THE CRYSTAL-STRUCTURE, PHASE-TRANSITION, AND DIELECTRIC-PROPERTIES OF K2BI3(PO4)3O, A NEW OXYPHOSPHATE

This new oxyphosphate compound exhibits a transition at about 745 K detectable by DSC and dielectric measurements. The structure was determined at ambient temperature and above the transition. In both cases, the cell is orthorhombic with a equals 13. 139(4), b equals 10. 413(6), c equals 9. 239(6) Angstrom at 295 K and a equals 13. 302(4), b equals 10. 506(7), c equals 9. 240(5) Angstrom at 795 K, Z equals 4. The structures were refined in space group Pnma to R values of 0. 048 and 0. 058 using 1472 and 1154 unique single-crystal reflections, respectively. The two structures are very similar and can be described as a three-dimensional network of distorted BiO//6 octahedra and PO//4 tetrahedra. At high temperature, one of the PO//4 tetrahedra is statistically disordered over two positions related by the mirror.