Rietveld refinement of langbeinite-type K2YHf(PO4)3

Potassium yttrium hafnium tris(orthophosphate) belongs to the langbeinite-family and is built up from [MO6] octahedra [in which the positions of the two independent M sites are mutually occupied by Y and Hf in a 0.605 (10):0.395 (10) ratio] and [PO4] tetrahedra connected via vertices into a three-dimensional framework. This framework is penetrated by large closed cavities in which the two independent K atoms are located; one of the K atoms is nine-coordinated and the other is 12-coordinated by surrounding O atoms. The K, Y and Hf atoms lie on threefold rotation axes, whereas the P and O atoms are located in general positions.

Potassium yttrium hafnium tris(orthophosphate) belongs to the langbeinite-family and is built up from [MO 6 ] octahedra [in which the positions of the two independent M sites are mutually occupied by Y and Hf in a 0.605 (10):0.395 (10) ratio] and [PO 4 ] tetrahedra connected via vertices into a threedimensional framework. This framework is penetrated by large closed cavities in which the two independent K atoms are located; one of the K atoms is nine-coordinated and the other is 12-coordinated by surrounding O atoms. The K, Y and Hf atoms lie on threefold rotation axes, whereas the P and O atoms are located in general positions.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2244).
Due to similar chemical properties of Zr and Hf and the close values of their ionic radii (for coordination number 6 they are 0.72 and 0.71 Å for Zr and Hf, respectively; Shannon, 1976) the cell parameters of K 2 YHf(PO 4 ) 3 are slightly smaller than of K 2 YZr(PO 4 ) 3 (a= 10.3346 (1) Å; Wulff et al., 1992). octahedra are joined to each other by three bridging orthophosphate tetrahedra forming {M 2 P 3 O 18 } groups. These groups form three-dimensional framework penetrated with large closed cavities. Two independent potassium atoms are located in each cavity. K1 atom is nine-coordinated, while K2 is twelve-coordinated.
Bond valence sums (BVS) were calculated using parameters for Hf, Y, P from Brese & O'Keeffe (1991) and for K from Brown & Altermatt (1985). The calculation were performed for formula sum K 2 YHf(PO 4 ) 3 taking into account occupancies of the octahedrally coordinatedd M positions. The sum of BVS of positively charged atoms is equal to 24.16 while the chemical charge of the remaining O atoms is equal to -24.

Experimental
Well-shaped tetrahedral crystals of K 2 YHf(PO 4 ) 3 were grown using a flux technique. A mixture of 4.52 g KPO 3 and 3.4 g K 4 P 2 O 7 (initial K/P molar ratio was set equal to 1.35) was melted in a platinum crucible at 1273 K. The melt was kept at this temperature for 1 h and after that the temperature was decreased to 1173 K. Dispersed in an agate mortar, a mixture of 1.36 g HfF 4 and 0,78 g YF 3 was added to the phosphate flux under stirring. The crystallization of the melts was performed from 1173 to 893 K at a rate of 30 K/h. The synthesized crystalline sample was separated from remaining glass by leaching with hot water. The dimensions of the crystals were found to be in a range 0.01-0.05 mm. The sample was ground in an agate mortar before performing powder XRD data collection. The recorded powder pattern indicated a single phase material.
supplementary materials sup-2 The element ratio was determined using ICP-AES analyses (Shimadzu ICPE-9000 spectrometer). The sample for measurements was prepared by dissolution of calculated amount of K 2 YHf(PO 4 ) 3 in sulfuric acid (98%) with final dilution by bidistilled water. Element ratio was found to be: 2.02:0.97:0.98:3.04 for K:Y:Hf:P which fits well with the theoretical values.