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.

Negative thermal expansion and local dynamics in Cu2O and Ag2O

Sanson, Andrea;Rocca, Francesco;Dalba, Giuseppe;Fornasini, Paolo;Grisenti, Rolly;
2006-01-01

Abstract

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.
2006
Sanson, Andrea; Rocca, Francesco; Dalba, Giuseppe; Fornasini, Paolo; Grisenti, Rolly; M., Dapiaggi; G., Artioli
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11572/91213
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