Polymorphs of CaSeO4 under pressure: a first-principles study of structural, electronic, and vibrational properties.

In this paper we report a theoretical study of the CaSeO4 compound at ambient pressure and under pressure. Here we made a structural analysis of its three known polymorphs—orthorhombic (Cmca), monoclinic monazite, and tetragonal scheelite—where direct comparison with experimental measurements is done. Besides, the electronic and vibrational structures are reported for the first time for those structures. In addition, the behavior of CaSeO4 as a function of pressure is studied, where phase transitions are investigated by considering a quasiharmonic approximation at 300 K. After a total energy study of 14 possible high-pressure phases of CaSeO4, the following sequence of pressure-driven structural transitions has been found: orthorhombic (Cmca) → tetragonal scheelite → monoclinic AgMnO4-type structure. It was observed that monazite is less stable as temperature increases, while the opposite occurs for the AgMnO4-type structure, this being a novel polymorph. This high-pressure structure is a distortion of th...