Structure redetermination and high pressure behaviour of AgCuO2☆
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 Ag0.5Cu0.5O had been synthesized [5], which is diamagnetic and shows mixed valence again, according to the formula AgICuIIIO2. A rough estimate of the plausibility of the charge distributions thus far found experimentally (CuIIO, AgIAgIIIO2 and AgICuIIIO2) can be obtained by inspection of the ionisation potentials. The difference of the sum of ionisation potentials of atom pairs ∑Ip(M1I/M2III)−∑Ip(M1II/M2II) 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 CuIIO and silver disproportionates to AgIAgIIIO2. 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.
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Experimental
Microcrystalline samples of AgCuO2 precipitated from combined saturated aqueous solutions of 1.205 g (5 mmol) AgNO3 (Merck, 99%) and 0.85 g (5 mmol) Cu(NO3)2·3H2O (Merck, 99%) upon adding a solution of 3.0 g KOH and 1.5 g K2S2O8 in 150 ml water at 90 °C. The precipitate was filtered off, washed with deionized water and dried in air at 70 °C. Another access to AgCuO2 is the oxidation of an aqueous suspension of Ag2Cu2O3 [6], under the same conditions.
Neutron powder diffraction patterns were
Results and discussion
Recently, the new ternary silver cuprate, AgCuO2, has been synthesized as a microcrystalline, black powder via a low temperature route [5]. According to EDX microprobe analyses the Ag/Cu ratio amounts to 1:0.99 (average over about five spots per sample, several samples checked). The thermal degradation, as recorded by thermal analysis, proceeds in two steps, at 280 °C and 310 °C, respectively, each being accompanied by a loss of oxygen with Ag2Cu2O3 forming intermediately at 280 °C [11]. The final
Summary
Different from CuO, but similar to Ag2O2, ternary AgCuO2 exhibits charge ordering and has to be regarded as a silver(I) oxocuprate(III). This assignment of valence states is conclusively supported by characteristic structural features and physical properties. The crystal structure is stable up to 36 GPa indicating no changes in the oxidation states (Ag+/Cu3+) occur in the investigated pressure range. The zero pressure bulk modulus (K0) has been estimated as 118.2 GPa.
Acknowledgements
The authors thank E. Bruecher and G. Siegle from MPI Stuttgart for magnetic and conductivity measurements, M. Mezouar and P. Bouvier for their help in high-pressure experiments in Beamline ID30 in ESRF, R.M. Ibberson from ISIS for recording neutron time of flight spectra and R. Dinnebier for helpful discussions concerning the fitting of the neutron data.
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Dedicated to Professor Fritz Franzen.