Structure redetermination and high pressure behaviour of AgCuO₂

Abstract

AgCuO₂ has been obtained via a low temperature route. The structure has been redetermined with Rietveld refinements using neutron powder diffraction data [C2/m, a=6.0756(3), b=2.8088(1), c=5.8728(3) Å, β=107.987(4)°, Z=2, R_p=6.36%, R_wp=7.22%]. The crystal structure consists of chains of ladder-like CuO₄-rectangles running along [010], interconnected by AgO₂-dumbbells. For physical and crystal chemical reasons the oxidation states +I for silver, and +III for copper, respectively, have been assigned. High-pressure X-ray powder diffraction experiments have been made. AgCuO₂ is stable with respect to decomposition and structural distortion up to 36.3 GPa. The zero pressure bulk modulus of AgCuO₂ is estimated as 118.2 GPa.

Introduction

The monoxides of the two lighter elements of the 11th group of the periodic table show amazing differences, both in structure and properties. The stable and paramagnetic CuO [1] contains copper on one independent crystallographic site, coordinated by 4+2 oxygen atoms forming a heavily elongated octahedron, in accordance with the Jahn–Teller theorem [2]. In contrast, AgO [3] is diamagnetic and has to be regarded as a charge disproportionated, pseudo ternary oxide [4]. The characteristic shapes of the first coordination spheres, linear for silver(I) and square planar for silver(III), have led to unambiguous assignments of the silver valence states at the respective crystallographic sites. Recently, the mixed silver copper monoxide Ag_0.5Cu_0.5O had been synthesized [5], which is diamagnetic and shows mixed valence again, according to the formula Ag^ICu^IIIO₂. A rough estimate of the plausibility of the charge distributions thus far found experimentally (Cu^IIO, Ag^IAg^IIIO₂ and Ag^ICu^IIIO₂) can be obtained by inspection of the ionisation potentials. The difference of the sum of ionisation potentials of atom pairs ∑I_p(M₁^I/M₂^III)−∑I_p(M₁^II/M₂^II) is largest for M₁=M₂=Cu, while it is smallest for the combination M₁=M₂=Ag. This is in agreement with the fact that, in the monoxides, copper forms Cu^IIO and silver disproportionates to Ag^IAg^IIIO₂. The corresponding sums for the mixed oxide AgCuO₂ yield intermediate values and differ only slightly for all three possible charge distributions. Hypothesizing that the charge distribution in these compounds might change under physical stress, we have performed X-ray diffraction experiments under high pressure.