Molecular orbital study on the onset of the pressure-induced transformations of the metastably compressed germanates

Abstract

In order to elucidate the onset of the pressure-induced transformations of germanates consisting of GeO4 tetrahedra at room temperature, we have investigated the stability of the crystal structures near the transition pressure in terms of the stability of the linkage of the tetrahedra. The stability of the linkage near the transition pressure is estimated from the results of the molecular orbital calculations for the model cluster H6Ge2O7, which mimics the linkage of tetrahedra, as functions of the bond length d(Ge–Obr) with bridging oxygen and bond angles Ge–Obr–Ge. The calculation shows that the total energy of the linkage becomes minimum at d(Ge–Obr)=1.758 A and <Ge–Obr–Ge=130.4°, and that it increases with the deviation from the energy minimum geometry. From the compression behavior of framework and chain germanates, we find that the linkage of the tetrahedra becomes unstable with increasing pressure, and that these germanates commonly transform into their high-pressure phases when the linkage of the tetrahedra largely departs from the energy minimum geometry. This suggests that the high-pressure transformations of the metastably compressed germanates are induced by the instability of the linkage of tetrahedra.