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Abstract
It has been widely assumed that the earth's lower mantle may be composed of magnesium and calcium silicate perovskites in abundance. The phases of these silicates may dominate the mineralogy of this region of the earth's interior, their material properties at high pressures and temperatures are considerable geophysical importance. Knowledge of the thermo dynamical properties and phase transitions can be directly used for a better understanding of geophysical data obtained from the seismic measurements. It is therefore, in the present investigation, the structural distortions and lattice dynamics of the high pressure perovskite phase of MgSiO3 and CaSiO3 has been studied by using lattice dynamical simulation method based on de Launey angular force (DAF) constant model. In this work, we are reporting the results of our theoretical investigation on the phonon properties of silicate perovskites MgSiO3 and CaSiO3 in their cubic phase. The calculated zone center phonon frequencies agree well with available results. The phonon spectrum of MgSiO3 and CaSiO3 is calculated along the three symmetry directions. Some zone boundary phonons in cubic phases of these silicates are coming out to be imaginary. The imaginary frequencies are associated with unstable mode. The unstable mode determines the nature of the phase transitions and the dielectric responses of the compounds and is in agreement with other study.