Extended X-Ray-Absorption Fine-Structure Studies of Electron-Lattice Correlations in Mixed-ValenceSm0.75Y0.25S

X-ray absorption spectrum of the Sm $L_{\mathrm{III}}$ edge has been measured in mixed-valence ${\mathrm{Sm}}_{0.75}$ ${\mathrm{Y}}_{0.25}$S. From analysis of the edge structure and extended x-ray absorption fine structure, the valence is determined and it is shown that the S neighbors of each Sm atom adopt an average distance rather than a dynamically distorted environment with two distances corresponding to the two valence states. From this it is concluded that the characteristic Sm $4f$-band width is not greatly modified by polaron effects.