Transition-Metal Oxides with Triangular Lattices. Generation of New Magnetic and Electronic Properties

The search for multifunctional materials as multiferroics to be applied in microelectronic or for new, chemically stable and nontoxic, thermoelectric materials to recover waste heat is showing a common interest in the oxides whose structures contain a triangular network of transition-metal cations. To illustrate this point, two ternary systems, Ba−Co−O and Ca−Co−O, have been chosen. It is shown that new phases with a complex triangular structure can be discovered, for instance, by introduction of Ga3+ into the Ba−Co−O system to stabilize Ba6Ga2Co11O26 and Ba2GaCo8O14, which both belong to a large family of compounds with formula [Ba(Co,Ga)O3−δ]n[BaCo8O11]. In the latter, both sublattices contain triangular networks derived from the hexagonal perovskite and the spinel structure. Among the hexagonal perovskite, the Ca3Co2O6 crystals give clear evidence where the coupling of charges and spins is at the origin of a magnetocapacitance effect. In particular, the ferrimagnetic to ferromagnetic transition, with a...