Temperature dependent structural studies and phase transition behavior of K2Th(PO4)2

Herein we report the details of crystal structure and phase transition behavior of a complex phosphate, K2Th(PO4)2 as observed from the variable temperature powder x-ray diffraction (XRD) and neutron diffraction (PND) studies. The ambient temperature monoclinic (P21/n) structure was observed to remain unchanged down to 6 K, while above 973 K, it transforms to a tetragonal (I41/amd) structure. This structural transition occurs with a volume discontinuity of about 13.4% and reverts back to monoclinic structure on cooling but with a large hysteresis in temperature (∼100 K). From the variation of unit cell parameters and structural differences as well as differential thermal analyses studies, the transition is concluded as a first order reconstructive type. The unit cell parameters of both structures gradually increases with increasing temperature but the coefficient of thermal expansion decreases appreciably after the transition (monoclinic, αV = 49.9 × 10−6/K in between 6 and 973 K; tetragonal, αV = 33.6 × 10−6/K, in between 673 and 1173 K). The details of crystal chemistry, phase transition and thermal expansion behaviors of K2Th(PO4)2 are explained, and compared with other isostructural compositions.