Synthesis and magnetic property of Cu₂GeO₄ containing edge-sharing CuO₂ chains

Polycrystals of Cu₂GeO₄ have been synthesized under high pressure (4 GPa) and high temperature (900°C) conditions. It has the tetragonally distorted spinel structure with CuO₆ octahedra elongated along the c axis due to the Jahn-Teller effect. The crystal structure is apparently three-dimensional, but the major superexchange path is assumed to be one-dimensional along the edge sharing chains. In fact, the magnetic susceptibility of Cu₂GeO₄ shows a broad maximum around 100 K and is well described as that of the S=1/2 Heisenberg antiferromagnetic chains with interaction J of 135 K. A small jump in the susceptibility curve is observed at 33 K, which is considered to be a sign of antiferromagnetic ordering. Moreover, specific heat measurements show a large sharp peak at the same temperature, suggesting a first-order phase transition. The effects of Mg substitution for Cu and Si substitution for Ge are also reported.