Localized-itinerant electronic transitions in oxides and sulfides

Comparison of the transition-metal oxides and sulfides highlights as a common feature the possibility of finding a displacive crystallographic transition at a cross-over from localized to itinerant electronic behavior. In metastable NiS with the B8(1) structure, for example, this cross-over manifests itself as a first-order expansion of the c-axis on cooling from a Pauli-paramagnetic metallic phase to an antiferromagnetic phase at a Neel temperature T-N = 264 K. A first-order lattice expansion also occurs in the perovskite NdNiO3 on cooling from a metallic to an insulating phase, but in this case the magnetic order below T-N indicates a segregation into ferromagnetic Ni-O-Ni (111) sheets containing itinerant electrons that are coupled by ionic bonding across an oxide-ion (111) plane to give a charge-density wave (CDW) with a propagation vector q(CDW) [111] and a spin-density wave (SDW) with q(SDW) approximate to 2q(CDW). In rhombohedral FeS, itinerant minority-spin electrons co-exist with localized majority-spin electrons into molecular orbitals. A low-spin to intermediate-spin transition in LaCoO3, occurs via short-range ordering of low-spin and high-spin to intermediate-spin trivalent cobalt, localized e electrons at the high-spin cobalt transforming to itinerant sigma* electrons in the intermediate spin state. As a final example, mixed-valent manganese oxides with perovskite structure are chosen to illustrate strong electron coupling to local Jahn-Teller deformations at a cross-over from localized to itinerant electronic behavior of cr-bonding electrons in the presence of pi-bonding electron configurations with localized spins S = 3/2. (C) 1997 Elsevier Science S.A.