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 B-1g optic mode softens up to this pressure and then becomes a hard A(g) 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.