Anomalous x-ray scattering study of GexSe1−xglassy alloys across the stiffness transition composition

Abstract

Anomalous x-ray scattering experiments on glassy Ge${}_{x}$Se${}_{1\ensuremath{-}x}$ were carried out at energies close to the Ge and Se $K$ absorption edges at concentrations between $x=0.15$ and $0.333$ in order to explore the correlation between the atomic structures in short and intermediate ranges and the stiffness transition which appears at approximately $x=0.20$ in this glassy system. The partial structure factors ${S}_{ij}(Q)$ and the corresponding partial pair-distribution functions ${g}_{ij}(r)$ were obtained using reverse Monte Carlo modeling. Although the ${S}_{ij}(Q)$ and ${g}_{ij}(r)$ spectra seem to gradually change with $x$, some indications are found related to the stiffness transition, in particular in the intermediate-range structure. First, the preshoulder position in ${S}_{\mathrm{SeSe}}(Q)$ largely shifts toward lower $Q$ values in the intermediate phase concentration region of the stiffness transition, while the prepeak positions in ${S}_{\mathrm{GeGe}}(Q)$ and ${S}_{\mathrm{GeSe}}(Q)$ remain almost unchanged. Second, the Ge-Se-Se bond angles are distributed at $\ensuremath{\sim}$90${}^{\ensuremath{\circ}}$ when the transition region is approached with decreasing $x$. No appreciable portions of the Ge-Se-Se-Ge sequences, i.e., Ge(Se${}_{1/2}$)${}_{4}$ tetrahedra connected by the Se${}_{2}$ dimer, are found in the Ge${}_{0.20}$Se${}_{0.80}$ glass. Instead, there is experimental evidence for a phase-separation tendency between directly connected Ge(Se${}_{1/2}$)${}_{4}$ tetrahedra and Se${}_{n}$ ($n\ensuremath{\geqslant}3$) chains in the intermediate phase concentration region of the stiffness transition. This may be due to avoid large stress in the Se${}_{2}$ dimer bonds of the Ge-Se-Se bond angle.