Negative thermal expansion in crystals with the delafossite structure: An extended x-ray absorption fine structure study of CuScO 2 and CuLaO 2

Extended x-ray absorption fine structure has been measured at the $K$ edge of copper in ${\text{CuScO}}_{2}$ and ${\text{CuLaO}}_{2}$ to investigate the local origin of negative thermal expansion (NTE) along the $c$ axis. A positive expansion of the Cu-O bond has been measured within the entire temperature interval to be contrasted with the negative expansion of the distance between average atomic positions measured by diffraction; the inadequacy of the riding correction to diffraction data has been evidenced. The weak temperature dependence of the parallel mean-square relative displacement (MSRD) indicates a rather stiff Cu-O bond, while the large perpendicular MSRD measures an intense relative motion of copper with respect to oxygen atoms perpendicular to the $c$ axis. The comparison between the two compounds enlightens the relevance of the correlation of perpendicular motion to quantitatively explain the apparent NTE of the Cu-O bond.