High pressure induced phase transformation of SiO2 and GeO2: difference and similarity

Authors report high pressure polymorphism of SiO2 and GeO2 at room and high temperatures. It was found that kinetics has a large effect on pressure induced phase transitions of SiO2 and GeO2. The high pressure behavior of SiO2 and GeO2 polymorphs depends on the starting material and pressure-temperature history. Studies show that SiO2 and GeO2 have a common sequence of high pressure, high temperature structural transformations when the same type of starting material (amorphous or a-quartz) was used: (amorphous or a-quartz)d-NiAsrutileCaCl2a-PbO2pyrite (Pa-3) types. In the case of cristobalite as starting material, the a-PbO2-type phase can be synthesized directly from the high pressure room temperature phase omitting rutile- or CaCl2-type structure. The crystallization of the d-NiAs phase in a narrow temperature interval 1000-1300 K can be used as an indicator of presence of a cation disordered network in both SiO2 and GeO2 materials at high pressure before heating. 37 refs.