Co-Doping Effect of Mn²⁺ and Eu³⁺ on Luminescence in Strontiowhitlockite Phosphors

Abstract

A new series of Sr-based phosphates, Sr_9−xMn_xEu(PO₄)₇, were synthesized using the high-temperature solid-state method in air. It was found that these compounds have the same structure as strontiowhitlockite, which is a β-Ca₃(PO₄)₂ (or β-TCP) structure. The concentration of Mn²⁺ ions required to form a pure strontiowhitlockite phase was determined. An unusual partial reduction of Eu³⁺ to Eu²⁺ in air was observed and confirmed by photoluminescence (PL) and electron spin resonance (ESR) spectra measurements. The PL spectra recorded under 370 nm excitation showed transitions of both 4f5d–4f Eu²⁺ and 4f–4f Eu³⁺. The total integral intensity of the PL spectra, monitored at 395 nm, decreased with increasing Mn²⁺ concentration due to quenching effect of Eu³⁺ by the Mn²⁺ levels. The temperature dependence of Eu²⁺ photoluminescence in a Sr_9−xMn_xEu(PO₄)₇ host was investigated. The conditions for the reduction of Eu³⁺ to Eu²⁺ in air were discussed.

1. Introduction

One of the primary objectives in addressing light-emitting diode (LED) issues is to identify an optimal host for harnessing the photoluminescence properties of cation activators. Another objective is to regulate emissions through chemical deposition. Research has clearly shown that certain phosphates [1,2], aluminates [3,4], silicates [5,6], glasses [7,8], and so on serve as excellent hosts for rare-earth elements and transition metals with photoluminescence properties in the visible region. However, each of these hosts has its drawbacks, such as a high synthesis temperature with reduced atmosphere [9], non-green chemistry preparation techniques [10], low isomorphic capacity, and variation in the substitution types [11]. In this context, β-Ca₃(PO₄)₂-type (or β-TCP) phosphors are considered suitable materials due to their wide capacity for substitutions of Ca²⁺ ions with either homovalent or heterovalent ions, resulting in excellent properties [12].

Calcium-based phosphates with a β-TCP structure are still of great interest. However, it has been shown that Sr-based phosphates with a β-Ca₃(PO₄)₂ structure exhibit photoluminescence properties that are several times higher [13,14]. The related mineral—strontiowhitlockite [15], or Sr₉Mg(PO₄)₆(PO₃OH)—is the strontium analogue of whitlockite or β-Ca₃(PO₄)₂ and also belongs to the cerite supergroup. The replacement of Ca²⁺ with Sr²⁺ ions leads to an increase in the photoluminescence properties [16] or their modification [17,18] due to the formation of more distorted luminescence center environments. It is worth noting that pure Sr₃(PO₄)₂ differs from the β-Ca₃(PO₄)₂ structure [19] and is known as a mineral palmierite family member. Furthermore, a comprehensive substitution of Ca²⁺ → Sr²⁺ was investigated in detail in [20]. It was shown that Sr-based phosphates form a β-Ca₃(PO₄)₂ structure only with the stoichiometric formula Sr₈M²⁺R³⁺(PO₄)₇ [21] or Sr₉R´³⁺(PO₄)₇ (where R³ is a rare earth element, and R´³⁺ is a tripositive ion, such as Sc³⁺ or Fe³⁺, [2,22]). Despite numerous articles on the photoluminescence properties of strontiowhitlockite-type phosphors, the full concentration series with Sr²⁺ → M²⁺ or Sr²⁺ → R³⁺ has not been discussed. However, a similar series on Ca-based phosphates, such as Ca_9−xM²⁺_xEu(PO₄)₇, was previously described in detail for M²⁺ = Zn²⁺ [23], Mg²⁺ [24], and Mn²⁺ [25].

The control of luminescence properties is achieved by using different pairs of ions in the host material, such as Tb³⁺/Eu³⁺ [26], Sm³⁺/Eu³⁺ [27], Ce³⁺/Mn²⁺ [28], Ce³⁺/Tb³⁺ [29], Eu²⁺/ Mn²⁺ [30], and others. The choice of these pairs is based on the existence of energy transfer processes between them, resulting in unique combinations of emitting colors. Another method to modify photoluminescence is by changing the oxidation state [31,32] of the ion-activator.

In this study, we investigate Sr-based phosphors with the β-TCP structure and common formula Sr_9−xMn_xEu(PO₄)₇. The influence of Mn²⁺ and Eu³⁺ co-doping on the photoluminescence properties as well as the impact of homovalent Ca²⁺ → Mn²⁺ substitution on the structure formation are also investigated. The abnormal self-reduction process of Eu³⁺ ions in the strontiowhitlockite host in air was observed.

2. Results and Discussion

2.1. PXRD and SHG Study

The PXRD patterns of SrMnxEu are shown in Figure 1. Unlike the similar Ca-based solid solution Ca_9−xMn_xEu(PO₄)₇ [25], an unbroken series of solid solutions with the strontiowhitlockite structure was not formed. However, the Sr₈MnEu(PO₄)₇ sample crystallized in the trigonal Sr₉Fe_1.5(PO₄)₇-type, or strontiowhitlockite-type, structure (space group (sp.gr.) R$\overline{3}$m, PDF-2 Card 51–427) (Figure 1). It appears that Mn²⁺ ions in the octahedra site played a critical role in the structure stabilization, similar to the Mg²⁺ ions in strontiowhitlockite. A similar structure formation was found in some related works, for example for Sr₉MnK(PO₄)₇:Eu²⁺,Ce³⁺ [33] or Sr₈MgCe(PO₄)₇:Eu²⁺,Mn²⁺ [34] on Sr-based phosphors.

Among the synthesized compounds, only the SrMn0.8Eu and SrMn1.0Eu samples were single-phased. Therefore, the limit content of Mn²⁺ ions required to form the strontiowhitlockite structure is x = 0.8. The observed reflections on the PXRD patterns for SrMn0Eu corresponded to the superposition of eulytite-typeSr₃Eu(PO₄)₃ (sp.gr. I$\overline{4}$3d, PDF-2 Card 48–410) and palmierite-type Sr₃(PO₄)₂ (sp.gr. R$\overline{3}$m, PDF-2 Card 85–502) structures. The samples with x = 0.2–0.4 were characterized by mixtures of phases with Sr₉Fe_1.5(PO₄)₇, Sr₃(PO₄)₂, and Sr₃Eu(PO₄)₃ structures (Figure 1). The quantitative analysis of the phase content calculated using the Jana2006 software is shown in

Table 1. The phase composition and the SHG signals for Sr_9−xMn_xEu(PO₄)₇ samples.

	Whitlockite-Type	Palmierite-Type	Eulytite-Type	SHG
	Sr9Fe1.5(PO4)7	Sr3(PO4)2	Sr3Eu(PO4)3
	sp.gr. $\mathit{R}\overline{3}$m	sp.gr. $\mathit{R}\overline{3}$m	sp.gr. $\mathit{I}\overline{4}$3d
	Centrosymmetric	Centrosymmetric	Non-Centrosymmetric
x = 0	0	45%	55%	1.1
x = 0.2	49%	23%	28%	0.7 ± 0.1
x = 0.4	67%	13%	20%	0.5 ± 0.1
x = 0.6	83%	0	17%	0.3 ± 0.1
x = 0.8	100%	0	0	0.1 ± 0.1
x = 1.0	100%	0	0	0.1 ± 0.1

.

The formation of the β–Ca₃(PO₄)₂-type structure has been described in previous studies [35,36]. These studies found that that samples with the general stoichiometric formula Sr₉R(PO₄)₇ (where R = La–Sm) did not crystallize in the strontiowhitlockite structure. However, the Ca-based phosphate series with the general formula Ca₉R³⁺(PO₄)₇ is known to crystallize in the β-Ca₃(PO₄)₂ structure [37].

The ionic radius of Sr²⁺ is significantly larger than that of Ca²⁺, resulting in structural defects that distort the β-Ca₃(PO₄)₂ structure. This distortion can lead to the formation of eulytite-type Sr₃R(PO₄)₃ phosphate, which contains an excess of Sr²⁺ ions. To stabilize the β-Ca₃(PO₄)₂ structure in Sr-based phosphates, small ions such as Mn²⁺, Zn²⁺, Mg²⁺ can be added [13,38]. Figure 2 shows the different structural sites. In the case of Sr²⁺ with Eu³⁺, they occupy the largest sites as SrO₁₀ and SrO₉. The smallest Mn²⁺ ion prefers to occupy the octahedral SrO₆ site in Sr₉Fe_1.5(PO₄).

The presence of the non-centrosymmetric eulytite-type Sr₃Eu(PO₄)₃ phase was confirmed through an SHG study. The SHG signal was dependent on the phase composition, which was determined using PXRD data. Consequently, the highest SHG signal was observed for SrMn0Eu, indicating a significant amount of the non-centrosymmetric eulytite-type phase. Increasing the concentration of Mn²⁺ in the SrMnxEu solid solution resulted in a reduction in the eulytite-type phase, which was also evident in the decrease in the SHG signal. For the SrMn0.8Eu and SrMn1.0Eu samples with the strontiowhitlockite structure, the SHG signals were comparable to the systematic errors in the measurement method.

2.2. ESR Analyze

No ESR signal was detected in the sample without Mn, i.e., SrMn0Eu, while the Mn²⁺-containing samples showed a wide, structureless signal with a g-factor of 1.997 (Figure 3). The shape of the ESR spectra remained unchanged as the temperature cooled down to 77 K. Furthermore, the signal intensity displayed non-linear behavior in relation to the Mn concentration (Figure 3, inset).

On one hand, the observed ESR signal was attributed to the presence of manganese ions. This is supported by the fact that the ESR of Mn²⁺ exhibited a signal in a region with a g-factor close to 2 [39,40,41,42]. However, the characteristic sextet pattern of Mn²⁺ was not observed in the studied samples.

Similar ESR spectra are often observed in Eu²⁺-doped compounds [43,44,45]. This observation is further supported by the non-linear increase in the ESR signal intensity with the increase in the Mn²⁺ concentration, particularly in the samples with x = 0.8 and 1.0 (Figure 3 inset). Simultaneously, the luminescence of Eu³⁺ was quenched (see below). These finding suggest the presence of Eu²⁺ ions in the samples. The broadening of the ESR signal may be attributed to the exchange interaction between manganese and europium ions, similar to what has been observed in silicates [45].

2.3. Diffuse Absorption

The diffuse absorption spectra of SrMnxEu are shown in Figure 4. The spectra for the samples with x = 0 and 0.6 exhibited a similar structure. However, the absorption spectrum of the SrMn1.0Eu showed a prominent edge starting at 400 nm and extending towards the shorter-wavelength region of the spectrum (Figure 4). This behavior can be explained by the formation of a single-phase sample for SrMn1.0Eu. The absorption bands of Mn²⁺ are typically found in the 370–440 nm spectral range and are attributed to d-d transitions. These absorption bands often have a low oscillator strength and can appear broadened when Mn²⁺ ions occupy multiple non-equivalent positions within the lattice [46,47]. Additionally, a prominent absorption edge is observed in this region. The combination of these factors can mask the absorption bands of Mn²⁺.

It is possible that manganese ions can exist in a different oxidation state. One hypothetical substitution scheme could be Mn²⁺ + Eu³⁺ ↔ Mn³⁺ + Eu²⁺. An indicator of the presence of Mn³⁺ ions in the lattice is the occurrence of a broad absorption band in the visible spectral region, typically peaking around 450–700 nm [48]. The broadening of this band can be attributed to the occupancy of different positions by Mn³⁺ ions. A wide plateau of low intensity can be observed in the visible region of the presented spectra (Figure 4). However, it is difficult to confidently conclude the presence of manganese 3+ solely based on the absorption spectra.

2.4. Photoluminescence Properties

The VUV excitation spectra of the Eu³⁺ 4f–4f luminescence, measured at the ⁵D₀ → ⁷F₂ band, are shown in Figure 5. In the SrMn0Eu sample undoped by Mn²⁺ ions, a broad band centered around 250 nm and a relatively sharp band around 150 nm were observed. The broad band at 250 nm was attributed to the charge transfer band from (CTB) O²⁻ to Eu³⁺, while the sharp band at 150 nm corresponded to host excitation. In the SrMnxEu solid solutions, the position of the band at 250 nm shifted to a shorter wavelength as the concentration of Mn²⁺ ions increased. Additionally, the intensity of the band at 150 nm significantly decreased. At 120–160 nm, Mn²⁺ ions typically exhibit intra-ionic 3d-4s transitions [49,50], which have a high oscillator strength. This indicates strong absorption bands in this wavelength range. Consequently, the strong absorption leads to non-radiative relaxation of excitations, resulting in the quenching of luminescence when excited in this specific region.

The photoluminescence properties of SrMnxEu solid solutions are sensitive to phase purity and chemical composition. Figure 6a shows the PLE spectra. The number and position of the observed bands, corresponding to 4f–4f transitions of the Eu³⁺ ions, remained unchanged for the samples with x = 0.2–0.8. The broad band ranging from 250 to 300 nm was attributed to the CTB, while the sharp peaks in the range of 300–500 nm arose from the f–f transitions of Eu³⁺. Specifically, the peaks located at 320, 361, 376, 382, 395, 416 and 465 nm corresponded to the ⁷F₀ → ⁵H₃, ⁵D₄, ⁵G_J, ⁵L₇, ⁵L₆, ⁵D₃, and ⁵D₂ transitions of Eu³⁺ ions [5,51,52,53]. All the spectral lines in the SrMnxEu host appeared to be wider when compared to those in other hosts that have been described. This could potentially be attributed to the presence of Eu³⁺ ions in the different environments. The presence of Mn²⁺ ions caused a notable reduction in the intensity of both the CTB and the standard transitions of Eu³⁺. The observed decrease in intensity was attributed to the increase in the Mn²⁺ concentration in the SrMnxEu solid solutions. This decrease could be attributed to the quenching effect by Mn²⁺ and the abnormal reduction of Eu³⁺ during synthesis. The proposed energy transfer schema is shown in Figure 6b, with the most intensive line observed at 395 nm.

The PL spectra for SrMnxEu, excited at 395 nm (Figure 7), exhibited characteristic lines corresponding to Eu³⁺ transitions. The sharp lines at 578, 594, 617, 650, and 700 nm corresponded to the transitions ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4), with the main band at 615 nm. The resulting emission was observed in the red region of the visible spectrum [54,55]. Previous studies have shown that the total integral intensity is higher for the host Sr₈MEu³⁺(PO₄)₇ (where M = Mg, Zn) compared to Ca-based phosphates [13], regardless of the synthesis method. In this work, an increase in the Mn²⁺ concentration led to a decrease in the total integral intensity. Additionally, a gap was observed for the samples with x = 0 and 0.2 (Figure 7a, insert).

A decrease in the total integral intensity of Eu³⁺ transitions was also observed for the single-phased SrMn0.8Eu and SrMn1.0Eu with a β-Ca₃(PO₄)₂-type (or strontiowhitlockite) structure. This can be explained by the energy transfer process from the Eu³⁺ to Mn²⁺ levels through nonradiative transitions to the excited ⁴T₁(G) state and emission to the ground ⁶A₁(S) state. It is possible that the Mn²⁺ emission overlapped with the ⁵D₀ → ⁷F_0,1,3 Eu³⁺ transition [56,57], which can be observed in the high-spectral-resolution PL spectra. Furthermore, the emission of Mn²⁺ could be decreased through a concentration-quenching process. A proposed schema of this process is shown in Figure 7b. Similar behavior in the quenching of Eu³⁺ photoluminescence by Mn²⁺ ions has been previously observed in Ca_9−xMn_xEu(PO₄)₇ [25] and in isostructural Ca₃(VO₄)₂:Eu³⁺, Mn²⁺ [58].

It is important to note that the spectral profile of the SrMn0Eu was significantly different compared to the others, as clearly seen in the high-spectral-resolution spectra (Figure 7b). The profiles for SrMn0.2Eu, SrMn0.4Eu, and SrMn0.6Eu were similar, indicating a similar oxygen environment. These samples consisted of two phases with β-TCP and eulytite types. This suggests that with the Mn²⁺ concentration at x = 0.2, Eu³⁺ primarily occupied sites in the whitlockite Sr₉Fe_1.5(PO₄)₇ and eulytite Sr₃Eu(PO₄)₃ structures with an excess of Sr²⁺ ions forming the Sr₃(PO₄)₂ phase.

Additional information about the phase composition and oxygen environment of the emission centers can be obtained thought consideration the forbidden electric dipole ⁵D₀ → ⁷F₀ transition of Eu³⁺ (Figure 8) [59]. The non-degenerate energy levels indicated the number of nonequivalent sites for Eu³⁺. For the sample with x = 0, this transition can be represented by two Gaussian components, indicating two non-equal oxygen environments for Eu³⁺. It should be noted that the Sr₃(PO₄)₂ structure exhibited two non-equal sites for Eu³⁺ occupation, but one of them was a ч. Therefore, the observed transition reflects the influence of the larger site. The average Eu–O distance in the polyhedral structure of Sr₃(PO₄)₂ was 2.7476 Å. In the Sr₃Eu(PO₄)₃, only one site was observed, with an average Eu–O distance of approximately 2.6754 Å. This point was clearly demonstrated in previous studies [23,60], where it was shown that for the ⁵D₀ → ⁷F₀ transition, the Eu–O distance has a direct correlation with the wavelength of the transition. Hence, line A corresponds to the Eu³⁺ environment in the Sr₃Eu(PO₄)₃, while line B corresponds to the Eu³⁺ in Sr₃(PO₄)₂ (Figure 8b).

Regarding the SrMn0.2Eu sample, the ⁵D₀ → ⁷F₀ transition can be described by four Gaussian components (Figure 8c). The increase in the number of components was attributed to the formation of the β-Ca₃(PO₄)₂-type structure (Figure 8c). Fitting line A remains unchanged in terms of the maximum and position, indicating the Eu³⁺ oxygen environment in the Sr₃Eu(PO₄)₃. Line B corresponds to the Eu³⁺ in the Sr₃(PO₄)₂ host. Additional lines C and D correspond to the Eu³⁺ in the strontiowhitlockite [13]. Therefore, some polyhedra in the strontiowhitlockite and Sr₃(PO₄)₂ hosts were approximately the same, which is why lines B and C have very closely centered maximum values.

The observed ⁵D₀ → ⁷F₀ transitions for SrMn0.4Eu and SrMn0.6Eu can be accurately fitted by three Gaussian components, indicating that Eu³⁺ ions were mainly involved in the Sr₃Eu(PO₄)₃- and strontiowhitlockite-type hosts. For the single-phased samples SrMn0.8Eu and SrMn1.0Eu, the ⁵D₀ → ⁷F₀ transition can be fitted by two Gaussian components. This suggests that there are two different environments for Eu³⁺ in the host [13], as is shown in Figure 2 for strontiowhitlockite with the presence of SrO₁₀ and SrO₉ sites.

According to the ESR data analysis, it was confirmed that Eu²⁺ ions were detected in the samples. As a result, the PL spectra were monitored for all the samples, employing an excitation wavelength of 370 nm (Figure 9). Notably, for the samples with x ≥ 0.2, distinct 4f5d–4f Eu²⁺ transitions were registered. The observed band appeared to be asymmetrical in shape and was predominantly centered around the wavelength of approximately 445 nm.

The intensity of the Eu²⁺ emission band at ~445 nm was higher for the x = 0.8 sample (Figure 9b) compared to the x = 0.2 sample (Figure 9a). Consequently, it can be inferred that the addition of Mn²⁺ to the samples resulted in an overall increase in the total integral intensity of the Eu²⁺ transitions. The presence of Eu²⁺ emissions in the PL spectra indicates the abnormal reduction of Eu³⁺ ions in the strontiowhitlockite host. Moreover, the increase in the Mn²⁺ doping in the SrMnxEu solid solutions led to an increase in the Eu²⁺ ion concentration and a more efficient reduction process.

2.5. Temperature Dependence of Photoluminescence

Upon cooling to 80 K, a broad band appeared in the PL spectra centered at 470 nm (Figure 10a line 3). The PLE spectra of this band are shown in Figure 10a (Figure 10a lines 1 and 2). Under monitoring at 470 nm, the PLE spectrum consisted of several bands at 285, 230, 200, and 170 nm (Figure 10a line 1), with the most intense band being observed at 200 nm. When monitoring at 595 nm, the PLE spectrum showed only one band peaked at 230 nm, corresponding to charge transfer effects.

Figure 10b demonstrates the temperature dependence of the luminescence intensity of an emission band centered at 470 nm under 230 nm excitation. The observed behavior of the dependence follows Mott’s law, with an activation energy (E) of 0.2 eV and a frequency factor (w₀) of 1.87∙10⁵ Hz. This luminescence band is potentially associated with 5d–4f transitions in the Eu²⁺ ions. Therefore, the activation energy in the temperature quenching curve of the luminescence could correspond to the energy difference between the high-energy excited 5d states of Eu²⁺ and the bottom of the conduction band.

With an increasing concentration of Mn²⁺ ions, the temperature dependence of luminescence underwent changes. At lower temperatures, the luminescence intensity of Eu²⁺ was observed to be quenched (Figure 10b). This phenomenon can be explained by the interaction between the Mn²⁺ ions and the surrounding environment. In the case of SrMn1.0Eu, the temperature dependence of the luminescence can be well described by the sum of two Mott’s functions, which provides valuable insights into the underlying mechanisms of luminescence:

$$I\left(T\right)=0.43{\left(1+w_0^1\exp \left(\frac{{-E}_1}{k_{B}T}\right)\right)}^{-1}+0.57{\left(1+w_0^2\exp \left(\frac{{-E}_2}{k_{B}T}\right)\right)}^{-1}$$

(1)

where $w_0^1=1.87\times {10}^5\mathrm{H}\mathrm{z}$, $w_0^2=1.21\times {10}^5\mathrm{H}\mathrm{z}$, $E_1=0.20$ eV, and $E_2=0.14$ eV. It was observed that the samples with a high Mn²⁺ content exhibited the presence of two distinct Eu²⁺ centers, which provides evidence for the existence of multiple Eu²⁺ centers within the SrMn1.0Eu sample.

2.6. The Decay Curves

The decay curves were collected for the single-phased samples of SrMn0.8Eu and SrMn1.0Eu (Figure 11). The curves were well fitted by the double exponent function:

I(t) = A_1·exp(−t/τ₁) + A_2·exp(−t/τ₂)

(2)

where I(t) is the intensity at time t, τ₁ and τ₂ are the decay times for the exponential components, and A₁ and A₂ are fitting constants. The average lifetimes were calculated using the following equation [61]:

$$\mathsf{\tau }=\frac{A_1{{\mathsf{\tau }}_1}^2+A_2{{\mathsf{\tau }}_2}^2}{A_1{\tau }_1+A_2{\mathsf{\tau }}_2}$$

(3)

The calculated average lifetime for the SrMn0.8Eu was 1.97 ms (A₁ = 0.3, τ₁ = 2.35, A₂ = 0.3, τ₂ = 0.69), while for the SrMn1.0Eu, it was 1.19 ms (A₁ = 0.11, τ₁ = 2.1, A₂ = 0.6, τ₂ = 0.59). The values were lower compared to other Eu³⁺-doped strontiowhitlockite samples [62]. The low decay time of the Eu³⁺ emission may indicate charge transfer processes from the Eu³⁺ levels.

2.7. The Abnormal Reduction Process

The presence of Eu²⁺ ions in the host, as indicated by the ESR and photoluminescence data, suggests that the Eu³⁺ reduced to Eu²⁺ during the synthesis in air. This reduction process has been observed previously in several works [25,32,63,64,65,66]. The authors propose that this abnormal reduction of Eu³⁺ to Eu²⁺ in air occurs through a charge compensation mechanism. In [67], the conditions for the reduction process in solid-state compounds were proposed. A detailed analysis of previously obtained data on Eu³⁺ spectra in different phosphate hosts reveals this abnormal reduction, which follows Pei’s rules with one additional modification (

Table 2. The conditions for the reduction of Eu³⁺ to Eu²⁺ in air.

Conditions	Present Work	Remarks
(1) No oxidizing ions should be present in the host.	There were no oxidizing ions in the SrMnxEu host.
(2) The dopant R3+ ions must replace host cations with a different oxidation state.	Eu3+ replaced Sr2+ ions in SrMnxEu host.	In the β-Ca3(PO4)2 structure, Eu3+ can also replace Ca2+ ions in the host. Eu2+ emission was found in some hosts.
(3) The host cations must have similar radii to the divalent R2+ ions.	rVIII(Eu2+) = 1.25 Å was close to rVIII(Sr2+) = 1.26 Å.	The similarity of the ionic radii explains the more common abnormal reduction in air in the Sr-based host compared to the Ca-based one.

).

Due to the ionic radii mismatch between Sr²⁺ and Eu³⁺ ions, Sr-based phosphates with a β-TCP-type structure are suitable for reducing Eu³⁺ to Eu²⁺ in air. Additionally, the synthesis products (NH₃, see Section 2.1, reaction) create a weak reducing atmosphere, further promoting the reduction of Eu³⁺ to Eu²⁺. One possible reduction scheme, based on diffuse absorption, is Mn²⁺ + Eu³⁺ ↔ Mn³⁺ + Eu²⁺, where Mn²⁺ acts as the reducing agent and Eu³⁺ as the oxidant. The reduction process occurs due to the susceptibility of the structure to the reducing agent NH₃ and the presence of Mn²⁺ ions.

2.8. Color Characteristics

One of the important characteristics of phosphors is their CIE coordinates. These coordinates can be determined from the emission spectral data of the samples. The calculated results are shown in Figure 12. For the Sr_8.2Mn_0.8Eu(PO₄)₇ sample monitored at 395 nm, the color coordinates (0.647; 0.351) fell in the red-orange region on the CIE diagram (Figure 12, point 1). When excited at 370 nm, the color coordinates (0.399; 0.270) were within the pink region (Figure 12, point 2).

3. Materials and Methods

3.1. Synthesis

The series of Sr_9−xMn_xEu(PO₄)₇ (named SrMnxEu, x = 0, 0.2, 0.4, 0.6, 0.8, 1) was synthesized using a high-temperature solid-state method. Stoichiometric mixtures of NH₄H₂PO₄ (99.9%), SrCO₃ (99.9%), MnCO₃ (99.99%), and Eu₂O₃ (99.9%) were used as the starting materials. The amounts of reactants were calculated based on the following reaction:

(18 − 2x) SrCO₃ + 2x MnCO₃ + 14 NH₄H₂PO₄ + Eu₂O₃ → 2 Sr_9−xMn_xEu(PO₄)₇ + 18 CO₂ + 14 NH₃ +21 H₂O

The required amounts were mixed in an agate mortar with acetone for better homogenization. The resulting mixture was then transferred to an alundum crucible for stepwise heating:

• Slow heating up to 200 °C for 8 h, followed by annealing for 8 h in air.
• Heating to 1100 °C for 12 h, followed by annealing for 24 h in air.

This slow heating method was chosen to guarantee the uniform removal of volatile by-products from the reaction. The powder X-ray diffraction (PXRD) patterns of the precursors were checked using the JCPDS PDF-2 database, which did not show any reflections of impurity phases.

3.2. Characterization

Powder X-ray diffraction (PXRD) patterns of SrMnxEu were collected on a Thermo ARL X’TRA powder diffractometer (CuKα radiation, λ = 1.5418 Å, Bragg–Brentano geometry, Peltier-cooled CCD detector). The PXRD data were collected over the 5°–70° 2θ range with steps of 0.02°. The Le Bail decomposition [68] was applied for the PXRD analysis using the JANA2006 software [69].

The second harmonic generation (SHG) signal was measured with a Q-switched YAG: Nd laser at λ_ω =1064 nm in the reflection mode.

For collecting the electron spin resonance (ESR) spectra, the powder samples were placed in a quartz tube and measured using an RE–1306 X–band ESR spectrometer (KBST, Smolensk, Russia) operating at a frequency of 9.3841 GHz at room temperature.

The VUV excitation luminescence was recorded using an MDR-2 monochromator (LOMO, Saint Petersburg, Russia) equipped with a grate of 1200 lines per mm. A Hamamatsu photomodule operating in the photon counting mode was used for the detection. Excitation was carried out using a Hamamatsu L7293-50 deuterium lamp with a magnesium fluoride window coupled with a VMR-2 vacuum monochromator. The excitation spectra were corrected using sodium salicylate. The registration of the temperature was performed using a type K thermocouple.

The diffuse absorption spectra were registered with a Lambda 950 spectrophotometer equipped with integrated sphere in the transmittance regime (Perkin-Elmer, New-York city, NY, USA).

The photoluminescence emission (PL) and excitation (PLE) spectra were recorded under excitation in the UV-Vis spectral region using a specialized laboratory set-up. An ARC 150 W Xe lamp was used as an excitation source. The primary monochromator MDR-206 was used for the selection of the excitation wavelength. The PLE spectra were measured with a spectral resolution of 5 nm. The luminescence spectra were detected using an Oriel MS257 spectrograph using a 300 gr/mm or 2400 gr/mm grating with a spectral resolution of 1.5 nm and 0.32 nm, respectively (“low” and “high” resolution). A Marconi 30-11 CCD detector was used for the registration. The luminescence spectra were corrected for the spectral sensitivity of the registration channel. The measured excitation spectra were normalized on the excitation spectrum of yellow lumogen. All measurements were performed at room temperature.

The luminescence decay time was registered using a Perkin-Elmer LS-55 spectofluorimeter (Perkin-Elmer, New-York city, NY, USA) equipped with a Xe lamp with a 10 mks pulse duration. All measurements were performed at room temperature and corrected for the sensitivity of the spectrometer.

4. Conclusions

New solid solutions of Sr_9−xMn_xEu(PO₄)₇ with a strontiowhitlockite structure were synthesized using a high-temperature solid-state method in air. The concentration limit of Mn²⁺ ions in the host for the formation of the strontiowhitlockite (or β-TCP) phase was determined at x ≥ 0.8. The composition of the multi-phased samples was confirmed by X-ray and SHG analyses. The quenching of the Eu³⁺ emission was observed under a 395 nm excitation with the Mn²⁺ concentration. It was proposed that Eu³⁺ excitation was quenched through the Mn²⁺ levels, following potential reaction scheme Mn²⁺ + Eu³⁺ ↔ Mn³⁺ + Eu²⁺. The ESR and PL spectra measurements confirmed the abnormal partial reduction of Eu³⁺ → Eu²⁺ in air and the presence of Eu²⁺ ions in the host. Both 4f5d–4f Eu²⁺ and 4f–4f Eu³⁺ transitions were observed in the PL spectra under a 370 nm excitation. The intensity of the Eu²⁺ emission decreased with heating from 80 K to 270 K in Sr_9−xMn_xEu(PO₄)₇. The detailed analysis of the ⁵D₀ → ⁷F₀ transition showed the presence of several non-equal Eu³⁺ environments. The decay curves were measured, and it was found that the decay times for the Eu³⁺ levels were lower compared to other strontiowhitlockite-based phosphors. The conditions for the occurrence of Eu³⁺ → Eu²⁺ reduction in air were discussed.