Hochdruckumwandlungen von Cer(III)Orthovanadat(V), CeVO4 / High-Pressure Transformations of Cerium(III) Orthovanadate(V), CeVO4

Abstract

The polymorphism of CeVO4 was investigated at high pressures and temperatures in a Belttype high-pressure apparatus. In addition to the normal-pressure modification CeVO4— I with zircon-type structure two high-pressure modifications have been found, viz. monazite-type CeVO4—II and scheelite-type CeVO4—III. CeVO4—II is stable between 1 bar and 30 kbar at 1300 °C. Its region of existence decreases rapidly at lower temperatures. From the observed p,T-relations for the I-II and I-III transformations a triple point CeVO4—I,II,III at about 27 kbar, 500 °C can be estimated. For kinetic reasons, however, the experimental determination of phase relations becomes difficult at temperatures below 600 °C. The crystal structures of CeVO4— I and —II have been refined from single-crystal data. Crystallographic data for the three modifications are given and discussed (CeVO4—I: I 41/amd, a = 7.383(1)Å, c = 6.485(1)Å, Z = 4; CeVO4—II: P21/n, a = 7.003(1)Å, b = 7.227(1)Å, c = 6.685(1)Å, β = 105.13(1)°, Z = 4; CeVO4—III: I 41/α, a = 5.1645(2)Å, c = 11.8482(7)Å, Z = 4).