Configuration and Rearrangement Rate of Excess Oxygen Atoms in Transition-Metal Oxides Related to Superconductors

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Both La_2NiO_<4+delta> and La_2CuO_<4+delta> crystals have atomic arrangements of a K_2NiF_4 type, and the latter crystal only with excess oxygen atoms shows superconductivity at low temperatures. The configurational structure and rearrangement rate of the excess oxygen atoms as functions of delta, affecting the superconducting properties in the latter copper crystals, are attractive. Heat capacity, resistivity and thermoelectric power were measured for the both crystals in the present work.1) High-precision adiabatic calorimeter was constructed which works in the temperature range between 13 K and 500 K.2) Nickel compound with delta=0.00 showed remarkable two-dimensional nature in the magnetic interaction between electronic spins of nickel atoms. The possibility was indicated that the phase transitions at 80 K and 120 K are of the (super-lattice commensurate) -incommensurate- (normal commensurate) type. It was also indicated that the crystals in the delta range between 0.047 and 0.150 show different delta-T phase relations depending on the size of crystal particles. Low-temperature stable compounds were found to be formed at around delta=0.07 and 0.10, and peritectic or eutectic disordering phenomena of excess oxygen atoms were found to take place in the delta ranges between the compositions of stable compounds. Glass transition phenomena due to the freezing-in of diffusional motion of excess oxygen atoms were found.3) Copper compounds with 0<delta<0.035 showed three structural phase transitions below 320 K.It was indicated that the phase separation with respect to the composition of excess oxygen atoms take place above room temperature, and that the phase transitions are ascribed to the tilt of CuO_6 octahedra. A glass transition was observed for delta-0.05 and 0.07 crystals, and interpreted as due to freezing of the short-range rearrangement (not diffusional motion) of excess oxygen atoms.