Topological changes in glassyGeSe2at pressures up to9.3GPadetermined by high-energy x-ray and neutron diffraction measurements

Abstract

Monochromatic high-energy x-ray diffraction measurements employing microfocusing optics were performed on glassy $\mathrm{Ge}{\mathrm{Se}}_{2}$ in a diamond anvil cell at pressures up to $9.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. In addition, the method of isotopic substitution in neutron diffraction was applied to samples that had been densified by 4% via pressurization to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ 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 $9.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. With increasing pressure, the first sharp diffraction peak in the measured diffraction patterns at $\ensuremath{\sim}1.0\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$ 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 $8.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ which is in good agreement with molecular dynamics simulations. The results are consistent with the occurrence of two densification processes for glassy $\mathrm{Ge}{\mathrm{Se}}_{2}$, namely, a conversion from edge-sharing to corner-sharing tetrahedra and a gradual increase in the average local coordination number with increasing density.