Structural and Luminescence Behavior of Nanocrystalline Orthophosphate KMeY(PO4)2: Eu3+ (Me = Ca, Sr) Synthesized by Hydrothermal Method

KMeY(PO4)2:5% Eu3+ phosphates have been synthesized by a novel hydrothermal method. Spectroscopic, structural, and morphological properties of the obtained samples were investigated by X-ray, TEM, Raman, infrared, absorption, and luminescence studies. The microscopic analysis of the obtained samples showed that the mean diameter of synthesized crystals was about 15 nm. The KCaY(PO4)2 and KSrY(PO4)2 compounds were isostructural and they crystallized in a rhabdophane-type hexagonal structure with the unit-cell parameters a = b ≈ 6.90 Å, c ≈ 6.34 Å, and a = b ≈ 7.00 Å, c ≈ 6.42 Å for the Ca and Sr compound, respectively. Spectroscopic investigations showed intense 5D0 → 7F4 transitions connected with D2 site symmetry of Eu3+ ions. Furthermore, for the sample annealed at 500 °C, europium ions were located in two optical sites, on the surface of grains and in the bulk. Thermal treatment of powders at high temperature provided better grain crystallinity and only one position of dopant in the crystalline structure. The most intense emission was possessed by the KSrY(PO4)2:5% Eu3+ sample calcinated at 500 °C.


Introduction
In recent times, inorganic phosphors doped with lanthanide ions have attracted much attention due to their potential applications as phosphors, laser materials, or solid-state lighting [1][2][3][4][5][6]. The current white-light-emitting diodes (W-LEDs) suffer from a lack of red light, therefore, red phosphors are sought to fill deficiency. Generally, W-LEDs are fabricated by combining the yellow emission of YAG: Ce 3+ with the blue light of the diode [7]. The second method is to mix the blue, green, and red emissions generated from three or two blended powders. Rare-earth-doped (RE) phosphates are a good candidate for the formation of white LEDs due to their good physical and chemical stability, low cost of substrates, and reduced preparation temperatures compared to silicates, aluminosilicates, or nitrates [8][9][10][11][12][13].
One of the most promising phosphors are KMeRE(PO 4 ) 2 phosphates, where Me = Ca or Sr, and have a hexagonal structure with the unit cell parameters a = b ≈ 7.0 and c ≈ 6.4 Å [14][15][16][17][18]. The structure is closely related to rhabdophane (REPO 4 ·nH 2 O), in which half of the cationic sites of RE 3+ are filled by Me 2+ (Figure 1). Simultaneously, the electrostatic charge is compensated by the placement of K + in free spaces inside of channels running parallel to the c axis and formed by (RE, Ca)-PO 4 -(RE, Ca) chains [14]. According to the available literature, the simultaneous replacement of RE 3+ with K + and Ca 2+ reduces the group symmetry of PO 4 and RO 8 (R = Ca or RE) from D 2 to C 2 [17]. The KMeRE(PO 4 ) 2 phosphates show good luminescence and hence can be applied as efficient luminophores [19,20]. Recently, a series of KMeRE(PO 4 ) 2 (Me = Ca, Sr; RE = Y, La, Lu, Gd) have been investigated carefully [4,[21][22][23][24]. Wang et al. successfully synthesized KCaY(PO 4 ) 2 :Dy 3+ , Eu 3+ as a nanosized phosphor-generated warm-white light with a high color rendering index (CRI) [23]. Basic information about KCaY(PO 4 ) 2 :Eu 3+ has been reported in 2006 by Wang et al. [24]. Furthermore, the energy transfer from Eu 2+ to Mn 2+ in KCaY(PO 4 ) 2 was studied by Liu and co-workers [4]. Up until now, strontium ternary phosphates were activated by RE ions, such as Ce 3+ , Tb 3+ , Eu 3+ , and Mn 2+ [21,22]. Samples doped with Ce 3+ , Tb 3+ , and Mn 2+ exhibited high quantum efficiency (60%) and were obtained by a Pechini method [22]. Compounds are mostly obtained via a conventional ceramic method (sintering at high temperatures), however, results presented in [25][26][27] show that obtaining single-phase KMeRE(PO 4 ) 2 is difficult because of the limited thermal stability of such ternary phosphates at temperatures exceeding 1100-1200 • C. A relatively mild hydrothermal process (short-term heating carried out at 240 • C) has been applied for the synthesis of phase-pure KCaCe(PO 4 ) 2 , which resulted in obtaining nanocrystalline powders [27].
The aim of the presented study was to describe the structural and luminescence properties of nanocrystalline KMeY(PO 4 ) 2 (Me = Ca, Sr) powders doped with Eu 3+ and to discuss the obtained results from the point of view of the powders' morphology.

Materials and Methods
The powders of KMeY(PO 4 ) 2 :5% Eu 3+ (Me = Ca or Sr) were obtained by a short-lasting hydrothermal technique. In the first step, Y 2 O 3 (99.99% POCh, Gliwice, Poland) and Eu 2 O 3 (99.9%, POCh, Gliwice, Poland) were digested in dilute nitric acid. The obtained clear solutions were dried for several days over solid NaOH until solid nitrates crystallized. The starting sols for hydrothermal processes were prepared as follows. Rare earth nitrates and Ca(NO 3 ) 2 ·4H 2 O (≥99.0%, POCh, Gliwice, Poland) or Sr(NO 3 ) 2 (≥99.0%, Sigma-Aldrich, Steinheim, Germany) were dissolved in 10 cm 3 of distilled water, then K 3 PO 4 ·H 2 O (≥94%, Fluka, Steinheim, Germany) was added to the solution, and the obtained mixture was diluted to 50 cm 3 volume. The molar ratio of K:Me:Y:Eu metals in the obtained sols was 68:1:0.95:0.05. After one-day ageing, the sols were used in a hydrothermal process performed in a Magnum II autoclave (Ertec, Wrocław, Poland) with heating of the sols by microwaves of 600 W maximal power. The syntheses were controlled by temperature measurements performed at a bottom part of the reactor made of PTFE. The syntheses were carried out for 2 h at a maximal temperature of 250 • C and a pressure of 30 bar. As they were obtained, fine precipitates were centrifuged, washed several times with distilled water, and dried for 24 h at 120 • C. Further heating of the samples was carried out at 500 • C and 1000 • C for 10 h.
Phase analysis of obtained powders was performed using the XRD technique and a Siemens D5000 diffractometer (Munich, Germany) equipped with a copper radiation cathode. The measurements were performed at a 2θ angle range of 5-60 • with a 0.02 • step and at least 4 s per step. The indexing of powder patterns was made using the computer program DICVOL [28]. Unit cell lattice constants of KMeY(PO 4 ) 2 :5%Eu were refined using the Checkcell software (LMGP, Grenoble, France) [29].
Excitation spectra were measured on a McPherson spectrometer (McPherson Inc. Chelmsford, MA, USA) equipped with a 150 W Xe lamp and 150 W deuterium lamp (for VUV part) as excitation sources, and a Hamamatsu R928 photomultiplier (Hamamatsu Photonics, Hamamatsu, Japan) as a detector. The normalization of the XRD and excitation spectra was performed at the highest point of the spectrum. The emission spectra were measured using a Hamamatsu PMA12 multichannel analyzer (Hamamatsu Photonics, Hamamatsu, Japan) as a detector, and 395 nm line from tunable pulsed Ti: Sapphire laser pumped by second harmonic (λ = 532 nm) of Nd: YAG laser as an excitation source (Lotis TII, Minsk, Belarus). Decay time measurements were performed using the same excitation as for emission spectra, while decay curves were collected on a Tektronix MSO-3052 digital oscilloscope (Tektronix, Beaverton, OR, USA). The low temperature measurement was carried out at 77 K using a quartz dewar.

Structure and Morphology
The analysis of XRD patterns of obtained phosphates confirmed the crystalline characteristics of the powders. All of the samples were isostructural to the rhabdophane-type KCaY(PO 4 ) 2 phosphate (ICDD card No. 51-1632) ( Figure 2). The XRD pattern of dried samples showed pure structures after the hydrothermal process ( Figure S1). Due to the presence of hydroxyl groups originating from zeolitic water molecules, which can easily quench luminescence, samples were annealed at 500 • C and 1000 • C. The annealing process does not change the structure of prepared phosphates. Minor reflections of a secondary phase were registered in the XRD pattern of KCaY(PO 4 ) 2 :5%Eu 3+ sintered at 500 • C. They most likely derive from the YPO 4 xenotime phase; however, due to the low intensity of reflections, the presence of the phase can be not confirmed with reliability. The refined hexagonal unit cell parameters of the phosphates sintered at 1000 • C are listed in Table 1.   3+ and KSrY(PO 4 ) 2 :5% Eu 3+ samples, respectively. As can be seen, the obtained compounds possess rounded grains with an average particle length size (maximum particle dimension) D [1,0] equal to 12 ± 2 nm and 18 ± 4 nm for Ca and Sr phosphates, respectively. The powders consist of single grains, as well as bigger objects created by the clumping of a few grains. Agglomerates have an irregular shape made from connected spherical single nanocrystals. The replacement of calcium ions by the strontium one does not change the shape of obtained nanopowders.    Figure 6a,b present infrared spectra in the middle (MIR) and far (FIR) infrared range, and Figure 7 shows the Raman spectra. The spectra measured for the strontium derivative are identical, so we decided to only show spectra for the calcium compound as the representative. They were also similar to those reported earlier by Tie et al. [17]. In general, the bands observed in these spectra agree with those reported earlier for other orthophosphates [30][31][32][33]. The particular types of vibrations appear in the typical range. The ν 3 (F 2 ) = ν as (PO 4 ) vibrations were observed at 1130 and 1090 cm −1 , where a strong multiplet consisting of 2-3 bands appeared. In the range characteristic for ν 1 (A 1 ) = ν s (PO 4 ) vibrations, a strong single Raman band was observed. All bending δ(PO 4 ) vibrations were observed in the range of 350-650 cm −1 , in which nine Raman and five IR bands could be identified. The IR bands of average to strong intensities in the ranges of 250-350 cm −1 and 150-250 cm −1 correspond to the T'(K + ) and T'(Ca 2+ ) translations, respectively. The analysis of these vibrations was performed in Reference 4 on the basis of the D 2 site symmetry of this unit, although factor group splitting was not taken into account.
Its primitive unit cell contains three Ca 2+ /Y 3+ ions equally sharing the 3c (D 2 site) position, and three PO 4 3− ions with P and O atoms in the 3d (D 2 ) and 12k (C 1 ) positions, respectively. The K + ions could not be used for calculations because their occupancy in the primitive cell was partial. For this reason, only a partial factor group analysis can be presented.

Optical Studies
The excitation spectra of KMeY(PO 4 ) 2 :5% Eu 3+ were measured by observing the transition at λ em = 612 nm (Figure 8). The spectra consisted of a broad and intense band centered at 235 nm, which was assigned to the charge transfer transition from oxygen to europium ions (CTS-charge-transfer state) [34]. Besides the CTS, there were also sharp and narrow bands, which derived from the 4f-4f transition of Eu 3+ [35]. The bands were assigned to the 7 F 0 → 5 H 3-7 at 319 nm, 7 F 0 → 5 D 4 at 361.5 nm, 7 F 0 → 5 G 2-6 , 5 L 7 at 379 nm, 7 F 0 → 5 L 6 at 393 nm, 7 F 0 → 5 D 3 at 413.5 nm, 7 F 0 → 5 D 2 at 464 nm, and 7 F 0 → 5 D 1 at 532 nm. All spectra were normalized to the intensity of the charge transfer band. In the sample sintered at 1000 • C, the ratio between the CTS and 4f-4f bands increased. The room temperature emission spectra measured with excitation at λ = 393 nm consist of bands characteristic for Eu 3+ ions' emission ( Figure 9a) [36]. They can be assigned to the transition from the 5 D 0 excited level to 7 F J (J = 0, 1, 2, 3, 4) ground levels. The emission peak attributed to the 5 D 0 → 7 F 0 was located with the maximum at 578 nm, while the 7 F 1 was at 587 nm, 7 F 2 was at 612 nm, and 7 F 3 and 7 F 4 were at 653 nm and 704 nm, respectively. The highest emission intensity was exhibited by the KSrY(PO 4 ) 2 :5% Eu 3+ sample annealed at 500 • C (Figure 9b). The Ca 2+ analogous phosphate possessed lower emission intensity. Furthermore, heat treatment of samples at 1000 • C decreased the overall emission intensity for both KCaY(PO 4 ) 2 :5% Eu 3+ and KSrY(PO 4 ) 2 :5% Eu 3+ . This was probably connected with more efficient non-radiative processes. When the samples were annealed at a higher temperature, all Eu 3+ ions only occupied one position in the crystal structure of the investigated phosphate. As a consequence, the distance between Eu 3+ -Eu 3+ ions decreased causing concentration quenching. The enhancement of nonradiative processes with annealing temperatures confirmed registered decay curves (see Figures S4-S7). As shown in Figure 9a, the most intense was the electric dipole transition (ED) 5 D 0 → 7 F 2 . However, it should the emphasized that the intensity of 7 F 4 bands was anomalously intense. Similar behavior was reported for the first time in 1988 by Blasse [37]. He linked this situation with the D 4h site symmetry of Eu 3+ ions. The same observation has been presented by Ferreira in Na 9 [EuW 10 O 36 ]·14H 2 O [38]. However, in the investigated phosphates, Eu 3+ ions occupy Ca 2+ /Sr 2+ sites with D 2 symmetry. Bettinelli in 2011 explained that the distortion of EuO 8 polyhedra to D 2 symmetry was responsible for the enhancement of the 5 D 0 → 7 F 4 transition [39]. No significant changes were detected in the emission spectra recorded at 77 K ( Figure S2). With the temperature decreasing, emission bands become narrower; however, even at low temperatures, the peaks overlapped and they were not well separated. The 5 D 0 → 7 F 0 transition of the sample annealed at 500 • C consisted of two peaks coming from two optical positions of dopant in the host of the compounds under study. On the other hand, for samples obtained at 1000 • C, only one 7 F 0 band was presented. The localization of Eu 3+ ions on the surface of nanocrystals and inside the bulk can cause the occurrence of two dopant sites, and has been reported many times in the literature [40]. Presented emission and decay times also confirmed this hypothesis. The 5 D 0 → 7 F 0 transition of samples annealed at 500 • C consisted of two separate bands, while only one band was observed for samples prepared at 1000 • C. The magnification of the 5 D 0 → 7 F 0 transition of all samples is presented in Figure S3. These observations were in line with measured emission decay curves (see below).
For europium ions, the magnetic dipole transition ( 7 F 0 → 5 D 1 ) was almost independent of the local symmetry and type of matrix, while the electric dipole transition ( 7 F 0 → 5 D 2 ) was very sensitive to any change in the local environment. When europium ions occupied a site with a center of symmetry, the only allowed transition was the magnetic dipole, while for europium ions at non-centrosymmetric sites, the most intense transition was the electric dipole. Therefore, the ratio between these transitions is called the asymmetry parameter R and is defined as follows: The higher the value of the R parameter, the more non-centrosymmetric the site of Eu 3+ was. Calculated values of the R parameter are listed in Table 2 and Figure 9c. Increasing the annealing temperature of samples lowered the R parameter, which meant that distortion of the local surrounding of Eu 3+ ions decreased, and for KCaY(PO 4 ) 2 :5% Eu 3+ annealed at 1000 • C, it was close to being ideally octahedral. Room temperature decay curves measured for all samples are presented in Figures S4-S7, while the calculated decay times are listed in Table 2. The decay curves of samples annealed at 500 • C have biexponential characteristics, which confirmed the localization of europium ions on the nanograins surface and inside the bulk. The calculated values of decay time were typical for Eu 3+ ions [35,[41][42][43]; for example, the decay time of the Eu 3+ ions in the Y 2 O 3 compound equaled 2 ms, while for YAsO 4 :Eu 3+ it was around 4 ms. Increasing the annealing temperature caused a decrease in decay time, which was in good agreement with the observed behavior of emission intensity. The enhancement of non-radiative processes with annealing temperatures confirmed registered decay curves. The lifetime of the KCaY(PO 4 ) 2 :5% Eu 3+ decreased from 5.4 ms to 4.7 ms with increasing annealing temperatures from 500 • C to 1000 • C, respectively.

Conclusions
Nanocrystalline powders of the KMeY(PO 4 ) 2 :5% Eu 3+ (Me = Ca, Sr) composition were obtained at relatively mild hydrothermal conditions. The XRD study confirmed the crystal structure of the investigated ternary phosphates, and TEM images showed that irregular agglomerates consisted of particles with an average size below 20 nm. Careful study of the IR and Raman spectra allowed all the vibrations, translations, and librations to be assigned. K(Ca, Sr)Y(PO 4 ) 2 activated with 5% of Eu 3+ ions showed intense red emission, with a dominance of electric dipole transitions from the 5 D 0 to 7 F 2 level. The local symmetry of Eu 3+ ions in the investigated phosphates KMeY(PO 4 ) 2 :5% Eu 3+ (Me = Ca, Sr) was D 2 , which caused the abnormally high intensity of the 5 D 0 → 7 F 4 transition. For samples annealed at the temperature of 500 • C, Eu 3+ ions were on the surface of the nanocrystals and inside the bulk. Increasing the annealing temperature to 1000 • C resulted in better crystallinity of the sample and only one location of doping ions. Due to this, the distance between Eu 3+ -Eu 3+ ions was reduced, which caused concentration quenching. For this reason, the most intense emission was observed for the samples heat-treated at 500 • C. These results were consistent with the recorded decay times.

Conflicts of Interest:
The authors declare no conflict of interest.