The local thermal behavior of the ${\mathrm{Ag}}_2\mathrm{O}$ framework structure has been studied by extended x-ray absorption fine structure. The average Ag-O nearest-neighbor distance expands upon heating, while the Ag-Ag next-nearest-neighbor distance contracts. An original implementation of the cumulant analysis shows that the Ag-O expansion is a joint effect of potential anharmonicity and geometrical deformation of the ${\mathrm{Ag}}_4\mathrm{O}$ basic tetrahedral units. Accordingly, the negative thermal expansion of the lattice parameter in ${\mathrm{Ag}}_2\mathrm{O}$ cannot be explained uniquely in terms of rigid unit modes.

• Received 30 April 2002

DOI:https://doi.org/10.1103/PhysRevLett.89.025503