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

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.