Phase stability and pressure-induced structural transitions at zero temperature in ZnSiO(3) and Zn(2)SiO(4).

ABSTRACT

Using density functional total energy calculations the structural phase stability and pressure-induced structural transition in different polymorphs of ZnSiO(3) and Zn(2)SiO(4) have been studied. Among the considered monoclinic phase with space groups (P 2(1)/c) and (C 2/c), rhombohedral [Formula see text] and orthorhombic (Pbca) modifications the monoclinic phase (P 2(1)/c) of ZnSiO(3) is found to be the most stable one. At high pressure monoclinic ZnSiO(3) (C 2/c) can co-exist with orthorhombic (Pbca) modification. Differences in equilibrium volume and total energy of these two polymorphs are very small, which indicates that it is relatively easier to transform between these two phases by temperature, pressure or chemical composition. It can also explain the experimentally established result of metastability of the orthorhombic phase under all conditions. The following sequence of pressure-induced structural phase transitions is found for ZnSiO(3) polymorphs monoclinic [Formula see text] monoclinic [Formula see text] rhombohedral [Formula see text]. Among the rhombohedral ([Formula see text]), tetragonal [Formula see text], orthorhombic (Pbca), orthorhombic (Imma), cubic [Formula see text] and orthorhombic (Pbnm) modifications of Zn(2)SiO(4), the rhombohedral phase is found to be the ground state. For this chemical composition of zinc silicate the following sequence of structural phase transitions is found rhombohedral [Formula see text] tetragonal [Formula see text] orthorhombic [Formula see text] orthorhombic (Imma) [Formula see text] cubic [Formula see text] orthorhombic (Pbnm). Based on the analogy of crystal structures of magnesium and zinc silicates and using the lattice and positional parameters of Mg(2)SiO(4) as input, structural properties of spinel Zn(2)SiO(4) have also been studied.