Martensitic transition in single-crystalline α-GeO2 at compression

We present a structural study of single crystalline quartz-like alpha-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 P2(1)/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 alpha-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. (C) 2000 MAIK "Nauka / Interperiodica".