn2+ concentration was explained by energy loss during Ce3+ → Mn2+ energy transfer process.	[24]
Ca10K(PO4)7:Eu2+, Mn2+	Eu2+/Mn2+	Two broad emission bands (λex = 347 nm): 425–500 nm/467 nm (Eu2+ emission) 600–700 nm/634 nm (Mn2+ emission)	Decreasing of PL intensity with increasing of Mn2+ concentration. The concentration quenching was observed above 7 mol. % Mn2+; resonant type of Eu2+ → Mn2+ energy transfer process via a dipole–quadrupole mechanism.	[14]
Ca9MgK(PO4)7:Eu2+, Mn2+	Eu2+/Mn2+	Two broad emission bands (λex = 347 nm) 425–500 nm/467 nm (Eu2+ emission) 600–700 nm/634 nm (Mn2+ emission)	Resonant type of Eu2+ → Mn2+ energy transfer with mechanism via a dipole–quadrupole interaction.	[25]
Ca8ZnCe(PO4)7:Eu2+, Mn2+	Ce3+/Eu2+/Mn2+	Three broad bands peaked (λex = 285 nm) 320–420 nm/375 nm (Ce3+ emission) 450–575 nm/500 nm (Eu2+ emission) 580–700 nm/645 nm (Mn2+ emission)	The energy transfers of Ce3+→Eu2+/Mn2+ and Eu2+ → Mn2+ ions were investigated. The emitting color can be adjusted from violet-blue to green/red-orange/white by doping/co-doping.	[26]

.

The idea of the present research was to combine the emission from Eu³⁺ and Mn²⁺ ions in the stable and easily synthesized host to obtain ideal red phosphor. The structure’s features were studied using synchrotron X-ray diffraction. The abnormal reduction of Eu³⁺ in air was observed according to PL spectra and confirmed by XPS data. Moreover, a strong quenching of Eu³⁺ emission was detected in Ca_9−xMn_xEu(PO₄)₇, which is the opposite to other Ca_9−xM_xEu(PO₄)₇ phosphates with divalent metals, such as Mg²⁺ or Zn²⁺. The mechanisms of self-reduction and quenching are discussed in detail.

2. Materials and Methods

The series of phosphates Ca_9−xMn_xEu(PO₄)₇ was synthesized by high-temperature solid-state route from simple oxides MnO₂ (99.9%), Eu₂O₃ (99.9%), calcium hydrogen phosphate CaHPO₄·2H₂O (99.9%) and calcium carbonate CaCO₃ (99.9%). The reagents of standard grade were checked for purity and used without further purification. The raw materials were weighted and thoroughly grounded. The syntheses were carried out in air in alundum crucibles at 1100 °C for 50 h. Phase analysis using JCPDS PDF-4 database (ICDD, Newtown Square, PA, USA) revealed that the synthesized samples did not contain any reflections of the initial or impurity phases.

The chemical composition of Ca₈MnEu(PO₄)₇ was determined by energy-dispersive X-ray spectrometry (EDX) using scanning electron microscope (SEM) Tescan VEGA3 (Oxford Instruments, Abingdon, UK) equipped with an Oxford Instruments X-Max 50 silicon drift. The EDX analysis results were based on the Ca_K, Mn_K, Eu_L and P_K edge lines. The oxygen content was not quantified by EDX.

Powder X-ray diffraction (PXRD) patterns were obtained using Thermo ARL X’TRA (Bragg–Brentano geometry, Scintillator detector, CuKα radiation, λ = 1.5418 Å, Thermo Fisher Scientific, Waltham MA, USA). PXRD data were collected in 2θ range from 5° to 75° with 0.02° step at room temperature.

Synchrotron PXRD data for Ca₈MnEu(PO₄)₇ were measured with a large Debye-Scherrer camera (home-made, NIMS, Tsukuba, Japan) at the BL15XU beamline of SPring-8. The intensity data were collected in 2θ range from 1° to 60° with step 0.003°. The incident beam was monochromatized at λ = 0.65298 Å. The samples were packed into Lindemann glass capillaries with an inner diameter of 0.1 mm, which were rotated during the measurement. The absorption coefficients were also measured. Rietveld analysis [27] was performed using JANA2006 software (by Petricek, V., Dusek, M. & Palatinus, L. Institute of Physics, Academy of Science of the Czech Republic, Praha) [28].

The second harmonic generation (SHG) signal was measured with a Q-switched YAG:Nd laser (home-made, Moscow, Russia) at λ_ω =1064 nm in the reflection mode. The powder of α-SiO₂ (3–5 μm particles size) was used as a standard to calibrate the intensity of the SHG signal (I_2ω). The final SHG value is a relation: I_2ω(sample)/I_2ω(SiO₂).

Differential scanning calorimetry (DSC) measurements were performed on an NETZSCH DSC 204 F1 calorimeter (NETZSCH, Selb, Germany) in the temperature range from 323 to 973 K with heating rate 10 K·min⁻¹ in nitrogen flow of 40 mL·min⁻¹.

The electrical conductivity, dielectric permittivity ε and dielectric loss tangent tg δ in air were measured by the double-contact method in the frequency range of 1–106 Hz at 300–1270 K (heating rate of 5 K·min⁻¹), with the help of precision voltmeter Solartron 7081 (Schlumberger, Houston, TX, USA) and frequency response analyzer Solartron 1260 (Schlumberger, Houston, TX, USA). Ceramic pellet from Ca₈MnEu(PO₄)₇ (1.5 mm thick and 5–6 mm in diameter) was prepared by pressing and sintering at 1473 K for 12 h. Pt paste was put on the flat surfaces of the pellet, and then it was heated at 1023 K for 4 h to produce platinum electrodes.

Magnetic measurements were performed on a SQUID magnetometer (Quantum Design, MPMS-XL-7T, Quantum Design, San Diego, CA, USA) from 400 K to 2 K at an applied field of 10 kOe. Isothermal magnetization measurements, M vs. H, were performed from 70 kOe to 0 Oe at T = 2 K.

The surface chemical analysis of Ca_9−xMn_xEu(PO₄)₇ x = 0.2 and x = 1.0 phosphates was performed by XPS using a Axis Ultra DLD (Kratos Analytical, Manchester, UK) spectrometer with monochromatic AlK_α source X-rays (1486.6 eV). The measurements were performed at pressure better than 5 × 10⁻⁷ Pa. The area of the surface analyzed was ~300 × 700 μm², which provided statistically reliable average results that represented the general surface of the compact powder. The resolution of the spectrometer measured as the full width at half maximum (FWHM) of the Au4f_7/2 line was about 0.7 eV. The experiments were performed with charge neutralization and use of the C1s level (285.0 eV) arising from the saturated hydrocarbon contamination on the sample surface as the binding energy (E_b, eV) scale reference. Selected region spectra were recorded covering the Ca2s, Ca2p, P2s, P2p, Eu3d, Eu4p, Eu4d, Mn2p, O1s and C1s photoemission peaks. The XPS spectra were measured with an energy step size of 1 eV and a pass energy of 160 eV. The high-resolution XPS spectra were performed with an energy step size of 0.1 eV and a pass energy of 40 eV.

Luminescence excitation spectra and emission spectra under excitation in the UV region were measured using a 150 W xenon lamp (Oriel Instruments, Stratford, CT, USA) as an excitation source, an MDR-206 primary monochromator (Lomo, Saint-Petersburg, Russia) and a LOT-Oriel MS-257 spectrograph (Oriel Instruments, Stratford, CT, USA) equipped with a Marconi CCD detector (Marconi Applied Technologies Limited, Chelmsford, UK). Samples were mounted into a Cryotrade LN-120 vacuum optical cryostat (Cryotrade engineering, Moscow, Russia).

3. Results

3.1. SEM Observations

The SEM image of Ca₈MnEu(PO₄)₇ is shown in Figure 1. The sample consists of small particles from 2–5 μm which are slightly agglomerate with each other. According to the EDX data, the ratio between Ca: Mn: Eu: P in Ca₈MnEu(PO₄)₇ was determined as 7.98 ± 0.62: 1.01 ± 0.08: 0.99 ± 0.05: 7.01 ± 0.85. This ratio, defined by EDX data, is close to the expected composition.

3.2. SHG, DSC and Dielectric Spectroscopy Measurements

The SHG signal of Ca₈MnEu(PO₄)₇ shows a very weak response. The value of the SHG signal (I_2ω) relative to the quartz standard I_2ω (SiO₂) was I_2ω/I_2ω (SiO₂) < 0.1, which corresponds to the sensitivity limit of the device. Such a small SGH signal value is attributed to a centrosymmetric structure. Previously studied Ca₈M²⁺Eu(PO₄)₇ phosphates with M²⁺ = Zn²⁺ [29], Mg²⁺ [30], Cd²⁺ [31] showed similar small SGH values, and their structures were defined as centrosymmetric with the space group R$\overline{3}$c. The studied Ca₈MnEu(PO₄)₇ phosphate complements the group of centrosymmetric β-TCP-type compounds.

For the series of phosphates Ca_9−xMn_xEu(PO₄)₇, the SHG signal shows a trend to decrease from 0.4 for Ca₉Eu(PO₄)₇ (x = 0) to 0 for Ca₈MnEu(PO₄)₇ (x = 1). This trend was observed in other β-TCP solid solutions, Ca_9−xMg_xRE(PO₄)₇, RE = Dy³⁺ [32], Eu³⁺ [33] and Ca_9−xZn_xRE(PO₄)₇ with RE = Tb³⁺ [34], Ho³⁺ [35], Eu³⁺ [29], La³⁺ [36] according to symmetry inhomogeneity of the β-TCP structure.

The fragments of DSC curves for Ca₈MnEu(PO₄)₇ are showed in Figure 2. DSC curves in the heating and cooling cycles point to the presence of only one peak at 501 and 520 °C, respectively. These peaks are attributed to the first-order phase transition and have a reversible character.

The temperature dependencies of the dielectric permittivity ε(T) and the dielectric loss tangent tg δ(T) at different frequencies for Ca₈MnEu(PO₄)₇ are shown in Figure 3a and Figure 3b, respectively. ε(T) increases with heating and reaches the phase transition at 525–575 °C with maximum at 550 °C (Figure 3a). A monotonous increase of ε(T) to around Curie temperature (T_c) seems to be the characteristic behavior. The location of the maximum on the curves does not depend on the frequency (Figure 3a). Such a maximum can characterize both ferroelectric and antiferroelectric phase transitions. However, the absence of an anomaly in tgδ(T) curve at a temperature of 500–1200 °C (Figure 3b) indicates the antiferroelectric character of the phase transition [37,38].

The absence of the SHG signal and the presence of an antiferroelectric phase transition on ε(T) along with effects on DSC curves confirm the centrosymmetric structure of Ca₈MnEu(PO₄)₇. Since the polar and nonpolar space groups R3c and R$\overline{3}$c in the β-TCP-type compounds are practically indistinguishable from PXRD data [33], previously it was proved by electron diffraction that Ca₈MgEu(PO₄)₇ [37] crystallizes in the centrosymmetric group R$\overline{3}$c, and during the phase transition, the symmetry changes from R$\overline{3}$c to R$\overline{3}$m [37].

The temperature position of the phase transition in Ca₈MnEu(PO₄)₇ exceeds these values for Ca₈ZnEu(PO₄)₇ (T_c ~ 547 °C) [29] and Ca₈MgEu(PO₄)₇ (T_c ~ 507 °C) [33]. This fact is due to the difference in the values of the ionic radii of M²⁺ in Ca₈MEu(PO₄)₇. The phase transition occurs at lower temperatures when the smaller ion is placed in the M5 site. Since Mn²⁺ is the largest among these ions (Zn²⁺, Mg²⁺, Mn²⁺), T_c shows the biggest value. However, the replacement Ca²⁺ → Mn²⁺ does not significantly affect the phase transition temperature, which is 573 °C for Ca₉Eu(PO₄)₇ [35].

The temperature dependence of the electric conductivity (σ) of Ca₈MnEu(PO₄)₇ at 50 kHz is shown in Figure 4 in the Arrenius coordinates log(σ)–(10³/T). The electroconductivity of Ca₈MnEu(PO₄)₇ is rising with the temperature increasing. The abrupt change in σ at 820–860 K is due to the rearrangement at the antiferroelectric/paraelectric phase transition (R$\overline{3}$c ↔ R$\overline{3}$m). Since the conduction temperature during heating is higher than during cooling, such a change in σ also indicates a first-order phase transition. The temperature behavior of the electroconductivity in Ca₈MnEu(PO₄)₇ is similar to other phosphates with the common formula Ca₈M²⁺RE³⁺(PO₄)₇ [30,31,39] and is a consequence of the mobility of Ca²⁺ ions [40].

3.3. PXRD Study

PXRD pattern of Ca₈MnEu(PO₄)₇ is similar to other compounds with the β-TCP-type structure (Figure 5). The absence of any impurity reflections on the PXRD pattern shows that Eu³⁺ and Mn²⁺ ions were completely involved in the structure. β-Ca₃(PO₄)₂ structure (sp.gr. R3c, Z = 6) is a rather rigid structure and consists of isolated tetrahedra PO₄ that connect CaO_n polyhedra into a 3D frame by common vertices [41]. The Ca²⁺ ions are located in sites M1–M5, where M1–M3 and M5 sites are fully occupied, while M4 sites are partly filled and M6 sites are fully vacant.

No vacancies in the anionic sublattice can appear in the β-Ca₃(PO₄)₂-type structure, even during heterovalent substitutions, when [PO4]³⁻ is replaced by [GeO₄]⁴⁻ [42] or [SO₄]²⁻ [43], for instance. Formation of phase-pure phases in these cases requires charge compensation. So, phosphorus atoms fully occupy three (P1, P2 and P3) tetrahedra sites. The symmetry changing R3c → R$\overline{3}$c results in an equivalence of M1 and M2 sites in the cationic sublattice and P2O₄ and P3O₄ tetrahedra in the anionic sublattice. M5, M3 and P1 sites are located in the center of symmetry, and P1 is in the half-occupied special position 12c. Atoms O1 and O2 are located in half-occupied positions 12c and 36f, respectively.

The atomic coordinates for Ca₈MgEu(PO₄)₇ were used as a starting model for synchrotron data refinement for Ca₈MnEu(PO₄)₇. Manganese ions were refined in the octahedral M5 site, while Eu ions were distributed through M1 and M3 sites with the preference occupation in the M1 site (Table S1 of the Supporting Information). After the structure refinement in an R$\overline{3}$c model, a good agreement between the calculated and the experimental synchrotron PXRD patterns was observed, as it can be seen from the Figure 5 difference plot. Figure 5 shows fragments of the observed, calculated and difference synchrotron PXRD patterns of Ca₈MnEu(PO₄)₇. Other numerical characteristics showing the quality of the structure refinements are given in

Table 2. Crystallographic data for Ca₈MnEu(PO₄)₇ (SG R$\overline{3}$c, Z = 6, T = 293 K).

Sample Composition	Ca8MnEu(PO4)7
Lattice parameters: a, Å	10.39826(1)
c, Å	37.17350(5)
Unit cell volume V, Å3	3480.851(7)
Calculated density, g/cm3	3.413
Data Collection:
Diffractometer	BL15XU beamline of SPring-8
Radiation/Wavelength (λ, Å)	Synchrotron/0.65298
Absorption coefficient, μ (mm−1)	4.381
F(000)	3462
2θ range (°)	2.040–60.237
Step scan (2θ)	0.003
Imax	606,480
Number of points	19,391
Refinement:
Refinement	Rietveld
Background function	Legendre polynoms, 15 terms
No. of reflections (all/observed)	945/922
No. of refined parameters/refined atomic parameters	43/34
R and Rw (%) for Bragg reflections (Rall/Robs)	5.29/5.95 and 5.05/4.81
RP and RwP; Rexp	2.06, 3.31, 0.63
Goodness of fit (ChiQ)	5.25
Max./min. residual density(e) (Å3)	0.65/−0.88

. The fractional atomic coordinates, isotropic atomic displacement parameters and cation occupancies are listed in Table S1 of the Supporting Information. The main interatomic distances are listed in Table S2 of the Supporting Information. CCDC 2237297 contains the supplementary crystallographic data for this paper.

Figure 6a shows PXRD patterns of Ca_9−xMn_xEu(PO₄)₇ solid solutions. All the diffraction peaks are matched with β-TCP (PDF#4 Card No. 00-09-0169). Moreover, a shifting of the reflections with the increasing of manganese concentration can be observed (Figure 6b). The reflections move toward larger angles according to Bragg’s rule and difference between the ionic radii of Ca²⁺ (r_IV = 1.00 Å) and Mn²⁺ (r_IV = 0.83 Å). The absence of the impurity phases in PXRD patterns and reflections shifting show the successful incorporation of Mn²⁺ ions in the β-TCP structure in all samples.

3.4. Magnetic Measurements

The inverse magnetic susceptibilities follow the Curie–Weiss law. Between 200 and 395 K, the inverse magnetic susceptibilities are fit by the Curie–Weiss equation:

χ(T) = μ²_effN(3k_B(T − θ))⁻¹,

where μ_eff is an effective magnetic moment, N is Avogadro’s number, k_B is Boltzmann’s constant and θ is the Curie–Weiss temperature. The fitting parameters for Ca₈MnEu(PO₄)₇ were μ_eff = 7.076(2) (μ_B/f.u.), μ_calc = 6.823 (μ_B/f.u.) and θ = −27.2(2) K. The μ_eff value was in good agreement with the theoretical value, where μ_calc is calculated using 3.4μ_B for Eu³⁺ [44] (Figure 7).

The M vs. H curve of Ca₈MnEu(PO₄)₇ followed very well the Brillouin function with g = 2, S = 5/2 and T = 2 K, as expected for a free Mn²⁺ cation (Figure 8). The magnetization at 2 K and 70 kOe reached 5.047 (μ_B/f.u.).

3.5. XPS Study

In the survey XPS scans of Ca_9−xMn_xEu(PO₄)₇ x = 0.2 and x = 1.0 samples (Figure 9a), the lines of calcium, europium, manganese, phosphorus, carbon and oxygen were observed. Ca2p XPS spectra (Figure 9b) were used for energy calibration of samples’ spectra to eliminate the charging effect.

In the Eu3d spectra of the samples (Figure 9c), the doublet of lines Eu3d_5/2 and Eu3d_3/2 was observed with 1135.0 and 1164.5 eV binding energies. These values are typical for the Eu³⁺ ion [45]. The additional Eu3d_5/2 component with 1124.4 eV binding energy can be distinguished in the Ca₈MnEu(PO₄)₇ spectrum. This component is attributed to Eu²⁺ [46,47,48,49]. The quantity of divalent europium in Ca₈MnEu(PO₄)₇ is about 5% of the total europium content. At the same time, in [45,50,51], the appearance of such low-energy components is associated with shake-down satellites from the main lines of trivalent europium and indicates that the intensity of such satellites can vary depending on the specific compound of trivalent europium.

The Mn2p XPS shape of the Ca₈MnEu(PO₄)₇ (Figure 9d) is typical of the divalent state of manganese [52]. The spectrum exhibits pronounced shake-up satellites characteristic of divalent manganese, which are shifted from the main peaks by approximately 6 eV towards higher binding energies. Similarly, in the Mn3s spectrum of Ca₈MnEu(PO₄)₇ (Figure 10), the splitting typical for divalent manganese is observed (Table 2). The Mn2p and Mn3s spectra of Ca_8.8Mn_0.2Eu(PO₄)₇ are close in shape to Ca₈MnEu(PO₄)₇. However, due to the significantly lower content of manganese in this sample, the spectra are observed to be noticeably noisier. The comparison of the parameters Mn2p and Mn3s spectra with reference data are listed in

Table 3. Relative position of the satellite (ΔMn2p_sat) in Mn2p XPS spectra and splitting (ΔMn3s) Mn3s XPS spectra of the studied Ca_9−xMn_xEu(PO₄)₇ (x = 0.2 and 1.0) samples and reference manganese oxides, eV.

Sample	ΔMn2psat	ΔMn3s	Reference
Ca8MnEu(PO4)7	~6.0	6.1	This work
Ca8.8Mn0.2Eu(PO4)7	~6.0	5.8	This work
MnO	~6.0	6.1	[53,54]
Mn2O3	10.1	5.4	[55]
MnO2	11.8	4.4	[56]

.

3.6. Photoluminescent Properties

Normalized photoluminescence excitation (PLE) spectra for one of the samples are shown in Figure 11. PLE spectra monitored at 440 nm exhibit an unresolved broad band from 300 to 400 nm, peaking at 365 nm, which originated from the Eu²⁺ 4f-5d-allowed transition (Figure 9). At 620 nm, the PLE spectrum consists of sharp lines attributed to transitions of Eu³⁺ from the ground ⁷F₀ level to excited levels. The bands are located at 318 nm (⁷F₀ → ⁵H₃), 362 nm (⁷F₀ → ⁵D₄), 378 nm (⁷F₀ → ⁵G_J), 382 nm (⁷F₀ → ⁵L₇), 395 nm (⁷F₀ → ⁵L₆), 416 nm (⁷F₀ → ⁵D₃) and 465 nm (⁷F₀ → ⁵D₄), and the area at 250–300 nm is attributed to the charge transfer band (Figure 9). The other samples from the series show the same spectra, and the main difference is in the intensity of the spectra.

Figure 12 shows PL spectra of Ca_8.2Mn_0.8Eu(PO₄)₇ at different excitation wavelengths. The broad unresolved emission band from 400 to 700 nm appearing under 370 nm excitation can be attributed to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ [57]. The band is asymmetrical and peaked at 440 nm. It arises from different crystallographic sites occupied by Eu atoms in the Ca_8.2Mn_0.8Eu(PO₄)₇ structure. Since the space group in this sample is R3c, there are M1–M3 sites occupied by Eu atoms, and several components in the Eu²⁺ emission can be distinguished [21]. The emission bands from Eu³⁺ are also observed under λ_ex = 370 nm; however, their intensity is very low (Figure 12). The location of these lines can be determined at 591 nm (⁵D₀ → ⁷F₁), 615 nm (⁵D₀ → ⁷F₂), 652 nm (⁵D₀ → ⁷F₃) and 698 nm (⁵D₀ → ⁷F₄).

Under 395 nm, the excitation PL spectra consist of the typical bands of Eu³⁺ emission (Figure 12). The presence of two types of Eu emission is related to partial abnormal self-reduction in the β-TCP host in agreement with XPS data. The locations of these bands are 593 nm (⁵D₀ → ⁷F₁), 618 nm (⁵D₀ → ⁷F₂) 655 nm (⁵D₀ → ⁷F₃) and 701 nm (⁵D₀ → ⁷F₄). The insignificant shifting of the peaks from 370 nm excitation is attributed to poor resolution of Eu³⁺ emission at λ_ex = 370 nm. Moreover, the transitions from higher-level ⁵D₁ to ⁷F₁ (535 nm) and ⁷F₂ (555 nm) in terms of ground state can be observed (Figure 12).

PL spectra for Ca_9−xMn_xEu(PO₄)₇ solid solutions at 395 nm excitation with high resolution are shown in Figure 13. The spectra consist of transitions from ⁵D₀ excited level to ⁷F₀ (579 nm), ⁷F₁ (589 nm), ⁷F₂ (612 nm), ⁷F₃ (652 nm) and ⁷F₄ (697 nm) levels. The normalized integral intensity of luminescence can be observed from the inset in Figure 13. It can be seen that PL intensity dramatically decreases with Mn²⁺ doping. This trend contradicts with the other Ca_9−xM_xEu(PO₄)₇ (M = Zn²⁺, Mg²⁺) [29] solid solutions, where changing of the symmetry from polar R3c to nonpolar R$\overline{3}$c leads to increasing of the luminescence intensity. However, such behavior of PL intensity in Eu³⁺ and Mn²⁺ co-doped isostructural Ca₃(VO₄)₂ was observed in [58]. The quenching of Eu³⁺ emission by Mn²⁺ doping in the β-TCP host is caused by the energy transfer from Eu³⁺ to Mn²⁺; however, the effective emission from Mn²⁺ ions is absent (Figure 12 and Figure 13). The energy transfer can be relaxed by the ⁴T₁ energy level of Mn²⁺ and then nonradiative relaxation to the ⁶A₁ ground state of Mn²⁺ ions. Moreover, the emission from Mn²⁺ can be overlapped with the ⁵D₀ → ⁷F₃ transition of Eu³⁺. In addition, the substitution Ca²⁺ → Mn²⁺ is accompanied by the R3c → R$\overline{3}$c symmetry changing and the formation of defects in the structure, which may act as quenching centers of photoluminescence.

PL spectra for Ca_9−xMn_xEu(PO₄)₇ solid solutions at 370 nm excitation are shown in Figure 14a. The intensity of Eu³⁺ emission also decreases with rising of Mn²⁺ concentration. Simultaneously, the intensity of the band attributed to Eu²⁺ emission (at ~450 nm) increases. Such behavior can be clearly observed from the dependence of normalized integral intensity of Eu²⁺ and Eu³⁺ emission on Mn²⁺ concentration (Figure 14b). The position and profile of the Eu²⁺ band do not change with the Mn²⁺ concentration, which points to the invariability of the surrounding crystal field strength. Actually, since Eu and Mn atoms occupy different crystal sites in the β-TCP structure, the environment of Eu does not change. The rising of the Eu²⁺ band intensity (Figure 14a) can be attributed to the increasing of its concentration in the samples. This conclusion also follows from the XPS data.

To study the evolution of the β-TCP structure in Ca_9−xMn_xEu(PO₄)₇ solid solutions, the hypersensitive ⁵D₀ → ⁷F₀ transition was analyzed. Figure 15 shows an enlarged part of luminescence spectra with the ⁵D₀ → ⁷F₀ transition. For the sample with the smallest Mn²⁺ concentration (x = 0.2), the presence of nonsymmetrical well-separated bands can be observed (Figure 15a). These peaks reveal the nonequivalent environments of Eu atoms in the structure. The intensity of the ⁵D₀ → ⁷F₀ transition decreases with the rising of Mn²⁺ in Ca_9−xMn_xEu(PO₄)₇ and becomes indistinguishable in samples with x = 0.8 and 1.0. According to this, the analysis of the asymmetry ratio (R/O) can provide the reliable information of the structure’s evolution. R/O value can be calculated from the observed spectra using the formula [33]:

$$R/O=\frac{{{\displaystyle \int }}_{604 \mathrm{nm}}^{638 \mathrm{nm}}{}_{}{}^{5}D{}_0\to {}_{}{}^{7}F{}_2}{{{\displaystyle \int }}_{518 \mathrm{nm}}^{604\mathrm{nm}}{}_{}{}^{5}D{}_0\to {}_{}{}^{7}F{}_1}$$

The dependence of R/O on Mn²⁺ concentration is shown in Figure 15b. The decreasing of R/O to ~1 for Ca₈MnEu(PO₄)₇ is attributed to the decreasing of the local distortion of the Eu environment in agreement with the structural data.

4. Discussion

The abnormal reduction Eu³⁺ → Eu²⁺ in inorganic phosphors prepared using high-temperature solid-state reactions in air was observed in numerous studies [59,60,61]. Usually, this reduction leads to the coexistence of two types of europium oxidation states. There is no information on the full reduction of Eu³⁺ into Eu²⁺ in non-reduction media, so it is difficult to control the efficiency of reduction and luminescence intensity [62]. However, it should be noted that Mn ions in our study were fully reduced from Mn⁴⁺ (MnO₂ as initial phase) to the Mn²⁺ state, which was shown by XPS and PL measurements.

The conditions for the abnormal reduction in oxosalts phosphors obtained in air using a high-temperature solid-state reaction were proposed in [63]. These conditions meet the requirements in the β-TCP type host:

(1)

There are no oxidizing ions in the structure;

(2)

The β-TCP-type host is based on tetrahedral anion groups (PO₄³⁻);

(3)

The doped ions (Eu³⁺ and Mn⁴⁺) substitute the ions with lower valences (Ca²⁺) in the host;

(4)

The substituted cation (Ca²⁺) has ionic radii close to Eu²⁺ (see

Table 4. Calculated D_r values for Mn and Eu in different oxidation states in the β-TCP host.

Site	Dr Value, %, for Doped Ion
	Mn2+	Mn4+	Eu2+	Eu3+
Ca1–Ca3 CN8 (r =1.12 Å)	14 (r = 0.96 Å)	-	11 (r = 1.25 Å)	4 (r = 1.07 Å)
Ca5 CN6 (r =1.00 Å)	17 (r = 0.83 Å)	47 (r = 0.53 Å)	17 (r = 1.17 Å)	5 (r = 0.95 Å)

CN is a coordination number; r is the ionic radii in the corresponding CN.

below).

The possibilities of Eu abnormal reduction in the β-TCP-type structure can be explained by the following reasons.

The structures of phosphates with the β-TCP type are built from PO₄ tetrahedra which connect all of the polyhedra by common oxygen atoms into a 3D network. These O atoms are shared by the adjacent polyhedra and tetrahedra and also lined columns A and B in the β-Ca₃(PO₄)₂ structure. Well chemically bonded O atoms form a rigid structure. This rigid 3D structure of phosphates can shield and isolate the reduced Eu²⁺ and Mn²⁺ ions from the oxidizing attack of oxygen from the atmosphere.

Second, Mn⁴⁺ may be a luminescent center as well, and its red emission in octahedral sites is due to spin-forbidden ²E_g → ⁴A_2g transitions. However, due to charge imbalance of Mn⁴⁺ and Ca²⁺, for such substitution, the charge compensation scheme in the anionic part is required [42]:

$$[{\mathrm{PO}}_4]3^{-}+\frac{3}{2}{\mathrm{Ca}}^{2+}\to {\mathrm{Mn}}^{4+}+{[{\mathrm{EO}}_4]}^{4-}+\frac{1}{2}{V}_{\mathrm{ca}}$$

where V_Ca is a calcium vacancy, and [EO₄]⁴⁻ is an anion with four negative charges, such as GeO₄⁴⁻ or SiO₄⁴⁻, for instance. Since no charge compensation was applied, Mn⁴⁺ could transfer to the Mn²⁺ state, which is more suitable for isovalent substitution. Usually, to stabilize manganese in the +2 oxidation state in the β-TCP hosts, MnCO₃ is used as a raw material in a reduction atmosphere [13,14,62].

Third, there is a size mismatch between Mn⁴⁺ and Ca²⁺ in the β-TCP host. This mismatch can be estimated by the ionic radius percentage difference (D_r). For isomorphic substitution, this value could not exceed 30%. The calculation of the ionic radius percentage difference D_r can be made by the formula:

$$D_r=\frac{|R_h\left(CN\right)-R_d(CN)|}{R_h\left(CN\right)}$$

where dopant R_d(CN) and host R_h(CN) ions are in the corresponding coordination numbers (CN). D_r values for different sites in the β-TCP host are given in Table 4. From the above data, Mn²⁺ doping into the β-TCP host is more preferable. The absence of emission from Mn⁴⁺ in the octahedra environment (red emission [9,64]) in the studied PL spectra shows its full reduction.

The reduction of Eu³⁺ can be explained by a charge compensation model. According to the difference in the oxidation state of Ca²⁺ and Eu³⁺ ions, two Eu³⁺ substitute three Ca²⁺ to keep the electroneutrality in the β-TCP host. Hence, one vacancy $V_{Ca}^{″}$ with two negative charges locates in the M4 site, while two defects of the cation site $E{u}_{Ca}^{•}$ with a positive charge in each could be produced:

$$3{\mathrm{Ca}}^{2+}+2{\mathrm{Eu}}^{3+}\to V_{Ca}^{″}+2E{u}_{Ca}^{•}$$

In this substitution, $V_{Ca}^{″}$ acts as a donor of electrons, while $E{u}_{Ca}^{•}$ is an acceptor of electrons. Thus, electrons can be transferred from the vacancy as follows during thermal treatment:

$$V_{Ca}^{″}\to V_{Ca}^{\times }+2e^{–}$$

and the defect Eu³⁺ captured electrons and further reduced Eu³⁺ to Eu²⁺:

$$2E{u}_{Ca}^{•}+2e^{–} \to 2E{u}_{Ca}^{\times }$$

A schematic representation of such reduction is present in Figure 16. A similar mechanism was observed in other β-TCP hosts [65] showing that Eu³⁺ could not be completely reduced to Eu²⁺, even in the reduction atmosphere due to size mismatching.

The reduction of Mn⁴⁺ can be described as follows. During thermal treatment, interstitial oxygen $O_i^{″}$ can be formed due to presence of caves along the c-axis from PO₄ frameworks. In order to keep the charge balance, one Mn⁴⁺ is needed to substitute for two Ca²⁺ ions. So, one vacancy defect $V_{Ca}^{″}$ with two negative charges and one $M{n}_{Ca}^{··}$ defect with two positive charges would form. Since MnO₂ is a raw material, these mechanisms can be ascribed by the following:

$${\mathrm{MnO}}_2\to M{n}_{Ca}^{••}+V_{Ca}^{″}+O_0^{\times }+O_i^{″}$$

The cause of the full reduction of Mn⁴⁺ is the possibility of transferring negative charges both from $V_{Ca}^{″}$ and interstitial oxygen $O_i^{″}$ during thermal treatment:

$$\begin{gathered} V_{Ca}^{″}\to V_{Ca}^{\times }+2e^{–} \\ {O}_i^{″}\to O_0^{\times }+2e^{–} \end{gathered}$$

So, these electrons would be released to reduce Mn⁴⁺ ions in the Ca²⁺ octahedral M5 site:

$$M{n}_{Ca}^{••}+2e^{–}\to M{n}_{Ca}^{\times }$$

Such a reduction was previously observed in oxosalt phosphors [66,67].

5. Conclusions

Phosphates Ca_9−xMn_xEu(PO₄)₇ were obtained by high-temperature solid-phase synthesis. All synthesized samples are isostructural to the β-Ca₃(PO₄)₂. Differential scanning calorimetry and dielectric spectroscopy revealed an antiferroelectric first-order reversible phase transition. The structure of Ca₈MnEu(PO₄)₇ was refined by the Rietveld method (sp. gr. R$\overline{3}$c) using synchrotron X-ray diffraction. Ca²⁺ and Eu³⁺ ions jointly occupy two sites, M1 and M3, while Mn²⁺ completely occupies the M5 site. Magnetic measurements have shown that Ca₈MnEu(PO₄)₇ contains Mn²⁺ and Eu³⁺ ions. XPS data show the coexistence of europium in +3/+2 oxidation states and manganese in the sole +2 oxidation state. The luminescence of Eu³⁺ and Eu²⁺ ions was found in Ca_9−xMn_xEu(PO₄)₇. The presence of two types of Eu²⁺/³⁺ emission is associated with the partial abnormal self-reduction of europium in the β-Ca₃(PO₄)₂ matrix. The concentration of Eu²⁺ cations is low (~5% according to XPS) and does not affect the magnetic properties. The intensity of Eu³⁺ emission is dramatically decreased with the rising of Mn²⁺ in Ca_9−xMn_xEu(PO₄)₇ and attributed to the Eu³⁺ → Mn²⁺ energy transfer. The analysis of the ⁵D₀ → ⁷F₀ transition and R/O values points to the symmetry inhomogeneity (R3c → R$\overline{3}$c) in Ca_9−xMn_xEu(PO₄)₇, such as in other Ca_9−xM_xEu(PO₄)₇ solid solutions with divalent metals.