Influence of composition and thermal treatment on the properties of Cu2+xTa4O12+δ

Polycrystalline samples of Cu_2+xTa₄O_12+δ were prepared by solid-state reactions. Copper tantalate shows a remarkable compositional flexibility with respect to both the copper and oxygen stoichiometry. Single phase compounds could be synthesised for 0.125 ≤ x ≤ 0.5. Slowly cooled samples are green and possess a pseudo-tetragonal unit cell, which changes to a pseudo-cubic symmetry for x ≥ 0.45. Rapidly cooled aliquots are brown and have a (pseudo-) cubic structure. For both the slow-cooled and quenched samples a linear increase of the oxygen content with x was observed, the values of δ for the latter being significantly smaller. Magnetic measurements reveal a ferrimagnetic transition at 12.5 K, the strength of which is strongly reduced both by increasing the copper content and by quenching.