Anomalous x-ray scattering experiments on glassy Ge${}_x$Se${}_{1-x}$ 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 $S_{ij}\left(Q\right)$ and the corresponding partial pair-distribution functions $g_{ij}\left(r\right)$ were obtained using reverse Monte Carlo modeling. Although the $S_{ij}\left(Q\right)$ and $g_{ij}\left(r\right)$ 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 $S_{\mathrm{SeSe}}\left(Q\right)$ largely shifts toward lower $Q$ values in the intermediate phase concentration region of the stiffness transition, while the prepeak positions in $S_{\mathrm{GeGe}}\left(Q\right)$ and $S_{\mathrm{GeSe}}\left(Q\right)$ remain almost unchanged. Second, the Ge-Se-Se bond angles are distributed at $\sim$90${}^{\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${}_n$ ($n⩾3$) 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.

• Received 25 October 2010

DOI:https://doi.org/10.1103/PhysRevB.84.014201