On the role of transition metal elements as structure-stabilising agents in cuprate superconductors

Abstract We outline here several key aspects of the crystal chemistry of (high-valent) transition metal-stabilised cuprate superconductors. An examination of the chemistry of the mercurocuprate homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+ δ leads us to propose that the inherent instability of the analogous strontium system (HgSr 2 Ca n −1 Cu n O 2 n +2+ δ ) arises from bond-length mismatch at the interface between the rock-salt and perovskite layers. A simple set of principles can now be formalised for the stabilisation of strontium mercurocuprates via partial substitution of mercury by a high-valent transition metal within the crystal structure of interest. The chemistry of three different cuprate systems, in which high-valent transition metal ions adopt a pivotal role as structure-stabilising agents , are illustrated here. First, we demonstrate how a combination of structural stabilisation (by the substitution of rhenium onto the mercury site) combined with control of the copper oxidation state (via judicious control of the Nd 3+ :Ca 2+ ratio), allows the successful synthesis of superconducting strontium analogues of HgBa 2 CaCu 2 O 6+ δ in the (Hg 1− x Re x )Sr 2 (Nd 1− y Ca y )Cu 2 O 6+ δ system. Second, we report the successful preparation of superconducting Hg 1− x Cr x Sr 2 CuO 4+ δ thin films, by laser ablation and ex situ mercuration. Finally, we describe a new family of layered cuprates with the general chemical formula (Cu 1− x TM x )Sr 2 (Y 1.2 Ce 0.8 )Cu 2 O 10− δ , with TM=Ti, Cr, Mo, W, V and Re, in which these transition metal substituents, once again, fulfil a key role in structure stabilisation.