Martensitic transition in single-crystalline alpha-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 c onditions. The transition to a new crystalline phase with a distorted rutil e structure, occurring in the pressure interval 8 to 12 GPa, was establishe d. The structure of the new phase was identified from X-ray and electron di ffraction data as P2(1)/c monoclinic. Electron transmission and scanning mi croscopy provide direct evidence of the martensitic (or displacive) nature of the transition, indicating, in particular, the lamellar morphology and c rystallographic orientation relation between the initial alpha-quartz and f inal new monoclinic phases. Upon heating, the new monoclinic phase transfor ms to the rutile-type structure with a similar (and similarly oriented) oxy gen structure motif. Finally, we discuss the difference in high-pressure be havior of single-crystalline and polycrystalline samples transforming to th e new crystalline and amorphous phases, respectively. (C) 2000 MAIK "Nauka / Interperiodica".