Ferroelastic phase transition in rutile-type germanium dioxide at high pressure

Rutile-type GeO₂ was found to undergo a proper ferroelastic transition at 26.7(2) GPa from Raman spectroscopic measurements. The B1g optic mode softens up to this pressure and then becomes a hard Ag mode. The square of these mode frequencies varies linearly with pressure in accordance with the soft-mode theory of second-order phase transitions. The present results have enabled a systematic relationship between the square of the soft-mode frequency at ambient pressure and the critical pressure in the homologous series SiO₂, GeO₂, and SnO₂ to be identified.