Negative thermal expansion and local dynamics inCu2OandAg2O

High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals ${\mathrm{Cu}}_2\mathrm{O}$ and ${\mathrm{Ag}}_2\mathrm{O}$ as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to $240\mathrm{K}$ for ${\mathrm{Cu}}_2\mathrm{O}$, up to $470\mathrm{K}$ for ${\mathrm{Ag}}_2\mathrm{O}$) and anisotropic thermal displacements of M atoms ($\mathrm{M}=\mathrm{Cu}$,Ag). EXAFS measures a positive expansion of the nearest-neighbors $\mathrm{M}\text{−}\mathrm{O}$ pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The $\mathrm{M}\text{−}\mathrm{O}$ bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 $\mathrm{M}\text{−}\mathrm{M}$ next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid $\mathrm{M}\text{−}\mathrm{O}$ rods.