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".