Breve descrizione dei contenuti: High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure EXAFS measurements
have been performed on the iso-structural framework crystals Cu2O and Ag2O 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 K for Cu2O, up to 470 K for Ag2O and
anisotropic thermal displacements of M atoms M=Cu,Ag. EXAFS measures a positive expansion of the
nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much
stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than
against bending. According to EXAFS, out of the 12 M-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 M-O rods.