Negative thermal expansion in cuprite-type compounds : a combined synchrotron XRPD, EXAFS, and computational study of Cu2O and Ag2O

Cuprite-type oxides (Cu2O and Ag2O) are framework structures composed by two interpenetrated networks of metal-sharing M4O tetrahedra (M=Cu, Ag). Both compounds exhibit a peculiar negative thermal expansion (NTE) behaviour over an extended temperature range (9-240 K for Cu2O, 30-470 K for Ag2O). High-accuracy synchrotron powder diffraction and EXAFS measurements were performed from 10 K up to the decomposition temperature to understand the nature of the NTE effects. The critical comparison of the diffraction and absorption results concerning the temperature dependence of the interatomic distances and of the atomic vibrational parameters proves to be fundamental in defining the local dynamics of the atoms in the structure. Both techniques measure a strong transverse motion of the metal atoms perpendicularly to the O-M-O linear bonds. Furthermore, the analysis of the next-near-neighbors shell in the EXAFS data indicates a different temperature behaviour of the M-M interaction between metal atoms related to the same framework and with respect to metal atoms located on distinct interpenetrated frameworks. The presence of M-M bonds is supported by first-principles calculation of the charge density distribution in Cu2O and Ag2O.