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

A new series of Sr-based phosphates, Sr9−xMnxEu(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 Sr9−xMnxEu(PO4)7 host was investigated. The conditions for the reduction of Eu3+ to Eu2+ in air were discussed.


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-Ca 3 (PO 4 ) 2 -type (or β-TCP) phosphors are considered suitable materials due to their wide capacity for substitutions of Ca 2+ 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-Ca 3 (PO 4 ) 2 structure exhibit photoluminescence properties that are several times higher [13,14].The related mineralstrontiowhitlockite [15], or Sr 9 Mg(PO 4 ) 6 (PO 3 OH)-is the strontium analogue of whitlockite or β-Ca 3 (PO 4 ) 2 and also belongs to the cerite supergroup.The replacement of Ca 2+ with Sr 2+ 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.
The control of luminescence properties is achieved by using different pairs of ions in the host material, such as Tb 3+ /Eu 3+ [26], Sm 3+ /Eu 3+ [27], Ce 3+ /Mn 2+ [28], Ce 3+ /Tb 3+ [29], Eu 2+ / Mn 2+ [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−x Mn x Eu(PO 4 ) 7 .The influence of Mn 2+ and Eu 3+ co-doping on the photoluminescence properties as well as the impact of homovalent Ca 2+ → Mn 2+ substitution on the structure formation are also investigated.The abnormal self-reduction process of Eu 3+ ions in the strontiowhitlockite host in air was observed.
The formation of the β-Ca 3 (PO 4 ) 2 -type structure has been described in previous studies [35,36].These studies found that that samples with the general stoichiometric formula Sr 9 R(PO 4 ) 7 (where R = La-Sm) did not crystallize in the strontiowhitlockite structure.However, the Ca-based phosphate series with the general formula Ca 9 R 3+ (PO 4 ) 7 is known to crystallize in the β-Ca 3 (PO 4 ) 2 structure [37].
The ionic radius of Sr 2+ is significantly larger than that of Ca 2+ , resulting in structural defects that distort the β-Ca 3 (PO 4 ) 2 structure.This distortion can lead to the formation of eulytite-type Sr 3 R(PO 4 ) 3 phosphate, which contains an excess of Sr 2+ ions.To stabilize the β-Ca 3 (PO 4 ) 2 structure in Sr-based phosphates, small ions such as Mn 2+ , Zn 2+ , Mg 2+ can be added [13,38]. Figure 2 shows the different structural sites.In the case of Sr 2+ with Eu 3+ , they occupy the largest sites as SrO 10 and SrO 9 .The smallest Mn 2+ ion prefers to occupy the octahedral SrO 6 site in Sr 9 Fe 1.5 (PO 4 ).The presence of the non-centrosymmetric eulytite-type Sr 3 Eu(PO 4 ) 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 ob-served for SrMn0Eu, indicating a significant amount of the non-centrosymmetric eulytitetype phase.Increasing the concentration of Mn 2+ 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 SrMn0.8Euand SrMn1.0Eusamples with the strontiowhitlockite structure, the SHG signals were comparable to the systematic errors in the measurement method.

ESR Analyze
No ESR signal was detected in the sample without Mn, i.e., SrMn0Eu, while the Mn 2+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).

ESR Analyze
No ESR signal was detected in the sample without Mn, i.e., SrMn0Eu, while the Mn 2+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 Mn 2+ exhibited a signal in a region with a g-factor close to 2 [39][40][41][42].However, the characteristic sextet pattern of Mn 2+ was not observed in the studied samples.
Similar ESR spectra are often observed in Eu 2+ -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 Mn 2+ concentration, particularly in the samples with x = 0.8 and 1.0 (Figure 3 inset).Simultaneously, the luminescence of Eu 3+ was quenched (see below).These finding suggest the presence of Eu 2+ 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].

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 SrMn1.0Eushowed 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 Mn 2+ 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 Mn 2+ 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 Mn 2+ .
It is possible that manganese ions can exist in a different oxidation state.One hypothetical substitution scheme could be Mn 2+ + Eu 3+ ↔ Mn 3+ + Eu 2+ .An indicator of the presence of Mn 3+ 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 Mn 3+ 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.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 Mn 2+ exhibited a signal in a region with a g-factor close to 2 [39][40][41][42].However, the characteristic sextet pattern of Mn 2+ was not observed in the studied samples.
Similar ESR spectra are often observed in Eu 2+ -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 Mn 2+ concentration, particularly in the samples with x = 0.8 and 1.0 (Figure 3 inset).Simultaneously, the luminescence of Eu 3+ was quenched (see below).These finding suggest the presence of Eu 2+ 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].

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 SrMn1.0Eushowed 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 Mn 2+ 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 Mn 2+ 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 Mn 2+ .

Photoluminescence Properties
The VUV excitation spectra of the Eu 3+ 4f-4f luminescence, measured at the 5 D0 → 7 F2 band, are shown in Figure 5.In the SrMn0Eu sample undoped by Mn 2+ 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) O 2− to Eu 3+ , 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 Mn 2+ ions increased.Additionally, the intensity of the band at 150 nm significantly decreased.At 120-160 nm, Mn 2+ 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 nonradiative 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 Eu 3+ 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 Eu 3+ .Specifically, the peaks located at 320, 361, 376, 382, 395, 416 and 465 nm corresponded to the 7 F0 → 5 H3, 5 D4, 5 GJ, 5 L7, 5 L6, 5 D3, and 5 D2 transitions It is possible that manganese ions can exist in a different oxidation state.One hypothetical substitution scheme could be Mn 2+ + Eu 3+ ↔ Mn 3+ + Eu 2+ .An indicator of the presence of Mn 3+ 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 Mn 3+ 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.

Photoluminescence Properties
The VUV excitation spectra of the Eu 3+ 4f-4f luminescence, measured at the 5 D 0 → 7 F 2 band, are shown in Figure 5.In the SrMn0Eu sample undoped by Mn 2+ 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) O 2− to Eu 3+ , 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 Mn 2+ ions increased.Additionally, the intensity of the band at 150 nm significantly decreased.At 120-160 nm, Mn 2+ 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.

Photoluminescence Properties
The VUV excitation spectra of the Eu 3+ 4f-4f luminescence, measured at the 5 D0 → 7 F2 band, are shown in Figure 5.In the SrMn0Eu sample undoped by Mn 2+ 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) O 2− to Eu 3+ , 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 Mn 2+ ions increased.Additionally, the intensity of the band at 150 nm significantly decreased.At 120-160 nm, Mn 2+ 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 nonradiative 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 Eu 3+ 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 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 Eu 3+ 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 Eu 3+ .Specifically, the peaks located at 320, 361, 376, 382, 395, 416 and 465 nm corresponded to the 7 F 0 → 5 H 3 , 5 D 4 , 5 G J , 5 L 7 , 5 L 6 , 5 D 3 , and 5 D 2 transitions of Eu 3+ 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 Eu 3+ ions in the different environments.The presence of Mn 2+ ions caused a notable reduction in the intensity of both the CTB and the standard transitions of Eu 3+ .The observed decrease in intensity was attributed to the increase in the Mn 2+ concentration in the SrMnxEu solid solutions.This decrease could be attributed to the quenching effect by Mn 2+ and the abnormal reduction of Eu 3+ during synthesis.The proposed energy transfer schema is shown in Figure 6b, with the most intensive line observed at 395 nm.
Molecules 2023, 28, x FOR PEER REVIEW 8 of 18 of Eu 3+ 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 Eu 3+ ions in the different environments.The presence of Mn 2+ ions caused a notable reduction in the intensity of both the CTB and the standard transitions of Eu 3+ .The observed decrease in intensity was attributed to the increase in the Mn 2+ concentration in the SrMnxEu solid solutions.This decrease could be attributed to the quenching effect by Mn 2+ and the abnormal reduction of Eu 3+ 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 Eu 3+ transitions.The sharp lines at 578, 594, 617, 650, and 700 nm corresponded to the transitions 5 D0 → 7 FJ (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 Sr8MEu 3+ (PO4)7 (where M = Mg, Zn) compared to Ca-based phosphates [13], regardless of the synthesis method.In this work, an increase in the Mn 2+ 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 Eu 3+ 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 Eu 3+ to Mn 2+ levels through nonradiative transitions to the excited 4 T1(G) state and emission to the ground 6 A1(S) state.It is possible that the Mn 2+ emission overlapped with the 5 D0 → 7 F0,1,3 Eu 3+ transition [56,57], which can be observed in the high-spectral-resolution PL spectra.Furthermore, the emission of Mn 2+ 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 Eu 3+ photoluminescence by Mn 2+ ions has been previously observed in Ca9−xMnxEu(PO4) 7 [25] and in isostructural Ca3(VO4)2:Eu 3+ , Mn 2+ [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 Mn 2+ concentration at х = 0.2, Eu 3+ The PL spectra for SrMnxEu, excited at 395 nm (Figure 7), exhibited characteristic lines corresponding to Eu 3+ transitions.The sharp lines at 578, 594, 617, 650, and 700 nm corresponded to the transitions 5 D 0 → 7 F 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 Sr 8 MEu 3+ (PO 4 ) 7 (where M = Mg, Zn) compared to Ca-based phosphates [13], regardless of the synthesis method.In this work, an increase in the Mn 2+ 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 Eu 3+ transitions was also observed for the single-phased SrMn0.8Eu and SrMn1.0Eu with a β-Ca 3 (PO 4 ) 2 -type (or strontiowhitlockite) structure.This can be explained by the energy transfer process from the Eu 3+ to Mn 2+ levels through nonradiative transitions to the excited 4 T 1 (G) state and emission to the ground 6 A 1 (S) state.It is possible that the Mn 2+ emission overlapped with the 5 D 0 → 7 F 0,1,3 Eu 3+ transition [56,57], which can be observed in the high-spectral-resolution PL spectra.Furthermore, the emission of Mn 2+ 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 Eu 3+ photoluminescence by Mn 2+ ions has been previously observed in Ca 9−x Mn x Eu(PO 4 ) 7 [25] and in isostructural Ca 3 (VO 4 ) 2 :Eu 3+ , Mn 2+ [58].
primarily occupied sites in the whitlockite Sr9Fe1.5(PO4)7 and eulytite Sr3Eu(PO4)3 structures with an excess of Sr 2+ 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 5 D0 → 7 F0 transition of Eu 3+ (Figure 8) [59].The non-degenerate energy levels indicated the number of nonequivalent sites for Eu 3+ .For the sample with х = 0, this transition can be represented by two Gaussian components, indicating two non-equal oxygen environments for Eu 3+ .It should be noted that the Sr3(PO4)2 structure exhibited two nonequal sites for Eu 3+ 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 5 D0 → 7 F0 transition, the Eu-O distance has a direct correlation with the wavelength of the transition.Hence, line A corresponds to the Eu 3+ environment in the Sr3Eu(PO4)3, while line B corresponds to the Eu 3+ in Sr3(PO4)2 (Figure 8b).
Regarding the SrMn0.2Eusample, the 5 D0 → 7 F0 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 А remains unchanged in terms of the maximum and position, indicating the Eu 3+ oxygen environment in the Sr3Eu(PO4)3.Line B corresponds to the Eu 3+ in the Sr3(PO4)2 host.Additional lines C and D correspond to the Eu 3+ 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 5 D0 → 7 F0 transitions for SrMn0.4Eu and SrMn0.6Eucan be accurately fitted by three Gaussian components, indicating that Eu 3+ ions were mainly involved in the Sr3Eu(PO4)3and strontiowhitlockite-type hosts.For the single-phased samples SrMn0.8Eu and SrMn1.0Eu, the 5 D0 → 7 F0 transition can be fitted by two Gaussian components.This suggests that there are two different environments for Eu 3+ in the host [13], as is shown in Figure 2 for strontiowhitlockite with the presence of SrO10 and SrO9 sites.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 Mn 2+ concentration at x = 0.2, Eu 3+ primarily occupied sites in the whitlockite Sr 9 Fe 1.5 (PO 4 ) 7 and eulytite Sr 3 Eu(PO 4 ) 3 structures with an excess of Sr 2+ ions forming the Sr 3 (PO 4 ) 2 phase.
Additional information about the phase composition and oxygen environment of the emission centers can be obtained thought consideration the forbidden electric dipole 5 D 0 → 7 F 0 transition of Eu 3+ (Figure 8) [59].The non-degenerate energy levels indicated the number of nonequivalent sites for Eu 3+ .For the sample with x = 0, this transition can be represented by two Gaussian components, indicating two non-equal oxygen environments for Eu 3+ .It should be noted that the Sr 3 (PO 4 ) 2 structure exhibited two non-equal sites for Eu 3+ 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 Sr 3 (PO 4 ) 2 was 2.7476 Å.In the Sr 3 Eu(PO 4 ) 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 5 D 0 → 7 F 0 transition, the Eu-O distance has a direct correlation with the wavelength of the transition.Hence, line A corresponds to the Eu 3+ environment in the Sr 3 Eu(PO 4 ) 3 , while line B corresponds to the Eu 3+ in Sr 3 (PO 4 ) 2 (Figure 8b).
Regarding the SrMn0.2Eusample, the 5 D 0 → 7 F 0 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-Ca 3 (PO 4 ) 2 -type structure (Figure 8c).Fitting line A remains unchanged in terms of the maximum and position, indicating the Eu 3+ oxygen environment in the Sr 3 Eu(PO 4 ) 3 .Line B corresponds to the Eu 3+ in the Sr 3 (PO 4 ) 2 host.Additional lines C and D correspond to the Eu 3+ in the strontiowhitlockite [13].Therefore, some polyhedra in the strontiowhitlockite and Sr 3 (PO 4 ) 2 hosts were approximately the same, which is why lines B and C have very closely centered maximum values.
The observed 5 D 0 → 7 F 0 transitions for SrMn0.4Eu and SrMn0.6Eucan be accurately fitted by three Gaussian components, indicating that Eu 3+ ions were mainly involved in the Sr 3 Eu(PO 4 ) 3 -and strontiowhitlockite-type hosts.For the single-phased samples SrMn0.8Eu and SrMn1.0Eu, the 5 D 0 → 7 F 0 transition can be fitted by two Gaussian components.This suggests that there are two different environments for Eu 3+ in the host [13], as is shown in Figure 2 for strontiowhitlockite with the presence of SrO 10 and SrO 9 sites.According to the ESR data analysis, it was confirmed that Eu 2+ 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 Eu 2+ 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 Eu 2+ 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 Mn 2+ to the samples resulted in an overall increase in the total integral intensity of the Eu 2+ transitions.The presence of Eu 2+ emissions in the PL spectra indicates the abnormal reduction of Eu 3+ ions in the strontiowhitlockite host.Moreover, the increase in the Mn 2+ doping in the SrMnxEu solid solutions led to an increase in the Eu 2+ ion concentration and a more efficient reduction process.According to the ESR data analysis, it was confirmed that Eu 2+ 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 4f 5d-4f Eu 2+ 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 Eu 2+ 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 Mn 2+ to the samples resulted in an overall increase in the total integral intensity of the Eu 2+ transitions.The presence of Eu 2+ emissions in the PL spectra indicates the abnormal reduction of Eu 3+ ions in the strontiowhitlockite host.Moreover, the increase in the Mn 2+ doping in the SrMnxEu solid solutions led to an increase in the Eu 2+ ion concentration and a more efficient reduction process.

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•10 5 Hz.This luminescence band is potentially associated with 5d-4f transitions in the Eu 2+ 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 Eu 2+ and the bottom of the conduction band.
With an increasing concentration of Mn 2+ ions, the temperature dependence of luminescence underwent changes.At lower temperatures, the luminescence intensity of Eu 2+ was observed to be quenched (Figure 10b).This phenomenon can be explained by the interaction between the Mn 2+ 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: where  = 1.87 × 10 Hz,  = 1.21 × 10 Hz,  = 0.20 eV, and  = 0.14 eV.It was observed that the samples with a high Mn 2+ content exhibited the presence of two distinct Eu 2+ centers, which provides evidence for the existence of multiple Eu 2+ centers within the SrMn1.0Eusample.

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: 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]: 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 Eu 3+ -doped strontiowhitlockite samples [62].The low decay time of the Eu 3+ emission may indicate charge transfer processes from the Eu 3+ levels.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 (w 0 ) of 1.87•10 5 Hz.This luminescence band is potentially associated with 5d-4f transitions in the Eu 2+ ions.Therefore, the activation energy in the temperature quenching curve of the luminescence could correspond to the energy difference between the highenergy excited 5d states of Eu 2+ and the bottom of the conduction band.
With an increasing concentration of Mn 2+ ions, the temperature dependence of luminescence underwent changes.At lower temperatures, the luminescence intensity of Eu 2+ was observed to be quenched (Figure 10b).This phenomenon can be explained by the interaction between the Mn 2+ 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: where w 1 0 = 1.87 × 10 5 Hz, w 2 0 = 1.21 × 10 5 Hz, E 1 = 0.20 eV, and E 2 = 0.14 eV.It was observed that the samples with a high Mn 2+ content exhibited the presence of two distinct Eu 2+ centers, which provides evidence for the existence of multiple Eu 2+ centers within the SrMn1.0Eusample.

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: where I(t) is the intensity at time t, τ 1 and τ 2 are the decay times for the exponential components, and A 1 and A 2 are fitting constants.The average lifetimes were calculated using the following equation [61]:

The Abnormal Reduction Process
The presence of Eu 2+ ions in the host, as indicated by the ESR and photoluminescence data, suggests that the Eu 3+ reduced to Eu 2+ 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 Eu 3+ to Eu 2+ 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 Eu 3+ spectra in different phosphate hosts reveals this abnormal reduction, which follows Pei's rules with one additional modification (Table 2).

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 R 3+ ions must replace host cations with a different oxidation state.
In the β-Ca 3 (PO 4 ) 2 structure, Eu 3+ can also replace Ca 2+ ions in the host.Eu 2+ emission was found in some hosts.
(3) The host cations must have similar radii to the divalent R 2+ ions.
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 Sr 2+ and Eu 3+ ions, Sr-based phosphates with a β-TCP-type structure are suitable for reducing Eu 3+ to Eu 2+ in air.Additionally, the synthesis products (NH 3 , see Section 2.1, reaction) create a weak reducing atmosphere, further promoting the reduction of Eu 3+ to Eu 2+ .One possible reduction scheme, based on diffuse absorption, is Mn 2+ + Eu 3+ ↔ Mn 3+ + Eu 2+ , where Mn 2+ acts as the reducing agent and Eu 3+ as the oxidant.The reduction process occurs due to the susceptibility of the structure to the reducing agent NH 3 and the presence of Mn 2+ ions.

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 8.2 Mn 0.8 Eu(PO 4 ) 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).

Synthesis
The series of Sr 9−x Mn x Eu(PO 4 ) 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 NH 4 H 2 PO 4 (99.9%),SrCO 3 (99.9%),MnCO 3 (99.99%),and Eu 2 O 3 (99.9%)were used as the starting materials.The amounts of reactants were calculated based on the following reaction: 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:

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Slow heating up to 200 • C for 8 h, followed by annealing for 8 h in air.

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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 byproducts 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.
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

Figure 6 .
Figure 6.(a) The PLE spectra for Sr 9−x Mn x Eu(PO 4 ) 7 , monitored at 620 nm; (b) the proposed schema of energy transfer processes in host.

Figure 7 .
Figure 7. (a) The low-resolution and (b) high-resolution PL of Sr 9−x Mn x Eu(PO 4 ) 7 (λ exc = 395 nm) (the insert shows the integral intensity of the PL spectra on Mn 2+ ion concentration).

Table 1 .
The phase composition and the SHG signals for Sr 9−x Mn x Eu(PO 4 ) 7 samples.

Table 2 .
The conditions for the reduction of Eu 3+ to Eu 2+ in air.