Ab initio studies of high-pressure transformations in GeO2 - art. no. 134106

We performed ab initio calculations of eermanium dioxide to study its behav ior and possible similarity to silica under high pressure. At the rutile-Ca Cl2-type phase transition, the lattice constants, unit cell volume, and Gib bs free energy change continuously with increasing pressure and indicate th at this ferroelastic phase transition is close to the second order. Further phase transitions to denser packed phases of alpha -PbO2-type and pyrite P a (3) over bar are considered. The pressure dependence of the elastic const ants is determined. For zero-pressure and 30-GPa phonon dispersion relation s of rutile-type and CaCl2-type structures, respectively, are calculated. A softening of an optical mode, linearly coupled to a shear instability, is found. The present results confirm the analogy of the phase-transition sequ ence between silica and germanium dioxide at high pressures.