Pressure-induced structural modifications of metagermanates at room temperature : In situ XRD and EXAFS studies

Abstract

In situ X ray diffraction (XRD) and extended X ray absorption fine structure (EXAFS) measurements under high pressure and at room temperature were performed in order to verify the possibility of a pressure-induced amorphization of MgGeO 3 -high-clinoenstatite and CaGeO 3 -wollastonite. We found that the two metagermanates were transformed into new metastable phases without the amorphization. For MgGeO 3 -high-clinoenstatite, a new reflection, which could not be assigned as its known high-pressure polymorphs, could be observed above 23 GPa. For CaGeO 3 -wollastonite, EXAFS measurements under high pressure were conducted using MAX90. The observed pressure dependence of the Ge-O distance can suggest the existence of a metastable intermediate phase between wollastonite and perovskite. The intermediate phase corresponds to a rhodonite-like structure, which was proposed by a previous XRD experiments. These transitions, which were observed in the two metagermanates, suggest to be kinetically favored processes under room temperature compression. Such metastable pathways during phase transitions can be described as typical examples of the Ostwald step rule.