Martensitic transition in single-crystalline α-GeO2 at compression

Abstract

We present a structural study of single crystalline quartz-like α-GeO₂ compressed to pressures up to 12 GPa and subsequently quenched to ambient conditions. The transition to a new crystalline phase with a distorted rutile structure, occurring in the pressure interval 8 to 12 GPa, was established. The structure of the new phase was identified from X-ray and electron diffraction data as P2₁/c monoclinic. Electron transmission and scanning microscopy provide direct evidence of the martensitic (or displacive) nature of the transition, indicating, in particular, the lamellar morphology and crystallographic orientation relation between the initial α-quartz and final new monoclinic phases. Upon heating, the new monoclinic phase transforms to the rutile-type structure with a similar (and similarly oriented) oxygen structure motif. Finally, we discuss the difference in high-pressure behavior of single-crystalline and polycrystalline samples transforming to the new crystalline and amorphous phases, respectively.