Comparative Raman spectroscopic study on ilmenite-type MgSiO₃ (akimotoite), MgGeO₃, and MgTiO₃ (geikielite) at high temperatures and high pressures

Abstract

The Raman spectra of MgXO₃-ilmenites (X = Si, Ge, Ti) were recorded up to 773 K at ambient pressure and up to 20-30 GPa at room temperature. Temperature and pressure dependence of the force constant of X-O stretching bands revealed that the expansion and compression behavior of XO₆ octahedra differed in the three ilmenites. For SiO₆ and GeO₆ octahedra, the shorter Si-O or Ge-O bonds became more lengthened with temperature and more shortened with pressure than did the longer Si-O or Ge-O bonds. In contrast, for TiO₆ octahedra, the longer Ti-O bonds became more lengthened with temperature and more shortened with pressure than did the shorter Ti-O bonds. For SiO₆ and GeO₆ at high temperatures and TiO₆ at high pressures, the cation positions moved in the direction of the c axis and tended to approach the center of the octahedra, decreasing the distortion of XO₆. For SiO₆ and GeO₆ at high pressures and TiO₆ at high temperatures, the cations moved away from the center, increasing the distortion of XO₆. One of the anharmonic correction terms on isochoric specific heat was also elucidated. The anharmonic effects were related to the elastic Debye temperature of the three ilmenites.