Pressure-induced structural evolution of pyrite-type SiO_2

摘要

The equation of state and pressure-induced structural evolution of pyrite-type SiO_2 have been investigated based on synchrotron X-ray diffraction measurements in a diamond-anvil cell. The Rietveld refinement revealed that the oxygen coordinate x of pyrite-type SiO_2 increases with increasing pressure. The SiO_6 coordination polyhedra of pyrite-type SiO_2 is less compressible than the unit cell, and the increase in x induces a rotation of the SiO_6 coordination polyhedra to fill the blank space in the unit cell. Thus, the volume reduction in pyrite-type SiO_2 is achieved mainly by the rotation of the SiO_6 polyhedra, rather than by the compression of the SiO_6 polyhedra. In addition, the increase in x with increasing pressure enhances a distortion of the coordination polyhedra of pyrite-type SiO_2, implying that pyrite-type SiO_2 is not likely to transform into a fluorite-type structure at higher pressures.