Topological changes in glassy Ge Se2 at pressures up to 9.3 GPa determined by high-energy x-ray and neutron diffraction measurements

Monochromatic high-energy x-ray diffraction measurements employing microfocusing optics were performed on glassy Ge Se2 in a diamond anvil cell at pressures up to 9.3 GPa. In addition, the method of isotopic substitution in neutron diffraction was applied to samples that had been densified by 4% via pressurization to 10 GPa in a multianvil device and subsequently recovered to ambient conditions. The results reveal a steady increase with pressure of the average coordination number of Ge from 4.0(2) under ambient conditions to 4.5(2) at 9.3 GPa. With increasing pressure, the first sharp diffraction peak in the measured diffraction patterns at ∼1.0 Å-1 decreases in intensity and almost disappears while the amplitude of the peaks beyond the nearest neighbor in the measured total pair distribution functions gradually increases. Equation of state measurements show a gradual density increase of 33% from ambient pressure to 8.5 GPa which is in good agreement with molecular dynamics simulations. The results are consistent with the occurrence of two densification processes for glassy Ge Se2, namely, a conversion from edge-sharing to corner-sharing tetrahedra and a gradual increase in the average local coordination number with increasing density.