Abnormal Eu3+ → Eu2+ Reduction in Ca9−xMnxEu(PO4)7 Phosphors: Structure and Luminescent Properties

β-Ca3(PO4)2-type phosphors Ca9−xMnxEu(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 Ca9−xMnxEu(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 → 7FJ 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 Ca9−xMnxEu(PO4)7 phosphors with doping of the host by Mn2+ ions. The phenomena of abnormal reduction and quenching were discussed in detail.


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 Eu 3+ -doped inorganic red phosphors have been developed with the narrow emission due to the electric dipole 5 D 0 → 7 F 2 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 Eu 3+ 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];

Materials and Methods
The series of phosphates Ca 9−x Mn x Eu(PO 4 ) 7 was synthesized by high-temperature solid-state route from simple oxides MnO 2 (99.9%), Eu 2 O 3 (99.9%), calcium hydrogen phosphate CaHPO 4 ·2H 2 O (99.9%) and calcium carbonate CaCO 3 (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 Ca 8 MnEu(PO 4 ) 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 Ca 8 MnEu(PO 4 ) 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-SiO 2 (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 (SiO 2 ).
Differential scanning calorimetry (DSC) measurements were performed on an NET-ZSCH 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 Ca 8 MnEu(PO 4 ) 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−x Mn x Eu(PO 4 ) 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 µm 2 , 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).

SEM Observations
The SEM image of Ca 8 MnEu(PO 4 ) 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 Ca 8 MnEu(PO 4 ) 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.
Materials 2023, 16, x FOR PEER REVIEW
The fragments of DSC curves for Ca8MnEu(PO4)7 are showed in Figure 2. DS in the heating and cooling cycles point to the presence of only one peak at 501 an respectively. These peaks are attributed to the first-order phase transition and h versible character.

SHG, DSC and Dielectric Spectroscopy Measurements
The SHG signal of Ca 8 MnEu(PO 4 ) 7 shows a very weak response. The value of the SHG signal (I 2ω ) relative to the quartz standard I 2ω (SiO 2 ) was I 2ω /I 2ω (SiO 2 ) < 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 Ca 8 M 2+ Eu(PO 4 ) 7 phosphates with M 2+ = Zn 2+ [29], Mg 2+ [30], Cd 2+ [31] showed similar small SGH values, and their structures were defined as centrosymmetric with the space group R3c. The studied Ca 8 MnEu(PO 4 ) 7 phosphate complements the group of centrosymmetric β-TCP-type compounds.
The fragments of DSC curves for Ca 8 MnEu(PO 4 ) 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 dielectr loss tangent tg δ(T) at different frequencies for Ca8MnEu(PO4)7 are shown in Figure 3a an Figure 3b, respectively. ε(T) increases with heating and reaches the phase transition 525-575 °C with maximum at 550 °C ( Figure 3a). A monotonous increase of ε(T) to aroun Curie temperature (Tc) seems to be the characteristic behavior. The location of the max mum on the curves does not depend on the frequency (Figure 3a). Such a maximum ca characterize both ferroelectric and antiferroelectric phase transitions. However, the a sence of an anomaly in tgδ(T) curve at a temperature of 500-1200 °C (Figure 3b) indicat the antiferroelectric character of the phase transition [37,38].
The absence of the SHG signal and the presence of an antiferroelectric phase trans tion on ε(T) along with effects on DSC curves confirm the centrosymmetric structure Ca8MnEu(PO4)7. Since the polar and nonpolar space groups R3c and R3 c in the β-TC type compounds are practically indistinguishable from PXRD data [33], previously it w proved by electron diffraction that Ca8MgEu(PO4)7 [37] crystallizes in the centrosymme ric group R3 c, and during the phase transition, the symmetry changes from R3 c to R3 [37].
The temperature position of the phase transition in Ca8MnEu(PO4)7 exceeds the values for Ca8ZnEu(PO4)7 (Tc ~ 547 °C) [29] and Ca8MgEu(PO4)7 (Tc ~ 507 °C) [33]. This fa is due to the difference in the values of the ionic radii of M 2+ in Ca8MEu(PO4)7. The pha transition occurs at lower temperatures when the smaller ion is placed in the М5 site. Sin Mn 2+ is the largest among these ions (Zn 2+ , Mg 2+ , Mn 2+ ), Tc shows the biggest value. How ever, the replacement Ca 2+ → Mn 2+ does not significantly affect the phase transition tem perature, which is 573 °C for Ca9Eu(PO4)7 [35]. The temperature dependencies of the dielectric permittivity ε(T) and the dielectric loss tangent tg δ(T) at different frequencies for Ca 8 MnEu(PO 4 ) 7 are shown in Figures 3a and 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 Ca 8 MnEu(PO 4 ) 7 . Since the polar and nonpolar space groups R3c and R3c in the β-TCPtype compounds are practically indistinguishable from PXRD data [33], previously it was proved by electron diffraction that Ca 8 MgEu(PO 4 ) 7 [37] crystallizes in the centrosymmetric group R3c, and during the phase transition, the symmetry changes from R3c to R3m [37].
The temperature position of the phase transition in Ca 8 MnEu(PO 4 ) 7 exceeds these values for Ca 8 ZnEu(PO 4 ) 7 (T c~5 47 • C) [29] and Ca 8 MgEu(PO 4 ) 7 (T c~5 07 • C) [33]. This fact is due to the difference in the values of the ionic radii of M 2+ in Ca 8 MEu(PO 4 ) 7 . The phase transition occurs at lower temperatures when the smaller ion is placed in the M5 site. Since Mn 2+ is the largest among these ions (Zn 2+ , Mg 2+ , Mn 2+ ), T c shows the biggest value. However, the replacement Ca 2+ → Mn 2+ does not significantly affect the phase transition temperature, which is 573 • C for Ca 9 Eu(PO 4 ) 7 [35].
The temperature dependence of the electric conductivity (σ) of Ca 8 MnEu(PO 4 ) 7 at 50 kHz is shown in Figure 4 in the Arrenius coordinates log(σ)-(10 3 /T). The electroconductivity of Ca 8 MnEu(PO 4 ) 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 (R3c ↔ R3m). 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 Ca 8 MnEu(PO 4 ) 7 is similar to other phosphates with the common formula Ca 8 M 2+ RE 3+ (PO 4 ) 7 [30,31,39] and is a consequence of the mobility of Ca 2+ ions [40].  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)-(10 3 /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 (R3 с ↔ R3 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 Ca8M 2+ RE 3+ (PO4)7 [30,31,39] and is a consequence of the mobility of Ca 2+ ions [40].

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 Eu 3+ and Mn 2+ 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 CaOn polyhedra into a 3D frame by common vertices [41]. The Ca 2+ ions are located in sites M1-M5, where M1-M3 and M5 sites are fully occupied, while M4 sites are  The temperature dependence of the electric conductivity (σ) of Ca8MnEu 50 kHz is shown in Figure 4 in the Arrenius coordinates log(σ)-(10 3 /T). The el ductivity of Ca8MnEu(PO4)7 is rising with the temperature increasing. The abrup in σ at 820-860 K is due to the rearrangement at the antiferroelectric/paraelect transition (R3 с ↔ R3 m). Since the conduction temperature during heating is hig during cooling, such a change in σ also indicates a first-order phase transition. perature behavior of the electroconductivity in Ca8MnEu(PO4)7 is similar to oth phates with the common formula Ca8M 2+ RE 3+ (PO4)7 [30,31,39] and is a consequen mobility of Ca 2+ ions [40].

PXRD Study
PXRD pattern of Ca8MnEu(PO4)7 is similar to other compounds with the β-T structure ( Figure 5). The absence of any impurity reflections on the PXRD patte that Eu 3+ and Mn 2+ ions were completely involved in the structure. β-Ca3(PO4)2 (sp.gr. R3c, Z = 6) is a rather rigid structure and consists of isolated tetrahedra connect CaOn polyhedra into a 3D frame by common vertices [41].

PXRD Study
PXRD pattern of Ca 8 MnEu(PO 4 ) 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 Eu 3+ and Mn 2+ ions were completely involved in the structure. β-Ca 3 (PO 4 ) 2 structure (sp.gr. R3c, Z = 6) is a rather rigid structure and consists of isolated tetrahedra PO 4 that connect CaO n polyhedra into a 3D frame by common vertices [41].  [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 → R3c results in an equivalence of M1 and M2 sites in the cationic sublattice and P2O 4 and P3O 4 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 Ca 8 MgEu(PO 4 ) 7 were used as a starting model for synchrotron data refinement for Ca 8 MnEu(PO 4 ) 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 R3c 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 Ca 8 MnEu(PO 4 ) 7 . Other numerical characteristics showing the quality of the structure refinements are given in Table 2. 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.  [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 → R3 с 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 R3 с 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. 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.

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: 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 Ca 8 MnEu(PO 4 ) 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 Eu 3+ [44] (Figure 7).

XPS Study
In the survey XPS scans of Ca9−xMnxEu(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.

XPS Study
In the survey XPS scans of Ca9−xMnxEu(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.

XPS Study
In the survey XPS scans of Ca 9−x Mn x Eu(PO 4 ) 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 Eu3d5/2 and Eu3d3/2 was observed with 1135.0 and 1164.5 eV binding energies. These values are typical for the Eu 3+ ion [45]. The additional Eu3d5/2 component with 1124.4 eV binding energy can be distinguished in the Ca8MnEu(PO4)7 spectrum. This component is attributed to Eu 2+ [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 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 Eu 3+ ion [45]. The additional Eu3d 5/2 component with 1124.4 eV binding energy can be distinguished in the Ca 8 MnEu(PO 4 ) 7 spectrum. This component is attributed to Eu 2+ [46][47][48][49]. The quantity of divalent europium in Ca 8 MnEu(PO 4 ) 7 is about 5% of the total europium content. At the same time, in [45,50,51], the appearance of such lowenergy 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 Ca 8 MnEu(PO 4 ) 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 Ca 8 MnEu(PO 4 ) 7 (Figure 10), the splitting typical for divalent manganese is observed ( Table 2). The Mn2p and Mn3s spectra of Ca 8. 8 Mn 0.2 Eu(PO 4 ) 7 are close in shape to Ca 8 MnEu(PO 4 ) 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. 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 Ca8.8Mn0.2Eu(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.

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 Eu 2+ 4f-5d-allowed transition (Figure 9). At 620 nm, the PLE spectrum consists of sharp lines attributed to transitions of Eu 3+ from the ground 7 F 0 level to excited levels. The bands are located at 318 nm ( 7 F 0 → 5 H 3 ), 362 nm ( 7 F 0 → 5 D 4 ), 378 nm ( 7 F 0 → 5 G J ), 382 nm ( 7 F 0 → 5 L 7 ), 395 nm ( 7 F 0 → 5 L 6 ), 416 nm ( 7 F 0 → 5 D 3 ) and 465 nm ( 7 F 0 → 5 D 4 ), 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. Materials 2023, 16, x FOR PEER REVIEW 13 of 22 Figure 11. Normalized PLE spectra of Ca8.2Mn0.8Eu(PO4)7 at λem = 440 and 620 nm. Figure 12 shows PL spectra of Ca8.2Mn0.8Eu(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 4f 6 5d 1 → 4f 7 transition of Eu 2+ [57]. The band is asymmetrical and peaked at 440 nm. It arises from different crystallographic sites occupied by Eu atoms in the Ca8.2Mn0.8Eu(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 Eu 2+ emission can be distinguished [21]. The emission bands from Eu 3+ are also observed under λex = 370 nm; however, their intensity is very low (Figure 12). The location of these lines can be determined at 591 nm ( 5 D0 → 7 F1), 615 nm ( 5 D0 → 7 F2), 652 nm ( 5 D0 → 7 F3) and 698 nm ( 5 D0 → 7 F4). Under 395 nm, the excitation PL spectra consist of the typical bands of Eu 3+ emission ( Figure 12). The presence of two types of Eu emission is related to partial abnormal selfreduction in the β-TCP host in agreement with XPS data. The locations of these bands are 593 nm ( 5 D0 → 7 F1), 618 nm ( 5 D0 → 7 F2) 655 nm ( 5 D0 → 7 F3) and 701 nm ( 5 D0 → 7 F4). The insignificant shifting of the peaks from 370 nm excitation is attributed to poor resolution of Eu 3+ emission at λex = 370 nm. Moreover, the transitions from higher-level 5 D1 to 7 F1 (535 nm) and 7 F2 (555 nm) in terms of ground state can be observed (Figure 12).  The broad unresolved emission band from 400 to 700 nm appearing under 370 nm excitation can be attributed to the 4f 6 5d 1 → 4f 7 transition of Eu 2+ [57]. The band is asymmetrical and peaked at 440 nm. It arises from different crystallographic sites occupied by Eu atoms in the Ca 8.2 Mn 0.8 Eu(PO 4 ) 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 Eu 2+ emission can be distinguished [21]. The emission bands from Eu 3+ 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 ( 5 D 0 → 7 F 1 ), 615 nm ( 5 D 0 → 7 F 2 ), 652 nm ( 5 D 0 → 7 F 3 ) and 698 nm ( 5 D 0 → 7 F 4 ).  Figure 12 shows PL spectra of Ca8.2Mn0.8Eu(PO4)7 at different excitation wavelengths The broad unresolved emission band from 400 to 700 nm appearing under 370 nm excita tion can be attributed to the 4f 6 5d 1 → 4f 7 transition of Eu 2+ [57]. The band is asymmetrica and peaked at 440 nm. It arises from different crystallographic sites occupied by Eu atom in the Ca8.2Mn0.8Eu(PO4)7 structure. Since the space group in this sample is R3c, there ar M1-M3 sites occupied by Eu atoms, and several components in the Eu 2+ emission can b distinguished [21]. The emission bands from Eu 3+ are also observed under λex = 370 nm however, their intensity is very low (Figure 12). The location of these lines can be deter mined at 591 nm ( 5 D0 → 7 F1), 615 nm ( 5 D0 → 7 F2), 652 nm ( 5 D0 → 7 F3) and 698 nm ( 5 D0 → 7 F4 Under 395 nm, the excitation PL spectra consist of the typical bands of Eu 3+ emissio ( Figure 12). The presence of two types of Eu emission is related to partial abnormal sel reduction in the β-TCP host in agreement with XPS data. The locations of these bands ar 593 nm ( 5 D0 → 7 F1), 618 nm ( 5 D0 → 7 F2) 655 nm ( 5 D0 → 7 F3) and 701 nm ( 5 D0 → 7 F4). Th insignificant shifting of the peaks from 370 nm excitation is attributed to poor resolutio of Eu 3+ emission at λex = 370 nm. Moreover, the transitions from higher-level 5 D1 to 7 F (535 nm) and 7 F2 (555 nm) in terms of ground state can be observed (Figure 12). Under 395 nm, the excitation PL spectra consist of the typical bands of Eu 3+ emission ( Figure 12). The presence of two types of Eu emission is related to partial abnormal selfreduction in the β-TCP host in agreement with XPS data. The locations of these bands are 593 nm ( 5 D 0 → 7 F 1 ), 618 nm ( 5 D 0 → 7 F 2 ) 655 nm ( 5 D 0 → 7 F 3 ) and 701 nm ( 5 D 0 → 7 F 4 ). The insignificant shifting of the peaks from 370 nm excitation is attributed to poor resolution of Eu 3+ emission at λ ex = 370 nm. Moreover, the transitions from higher-level 5 D 1 to 7 F 1 (535 nm) and 7 F 2 (555 nm) in terms of ground state can be observed ( Figure 12).
PL spectra for Ca 9−x Mn x Eu(PO 4 ) 7 solid solutions at 395 nm excitation with high resolution are shown in Figure 13. The spectra consist of transitions from 5 D 0 excited level to 7 F 0 (579 nm), 7 F 1 (589 nm), 7 F 2 (612 nm), 7 F 3 (652 nm) and 7 F 4 (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 Mn 2+ doping. This trend contradicts with the other Ca 9−x M x Eu(PO 4 ) 7 (M = Zn 2+ , Mg 2+ ) [29] solid solutions, where changing of the symmetry from polar R3c to nonpolar R3c leads to increasing of the luminescence intensity. However, such behavior of PL intensity in Eu 3+ and Mn 2+ co-doped isostructural Ca 3 (VO 4 ) 2 was observed in [58]. The quenching of Eu 3+ emission by Mn 2+ doping in the β-TCP host is caused by the energy transfer from Eu 3+ to Mn 2+ ; however, the effective emission from Mn 2+ ions is absent (Figures 12 and 13). The energy transfer can be relaxed by the 4 T 1 energy level of Mn 2+ and then nonradiative relaxation to the 6 A 1 ground state of Mn 2+ ions. Moreover, the emission from Mn 2+ can be overlapped with the 5 D 0 → 7 F 3 transition of Eu 3+ . In addition, the substitution Ca 2+ → Mn 2+ is accompanied by the R3c → R3c symmetry changing and the formation of defects in the structure, which may act as quenching centers of photoluminescence. PL spectra for Ca9−xMnxEu(PO4)7 solid solutions at 395 nm excitation with high resolution are shown in Figure 13. The spectra consist of transitions from 5 D0 excited level to 7 F0 (579 nm), 7 F1 (589 nm), 7 F2 (612 nm), 7 F3 (652 nm) and 7 F4 (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 Mn 2+ doping. This trend contradicts with the other Ca9−xMxEu(PO4)7 (M = Zn 2+ , Mg 2+ ) [29] solid solutions, where changing of the symmetry from polar R3c to nonpolar R3 c leads to increasing of the luminescence intensity. However, such behavior of PL intensity in Eu 3+ and Mn 2+ co-doped isostructural Ca3(VO4)2 was observed in [58]. The quenching of Eu 3+ emission by Mn 2+ doping in the β-TCP host is caused by the energy transfer from Eu 3+ to Mn 2+ ; however, the effective emission from Mn 2+ ions is absent (Figure 13 and Figure 12). The energy transfer can be relaxed by the 4 T1 energy level of Mn 2+ and then nonradiative relaxation to the 6 A1 ground state of Mn 2+ ions. Moreover, the emission from Mn 2+ can be overlapped with the 5 D0 → 7 F3 transition of Eu 3+ . In addition, the substitution Ca 2+ → Mn 2+ is accompanied by the R3c → R3 c symmetry changing and the formation of defects in the structure, which may act as quenching centers of photoluminescence. PL spectra for Ca9−xMnxEu(PO4)7 solid solutions at 370 nm excitation are shown in Figure 14a. The intensity of Eu 3+ emission also decreases with rising of Mn 2+ concentration. Simultaneously, the intensity of the band attributed to Eu 2+ emission (at ~450 nm) increases. Such behavior can be clearly observed from the dependence of normalized integral intensity of Eu 2+ and Eu 3+ emission on Mn 2+ concentration (Figure 14b). The position and profile of the Eu 2+ band do not change with the Mn 2+ 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 Eu 2+ band intensity (Figure 14a) can be attributed to the increasing of its concentration in the samples. This conclusion also follows from the XPS data. PL spectra for Ca 9−x Mn x Eu(PO 4 ) 7 solid solutions at 370 nm excitation are shown in Figure 14a. The intensity of Eu 3+ emission also decreases with rising of Mn 2+ concentration. Simultaneously, the intensity of the band attributed to Eu 2+ emission (at~450 nm) increases. Such behavior can be clearly observed from the dependence of normalized integral intensity of Eu 2+ and Eu 3+ emission on Mn 2+ concentration (Figure 14b). The position and profile of the Eu 2+ band do not change with the Mn 2+ 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 Eu 2+ 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 Ca9−xMnxEu(PO4)7 solid solutions, the hypersensitive 5 D0 → 7 F0 transition was analyzed. Figure 15 shows an enlarged part of luminescence spectra with the 5 D0 → 7 F0 transition. For the sample with the smallest Mn 2+ 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 5 D0 → 7 F0 transition decreases with the rising of Mn 2+ in Ca9−xMnxEu(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]: The dependence of R/O on Mn 2+ 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. To study the evolution of the β-TCP structure in Ca 9−x Mn x Eu(PO 4 ) 7 solid solutions, the hypersensitive 5 D 0 → 7 F 0 transition was analyzed. Figure 15 shows an enlarged part of luminescence spectra with the 5 D 0 → 7 F 0 transition. For the sample with the smallest Mn 2+ 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 5 D 0 → 7 F 0 transition decreases with the rising of Mn 2+ in Ca 9−x Mn x Eu(PO 4 ) 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]:

Discussion
The abnormal reduction Eu 3+ → Eu 2+ in inorganic phosphors prepared using hightemperature 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 Eu 3+ into Eu 2+ 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 Mn 4+ (MnO2 as initial phase) to the Mn 2+ 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 (PO4 3-); (3) The doped ions (Eu 3+ and Mn 4+ ) substitute the ions with lower valences (Ca 2+ ) in the host; (4) The substituted cation (Ca 2+ ) has ionic radii close to Eu 2+ (see Table 4 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 Eu 2+ and Mn 2+ ions from the oxidizing attack of oxygen from the atmosphere.

Discussion
The abnormal reduction Eu 3+ → Eu 2+ in inorganic phosphors prepared using hightemperature 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 Eu 3+ into Eu 2+ 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 Mn 4+ (MnO 2 as initial phase) to the Mn 2+ 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 (PO 4 3− ); (3) The doped ions (Eu 3+ and Mn 4+ ) substitute the ions with lower valences (Ca 2+ ) in the host; (4) The substituted cation (Ca 2+ ) has ionic radii close to Eu 2+ (see Table 4 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 PO 4 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-Ca 3 (PO 4 ) 2 structure. Well chemically bonded O atoms form a rigid structure. This rigid 3D structure of phosphates can shield and isolate the reduced Eu 2+ and Mn 2+ ions from the oxidizing attack of oxygen from the atmosphere.
Second, Mn 4+ may be a luminescent center as well, and its red emission in octahedral sites is due to spin-forbidden 2 E g → 4 A 2g transitions. However, due to charge imbalance of Mn 4+ and Ca 2+ , for such substitution, the charge compensation scheme in the anionic part is required [42]: where V Ca is a calcium vacancy, and [EO 4 ] 4− is an anion with four negative charges, such as GeO 4 4− or SiO 4 4− , for instance. Since no charge compensation was applied, Mn 4+ could transfer to the Mn 2+ state, which is more suitable for isovalent substitution. Usually, to stabilize manganese in the +2 oxidation state in the β-TCP hosts, MnCO 3 is used as a raw material in a reduction atmosphere [13,14,62].
Third, there is a size mismatch between Mn 4+ and Ca 2+ 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: 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, Mn 2+ doping into the β-TCP host is more preferable. The absence of emission from Mn 4+ in the octahedra environment (red emission [9,64]) in the studied PL spectra shows its full reduction. CN is a coordination number; r is the ionic radii in the corresponding CN.
The reduction of Eu 3+ can be explained by a charge compensation model. According to the difference in the oxidation state of Ca 2+ and Eu 3+ ions, two Eu 3+ substitute three Ca 2+ 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 Eu • Ca with a positive charge in each could be produced: In this substitution, V Ca acts as a donor of electrons, while Eu • Ca is an acceptor of electrons. Thus, electrons can be transferred from the vacancy as follows during thermal treatment: V Ca → V × Ca +2eand the defect Eu 3+ captured electrons and further reduced Eu 3+ to Eu 2+ :

Ca
A schematic representation of such reduction is present in Figure 16. A similar mechanism was observed in other β-TCP hosts [65] showing that Eu 3+ could not be completely reduced to Eu 2+ , even in the reduction atmosphere due to size mismatching. Materials 2023, 16, x FOR PEER REVIEW 18 of 22 Figure 16. The scheme of abnormal reduction of Eu 3+ to Eu 2+ in the β-TCP-type host.
The reduction of Mn 4+ can be described as follows. During thermal treatment, interstitial oxygen ´´ can be formed due to presence of caves along the c-axis from PO4 frameworks. In order to keep the charge balance, one Mn 4+ is needed to substitute for two Ca 2+ ions. So, one vacancy defect ´´ with two negative charges and one ·· defect with two positive charges would form. Since MnO2 is a raw material, these mechanisms can be ascribed by the following: The cause of the full reduction of Mn 4+ is the possibility of transferring negative charges both from ´´ and interstitial oxygen ´´ during thermal treatment: So, these electrons would be released to reduce Mn 4+ ions in the Ca 2+ octahedral M5 site: The reduction of Mn 4+ 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 PO 4 frameworks. In order to keep the charge balance, one Mn 4+ is needed to substitute for two Ca 2+ ions. So, one vacancy defect V Ca with two negative charges and one Mn ·· Ca defect with two positive charges would form. Since MnO 2 is a raw material, these mechanisms can be ascribed by the following: The cause of the full reduction of Mn 4+ is the possibility of transferring negative charges both from V Ca and interstitial oxygen O i during thermal treatment: Such a reduction was previously observed in oxosalt phosphors [66,67].

Conclusions
Phosphates Ca 9−x Mn x Eu(PO 4 ) 7 were obtained by high-temperature solid-phase synthesis. All synthesized samples are isostructural to the β-Ca 3 (PO 4 ) 2 . Differential scanning calorimetry and dielectric spectroscopy revealed an antiferroelectric first-order reversible phase transition. The structure of Ca 8 MnEu(PO 4 ) 7 was refined by the Rietveld method (sp. gr. R3c) using synchrotron X-ray diffraction. Ca 2+ and Eu 3+ ions jointly occupy two sites, M1 and M3, while Mn 2+ completely occupies the M5 site. Magnetic measurements have shown that Ca 8 MnEu(PO 4 ) 7 contains Mn 2+ and Eu 3+ ions. XPS data show the coexistence of europium in +3/+2 oxidation states and manganese in the sole +2 oxidation state. The luminescence of Eu 3+ and Eu 2+ ions was found in Ca 9−x Mn x Eu(PO 4 ) 7 . The presence of two types of Eu 2+ / 3+ emission is associated with the partial abnormal self-reduction of europium in the β-Ca 3 (PO 4 ) 2 matrix. The concentration of Eu 2+ cations is low (~5% according to XPS) and does not affect the magnetic properties. The intensity of Eu 3+ emission is dramatically decreased with the rising of Mn 2+ in Ca 9−x Mn x Eu(PO 4 ) 7 and attributed to the Eu 3+ → Mn 2+ energy transfer. The analysis of the 5 D 0 → 7 F 0 transition and R/O values points to the symmetry inhomogeneity (R3c → R3c) in Ca 9−x Mn x Eu(PO 4 ) 7 , such as in other Ca 9−x M x Eu(PO 4 ) 7 solid solutions with divalent metals.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/ma16041383/s1, Table S1: Atomic coordinates, displacement parameters (Å 2 ) and site-occupancy factors (SOFs) in the structure of Ca 8 MnEu(PO 4 ) 7 ; Table S2  Data Availability Statement: Data will be available from the corresponding author on reasonable request.