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

Anomalous x-ray scattering experiments on glassy GexSe1-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-SeSe(Q) largely shifts toward lower Q values in the intermediate phase concentration region of the stiffness transition, while the prepeak positions in S-GeGe(Q) and S-GeSe(Q) remain almost unchanged. Second, the Ge-Se-Se bond angles are distributed at similar to 90 degrees 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 Ge0.20Se0.80 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 Se2 dimer bonds of the Ge-Se-Se bond angle.