Crystal growth, layered structure and luminescence properties of K2Eu(PO4)(WO4)

K2Eu(PO4)(WO4) has been prepared via the high-temperature solution growth (HTSG) method using K2WO4–KPO3 molten salts as a self-flux and characterized by single-crystal X-ray diffraction analysis, IR and luminescence spectroscopy. The structure of this new compound features a 2D framework containing [EuPO6]4− layers, which are composed of zigzag chains of [EuO8]n interlinked by slightly distorted PO4 tetrahedra. Isolated WO4 tetrahedra are attached above and below these layers, leaving space for the K+ counter-cations. The photoluminescence (PL) characteristics (spectra, line intensity distribution and decay kinetics) confirm structural data concerning one distinct position for europium ions. The luminescence color coordinates suggest K2Eu(PO4)(WO4) as an efficient red phosphor for lighting applications.


Introduction
Lanthanide-containing complex oxides based on phosphate, vanadate, molybdate and tungstate have been actively studied as phosphors for solid lighting technologies, particularly for white light emitting diodes (LEDs). 1-3 Among them, much attention has been paid to Eu 3+ -containing compounds due to their prominent photoluminescence (PL) in the red spectral region. This photoluminescence can be excited by (1) a direct excitation of Eu 3+ ions through intracongurational 4f 6 -4f 6 absorption transitions, (2) charge transfer transitions from ligand to europium(III) ion or (3) an energy transfer of absorbed energy from a host to rare-earth (RE) ions. Among these ways the 4f 6 -4f 6 transitions lead to low values of absorption crosssections because they are forbidden from the viewpoint of quantum mechanics. The wide band of O 2À / Eu 3+ chargetransfer transition has high intensity and is located at $225-300 nm for many oxide compounds. [4][5][6][7] However, absence of cheap semiconductor chips with intensive radiation at this short wavelength makes excitation through O 2À / Eu 3+ mechanism inconvenient for LED applications. The third takes place through absorption of light by structural moieties of the host with further transfer of absorbed energy to Eu 3+ -based emission centres. In case of molybdate and tungstate compounds this type of absorption is realized through O 2À / Mo 6+ or O 2À / W 6+ charge transfer providing a wide band in 250-350 nm range in the PL excitation spectra. 7-10 From LED application viewpoint suitable PL excitation can be achieved simultaneously through a direct f-f transition in rare earth ions and by light absorption of the host. It is worth noting that the most intensive absorption usually takes place in the energy region near to the host band gap. In case of complex oxides with molecular anions listed above, the typical band gap values fall within 3-5 eV energy region that is considered as the most convenient for phosphor elaboration. 1 One of the advantages of molybdate and tungstate hosts for rare earth ions is related with weak concentration quenching of luminescence caused by these ions, particularly Eu 3+ ones. This phenomenon is explained by quite inefficient energy transfer between Eu 3+ ions those ones located at the distances at about 4-5Å each from another. 8,11 Some of the structures discussed are layered and characterized by preferable directions for energy transfer. Layered crystal structure is inherent also to mixedanion compounds with general formula A 2 R(PO 4 )(MO 4 ), where A ¼ Na or K; R ¼ Y, Bi or RE; M ¼ Mo or W. [12][13][14][15][16][17][18] Although the rst structure of this family, Na 2 Y(PO 4 )(MoO 4 ), was reported more than three decades ago, 12 there are some gaps in the studies of layered phosphomolybdates concerning both crystal structure and their physicochemical properties. To the best of our knowledge there are no reports in the literature on synthesis, crystal structure and optical properties of K 2 Eu(PO 4 )(WO 4 ). Importantly, an isostructural compound K 2 Eu(PO 4 )(MoO 4 ) has been reported as an efficient phosphor possessing intensive red luminescence. 15 The luminescence properties of the mentioned above phosphomolybdate 15 has been studied in a light of bismuth by europium substitution in the K 2 Bi(PO 4 )(MoO 4 ) structure. 19 The further studies of K 2 Eu(PO 4 )(MoO 4 ) luminescence have shown that its quantum yield is close to 96% and 86% when the PL excitation is performed at 394 and 465 nm, respectively. 20 It is worth noting, the substitution of molybdenum by tungsten in K 2 Bi(PO 4 )(MoO 4 ) : Eu phosphor improves intensity of luminescence with best results achieved for K 2 Bi(PO 4 )(WO 4 ) : 0.8Eu. 19 The effect of anion substitution in similar layered compounds has been shown to be a driving force in separating emission centers and therefore enhancing the thermal stability and increasing the critical concentration of activator ions. 21 for Na 2Àn Y(MoO 4 ) 1+n (PO 4 ) 1Àn : Tb 3+ ,Eu 3+ has been used for improving the thermal stability of phosphors obtained. 22 In this light one should admit signicant difference in excitation and luminescence spectra for isostructural hosts containing molybdate 15,21,23 and tungstate groups. 24,25 To clarify this phenomenon more spectral data for phosphotungstates should be collected and analysed.
In the present paper we report single crystal growth, crystal structure and luminescence properties of the layered phosphor K 2 Eu(PO 4 )(WO 4 ).

Synthetic procedures
Single crystals of K 2 Eu(PO 4 )(WO 4 ) have been grown by the hightemperature solution growth method from K 2 WO 4 : KH 2 PO 4 -: Eu 2 O 3 molten mixture in the ratio 4.99 : 4.99 : 0.02. The precursors without further purication have been mixed and grinded together in an agate mortar and melted in a platinum crucible at 1100 C. On the next stage the high-temperature solution prepared has been kept at this temperature for 2 hours under stirring in order to reach the homogeneity. The molten mixture has been cooled down to 750 C at the rate of 80 C h À1 . At this stage the melt has been poured down on a copper sheet and a crystalline product has been le in the furnace for slow cooling to room temperature. The colourless plates have been leached out with hot deionized water and characterized by IR and single crystal X-ray diffraction.

Crystallography
A suitable single crystal of K 2 Eu(PO 4 )(WO 4 ) was selected and mounted on an Xcalibur, Eos diffractometer (Mo-K a radiation, l ¼ 0.71073). The crystal was kept at 293 K during data collection. Using Olex2, 26 the structure was solved with the SHELXT 27 structure solution program (Intrinsic Phasing method) and rened with the SHELXL 28 renement package (Least Squares minimisation). Crystallographic data and structure renement parameters for K 2 Eu(PO 4 )(WO 4 ) are summarized in Table 1.

Sample characterization
Investigations of the thermal behavior of the K 2 Eu(PO 4 )(WO 4 ) have been performed using a Shimadzu DTG-60H simultaneous thermogravimetry/differential thermal analyzer. The sample and the reference (a-Al 2 O 3 ) were heated up to 900 C in Pt crucibles under an air atmosphere at 10 C min À1 .
IR spectrum has been measured on a PerkinElmer Spectrum BX FTIR spectrometer in the frequency range 400-4400 cm À1 in KBr pellets.
The PL emission and excitation spectra of the samples have been recorded at room temperature using a DFS-12 spectrometer equipped with a FEU-79 photomultiplier. A powerful Xenon arc lamp (DXeL-1000) combined with a DMR-4 prism monochromator was used as source of the excitation light. All the spectra have been corrected on system response.
The PL kinetics have been measured with use of a MSA-300 multiscaler photon counter and a blue LED (l rad ¼ 465 nm) operating at pulse regimes as a source of the PL excitation.

Crystal structure
The dipotassium europium(III) phosphate(V) tungstate(VI) K 2 -Eu(PO 4 )(WO 4 ) crystallizes in the Ibca space group (orthorhombic crystal system) with eight K 2 Eu(PO 4 )(WO 4 ) formula units per unit cell (Table 1). There is one crystallographically unique europium cite in the Wyckoff special position 8d, showing a coordination sphere of eight oxygen atoms in the shape of a triangular dodecahedron (Fig. 1a).
The distortions of the coordination environment of europium, phosphorus and tungsten have been calculated with Shape 2.0 program 29 via the Continuous Shape Measure method. The value of S ¼ 2.908 was obtained for the Eu  Table 2). Thus, K 2 Eu(PO 4 )(WO 4 ) comprises non-condensed phosphate and tungstate tetrahedra. Each tetrahedrally coordinated phosphorus(V) and tungsten(VI) atoms are crystallographically unique and are located at the Wyckoff positions 8d and 8e, respectively. They are surrounded by four oxygen atoms forming bisphenoidally distorted tetrahedra. The small values of the S parameter (Table 2) indicate a slight deviation from ideal tetrahedra for both PO 4 and WO 4 . Despite the fact that both tetrahedral moieties exhibit C 2 site symmetry, phosphate tetrahedra are found to be more disported than tungstate ones.
Europium triangular dodecahedra are connected by common edges forming a zigzag chain along a axis (Fig. 1b). These [EuO 8 ] n zig-zag chains are linked by phosphate tetrahedra building a layer in ab-plane (Fig. 1b). Finally, the WO 4 tetrahedra are attached to the plane from both sides along b axis (Fig. 1c). Layers [EuPO 6 ] 4represent the nearest Eu/Eu contacts 3.9644(4)Å, while the other ones are much longer and belong to different layers (Fig. 2a). In comparison to K 2 -Eu(PO 4 )(MoO 4 ) structure 15 the shortest distance between neighbor Eu/Eu contacts are much shorter being 3.5Å. Potassium cations are found on crystallographically unique 16f Wyckoff positions, showing a coordination sphere of eight oxygen atoms in the shape of biaugmented trigonal prism (Table 2), which reside among the voids between the neighboring sheets. Fig. 1 (a) The nearest surrounding of europium cation in K 2 -Eu(PO 4 )(WO 4 ) structure; (b) 2D layer at ab plane; c) The crystal structure view along a axis.  Interestingly, potassium cations within the layer form a graphene-like sheets along direction (1 0 1) with the shortest K to K distance equal to 3.9869(1)Å, and the longest are 4.2492(1)Å (Fig. 2b).
Thermal analysis K 2 Eu(PO 4 )(WO 4 ) is characterized by high thermal stability in the temperature range of 20-900 C with slight weight loss less than 0.65% most probably explained by adsorbed water. Taking into consideration that melting point of K 2 Eu(PO 4 )(MoO 4 ) 15 is above 1050 C, the thermal stability of the tungstate-containing analogue may be expected to be higher. Fig. 3 shows IR spectrum of K 2 Eu(PO 4 )(WO 4 ) in the region of 400-1200 cm À1 where the most intensive absorption bands are located. From structural point of view, the titled compound is isostructural to the parent one K 2 Bi(PO 4 )(MoO 4 ). 14 Spectroscopic data illustrates the difference in the local symmetry of tetrahedral units. Thus, the wide band located at 1078 cm À1 with a shoulder at 1102 cm À1 is ascribed to the asymmetric stretching vibration n 3 (F 2 ) in PO 4 tetrahedra. 31 On the contrary, this band in IR spectrum of K 2 Bi(PO 4 )(MoO 4 ) is redshied toward 1055 cm À1 . The same situation is found for a band at 961 cm À1 with a shoulder at 1000 cm À1 which is ascribed to the symmetric stretching vibration n 1 (F 2 ) in PO 4 tetrahedra. The corresponding band is found at 945 cm À1 in the case of K 2 -Bi(PO 4 )(MoO 4 ) ( Table 3). The bands in a range of 887-792 cm À1 can be attributed to stretching vibrations in the WO 4 tetrahedra. The 500-650 cm À1 region shows three bonds expected for n 4 (F 2 ) of PO 4 tetrahedra bending vibrations: 618, 572 and 530 cm À1 . The characteristic bands observed for the title compound agree well with other isostructural compounds (Table 3).

Luminescence spectroscopy
The phosphotungstate K 2 Eu(PO 4 )(WO 4 ) reveals intensive red photoluminescence in case of excitation under UV and blue light excitation at room temperature. The corresponding spectra consist of relatively narrow emission bands which are related with 5 D 0 / 7 F J¼1À4 electronic transitions in Eu 3+ ions (Fig. 3). The most intensive bands located near 615 and 700 nm correspond to forced electric dipole transitions 5 D 0 / 7 F 2 and 5 D 0 / 7 F 4 , respectively. It should be pointed out that intensity of the bands of 5 D 0 / 7 F 4 transitions is abnormally high in respect to 5 D 0 / 7 F 1 ones for all spectra obtained. This phenomenon is not typical for Eu 3+ ions emission in solids but it has been observed earlier for some hosts, in particular phosphate and tungstate ones. [35][36][37][38] High intensity of the 5 D 0 / 7 F 4 transitions was explained earlier assuming the highly polarizable chemical environment for Eu 3+ emission centre. 32 In our case, the europium cations are surrounded by 4 phosphate and 2 tungstate tetrahedra (Fig. 1a). Taking into consideration the second coordination sphere of EuO 8 polyhedra one can admit more covalent character in case of P-O-Eu bonds in comparison to W-O-Eu ones. These structure-related peculiarities can be regarded as the source of higher polarizability of Eu 3+ environment in the K 2 Eu(PO 4 )(WO 4 ) and, consequently,   Table 4 Ratios between total a intensities of 5 D 0 / 7 F J transitions in Eu 3+ ions in K 2 Eu(PO 4 )(WO 4 ) and chromaticity coordinates l ex , nm R ¼ I( 7 F 2 )/I( 7 F 1 ) I( 7 F 4 )/I( 7 F 1 ) I( 7 F 4 )/I( 7 F 2 ) x y the reason for high emission intensity of the 5 D 0 / 7 F 4 transitions. Moreover, the ratios between the PL intensities of 5 D 0 / 7 F 4 and 5 D 0 / 7 F J¼1,2 transitions depend on excitation wavelength (calculated values are collected in the Table 4). This phenomenon can be explained by the inuence of electron-phonon coupling in two types of luminescence centers: a regular EuO 8 polyhedron and a defect-containing one. The high value of the asymmetry ratio, R ¼ I( 5 D 0 / 7 F 2 )/I( 5 D 0 / 7 F 1 ), in the Table 4 indicates that Eu 3+ cations are located at low-symmetry sites without inversion centre in accordance with structural data. Ratio of intensities I( 5 D 0 / 7 F 4 )/I( 5 D 0 / 7 F 2 ) changes slightly when l ex is switched from 380 to 393 nm that is also related with the impact of defect-containing luminescence centers.
The normalized PL excitation spectra of the Eu 3+ -related luminescence in the K 2 Eu(PO 4 )(WO 4 ) are shown in Fig. 4. The most intensive band peaking at 393 nm in the spectra is related with 7 F 0 / 5 L 6 transition. Less intensive bands are located near 319 ( 7 F 0 / 5 H J ), 360 ( 7 F 0 / 5 D 4 ), 375 ( 7 F 0 / 5 G J ), 380 ( 7 F 0 / 5 L 7,8 ), 415 ( 7 F 0 / 5 D 3 ), 465 ( 7 F 0 / 5 D 2 ), 534 and 543 nm ( 7 F 0-2 / 5 D 1 ). The wide band with maximum below 260 nm is ascribed to O 2À / Eu 3+ charge transfer that typically observed for Eu 3+ -containing oxide compounds, e.g. in case of the K 2 -Eu(PO 4 )(MoO 4 ) ones. 19,20 Minor changes in the PL excitation spectra can be seen in the regions of 7 F 0 / 5 L 6 and 7 F 0 / 5 D 2 electronic transitions. In case of registration at l em ¼ 594 nm the band maxima of these transitions are shied toward longer wavelength in respect to corresponding bands in the PL excitation spectra registered at l em ¼ 615 and 702.5 nm. These shis are about 0.02 eV in energy scale that is comparable with kT value at room temperature (0.026 eV). It has been found that under excitation at 465 nm and registration at 615 nm the PL kinetics curve can be tted by double exponential decay: I ¼ 10.6 Â exp(Àt/s 1 ) + 88.4 Â exp(Àt/t 2 ) with time constants s 1 ¼ 277 AE 5 ms and s 2 ¼ 1527 AE 2 ms. Average lifetime for K 2 Eu(PO 4 )(WO 4 ) has a value of 1379 ms when calculated with formula s avg ¼ (I 1 Â s 1 + I 2 Â s 2 )/(I 1 + I 2 ).
This value is higher than common ones for tungstatecontaining compounds, namely, 498 ms found for KEu(WO 4 ) 2 . 34 The increased PL lifetimes might be related to the charge transfer band lying at higher energies as it has been found for isostructural compound, K 2 Eu(PO 4 )(MoO 4 ) with PL emission component having s z 2050 ms at room temperature 20 when l ex ¼ 465 nm and l em ¼ 615 nm. Fig. 5 Signicant difference in average lifetime for tungstate and molybdatecontaining isostructural compounds may be also related with different energies of charge transfer bands. 20 Luminescence data are found to be in agreement with structural peculiarities of the K 2 Eu(PO 4 )(WO 4 ). The spectroscopic characteristics can be discussed in a light of one unique Eu position in a quite distorted eight-fold coordination and the arrangement of these polyhedra into 2D layers. At the same time, the presence of peak positions' shis in the PL excitation spectra, complex dependence of the asymmetry ratio R on excitation wavelength, and two components observed in kinetics of the PL decay cannot be omitted and requires additional study. Similar situation has been observed for other Eucontaining compounds where defects in oxygen environment of europium [35][36][37][38][39][40] caused by annealing during synthetic procedure leads to distinguish two types of luminescence centers. The rst one is associated with Eu in regular EuO 8 dodecahedra, while the second center may be related with oxygen vacancies. 37,41 The latter assumption is supported with previously reported data for oxide phosphors containing phosphate and tungstate groups. 42 Moreover, the complex nature of the PL decay, which is a superposition of fast and slow components is also more likely to be defect-related. Thus, the contribution of the fast component to the emission (I 1 $s 1 z 2900 r.u.), which is related with vacancy-containing centers is $45 times smaller than the slow component one (I 2 $s 2 z 135 000 r.u.).  The further studies of the PL properties especially at low temperatures are very necessary for clarifying noted assumption.
Due to intensive red luminescence of phosphotungstate K 2 -Eu(PO 4 )(WO 4 ) can be considered as a suitable phosphor for luminescent lighting application. The calculated values of chromaticity coordinates (x y) are collected in Table 4. The colour coordinates are close to those of the NTSC standard for red colour (0.67; 0.33) for all PL excitation studied. High intensity of 5 D 0 / 7 F 2 observed for K 2 Eu(PO 4 )(WO 4 ) can be considered for applications as luminescent down-shiing for white light emitting diodes.

Conclusions
In summary, K 2 Eu(PO 4 )(WO 4 ) crystals have been successfully prepared by the high-temperature solution growth, and its structure, thermal stability, chromaticity coordinates, luminescence spectra and decay kinetics have been investigated in detail. The crystal structure contains non-condensed phosphate and tungstate tetrahedra interlinked with condensed in a zigzag chain EuO 8 polyhedra, while potassium cations reside in interlayer space. The phosphor can be efficiently excited by light at the 360-480 nm spectral region and give rise to bright luminescence with the most intensive bands located near 615 and 700 nm correspond to 5 D 0 / 7 F 2 and 5 D 0 / 7 F 4 forced electric dipole transitions in europium cation, respectively. The abnormally high intensity of 5 D 0 / 7 F 4 transitions is ascribed to polarizability of Eu 3+ environment in the K 2 Eu(PO 4 )(WO 4 ). Luminescent properties are consistent with the structural characteristics of the studied crystals. In particular, the PL results conrm that all positions of europium ions in the crystal lattice are equivalent. The photoluminescence characteristics obtained indicate that titled compound has a potential application as a red-light emitting phosphor.

Conflicts of interest
There are no conicts to declare.