A monoclinic polymorph of KY(PO3)4

The title compound, potassium yttrium polyphosphate, KY(PO3)4, was synthesized using the flux method. The atomic arrangement consists of an infinite long-chain polyphosphate organization. Chains, with a period of four PO4 tetrahedra, run along the a-axis direction. Two other polymorphs of this phosphate are known, in space groups P21/n and C2/c.


Data collection
Enraf-Nonius CAD-4 diffractometer Absorption correction: multi-scan (SADABS; Sheldrick, 1996)  Data collection: CAD-4 EXPRESS (Duisenberg, 1992;Macíček & Yordanov, 1992); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXL97. to there high-temperature chemical stability, high yield intrinsic fluorescence and minimal trapping of excitation, rendering them attractive materials for investigations of the energy transfer phenomena and fluorescence quenching (Malinowski, 1990;Malinowski et al., 1988). The literature dealing with these compounds was rather confusing for some time, between cyclic or chain condensed phosphates, but it is currently well established that the M I Y(PO 3 ) 4 compounds are polyphosphates with infinite chain and M I YP 4 O 12 are cyclotetraphosphates (with M I = monovalent cation) (Durif, 1995). In our laboratory we have synthesized the potassium and yttrium polyphosphates to establish the solid-liquid equilibrium diagram of the KPO 3 -Y(PO 3 ) 3 system (Jouini et al., 2003). Three allotropic phases with the space groups P2 1 , P2 1 /n and C2/c were isolated and characterized. The three monoclinic allotropes are: i) KY(PO 3 ) 4 polyphosphate with the P2 1 space group, isostructural with KNd(PO 3 ) 4 (Hong, 1975). ii) KY(PO 3 ) 4 polyphosphate belongs to P2 1 /n space group, and is isostructural with TlNd(PO 3 ) 4 (Palkina et al., 1977). In these two forms the phosphate anion has a chain structure. iii) The third allotropic form is KYP 4 O 12 which crystallizes in the C2/c space group, only this structure was investigated (Hamady, 1995). This paper is devoted to the crystal structure of the first polymorph KY(PO 3 ) 4 (P2 1 ). The atomic arrangement of this srtucture is characterized by a three-dimensional framework built of (PO 3 ) n chains that are formed by corner-sharing of PO 4 tetrahedra (Figs 1,2). The chains run along the a axis , with four PO 4 tetrahedra in a repeating unit. KY(PO 3 ) 4 is isostructural with KNd(PO 3 ) 4 , but not with CsLa(PO 3 ) 4 (Sun et al., 2004) although they belong to the same space group, P2 1 . In the latter, the infinite screw (PO 3 ) n chains are repeated after every eighth PO 4 group along the b axis. The chains (two per unit cell) are joined to each other by YO 8 polyhedra (Fig 3.), no O atom is shared with the adjacent YO 8 polyhedra. The K atoms are in an eightfold coordination.

Experimental
Single crystal of KY(PO 3 ) 4 was prepared by flux method. Homogeneous solution of potassium carbonate K 2 CO 3 (6 g) and yttrium oxide Y 2 O 3 (0.5 g) containing a large excess of orthophosphoric acid H 3 PO 4 (16 ml, 85% concentration) was heated in a vitreous carbon crucible at 473 K for 1 day. Then the temperature of the furnace was slowly raised to the predermined temperature in the range of 573-623 K for 7 days. Crystals were separated from the excess phosphoric acid by washing the product in boiling water.

Refinement
The highest peak and the deepest hole are located 0.09Å and 0.85 Å from Y.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.