Charge, Spin and Orbital States of Transition Metal Oxides with Square Pyramidal Coordination

(1)Charge order transition in NaV_2O_5. We found novel charge order transition at 35 K in NaV_2O_5 with the V_2O_5-framework, so-called trellis lattice, formed by sharing the edges and corners of VO_5 square pyramids. The determined charge ordered pattern has a zigzag type of V^<4+> and V^<5+> in the a-b plane and the stacking sequence of AAA'A' along the c-axis. We found that various stacking sequences appear under high pressure. The obtained phase diagram is very similar to the devil's flower type phase diagram theoretically predicted from ANNNI model in frustrated spin system. (2)Study on β-vanadium bronzes, β-A_<1/3>V_2O_5 (A=Li,Na,Ag,Ca,Sr,Pb). The V_2O_5-framework of β-A_<1/3>V_2O_5 is formed by octahedral (V1)O_6 and (V2)O_6 and square pyramidal (V3)O_5, where V1, V2 and V3 are independent ciystallographical vanadium sites. All compounds except A=Pb show metal-insulator transitions accompanied by the charge ordering. The charge ordered phase is suppressed under high pressure and the superconducting phase appears adjacent to the charge ordered phase in A^+-compounds. The presence/absence of superconductivity in A^+-/A^<2+>-compounds indicates an optimum doping level for the superconductivity. (3)The A-site ordered perovskite manganites, RBaMn_2O_6 (R=rare earth metal). We successfully synthesized the A-site ordered perovskite manganites, RBaMn_2O_6, which have been desired to study the A-site randomness effect on the charge, orbital and spin coupled phenomena in perovskite manganese oxides. At first we synthesized the intermediate compounds, oxygen-deficient perovskites RBaMn_2O_5 with square pyramidal MnO_5. By oxidizing RBaMn_2O_5 at relatively low temperature about 600 K, we succeeded in preparing RBaMn_2O_6. RBaMn_2O_6 shows novel charge, orbital and spin coupled phenomena different from those of ordinary disordered manganites.