Takeo Tojo

Metal-insulator transition at 230 K in a new thiospinel CuIr2S4

Abstract A new thiospinel CuIr2S4 has been synthesized. We have discovered a metal-insulator transition at 230 K. The physical properties of this compound have been investigated with special emphasis on heat capacity measurements in the range 13·8 K < T < 300K using an adiabatic calorimeter. The metal-insulator transition is accompanied by a structural phase transition from tetragonal symmetry (at the lower temperature) to cubic symmetry (at the higher temperature). The phase transition has been observed to be first order. A sharp heat capacity anomaly is exhibited at 230·8 K. The enthalpy and entropy of the transition are 3·50 kJ mol−1 and 15·6 J K−1 mol−1 respectively. A hysteresis effect at the phase transition was detected clearly in the heat capacity measurements. The metallic phase of CuIr2S4 shows Pauli paramagnetism which indicates a rather high density of states at the Fermi level, D(∊F) = 0·67 states/eV atom. The insulating phase of CuIr2S4 reveals semiconductive behaviour with an activation ene...

Excess Heat Capacity in Yttria Stabilized Zirconia

The heat capacity of 9.70 and 11.35 mol% yttria stabilized zirconia ((ZrO2)1−x(Y2O3)x; x=0.0970, 0.1135) was measured by adiabatic calorimetry between 13 and 300 K, and some thermodynamic functions were calculated and given in a table. A large excess heat capacity extending from the lowest temperature to room temperature with a broad maximum at about 75 K was found in comparison with the heat capacity calculated from those of pure zirconia and yttria on the basis of simple additivity rule. The shape of the excess heat capacity is very similar to the Schottky anomaly, which may be attributed to a softening of lattice vibration. The amount of the excess heat capacity decreased with increasing yttria doping, while the maximum temperature did not vary. The relationships among the excess heat capacity, defect structure and interatomic force constants, and also the role of oxygen vacancy were discussed.

Molecular dynamics study on lattice vibration and heat capacity of yttria-stabilized zirconia

Abstract Thermodynamic properties were studied by molecular dynamics simulation for the samples of 8, 10, 12 and 14 mol% Y2O3 doped ZrO2. The phonon density of states and the heat capacity were calculated. The calculation showed extra vibrational modes at low frequencies caused by structural defects in the crystal, and confirmed the excess heat capacity around 25 K which had been previously detected by heat capacity measurements. The composition dependence of the excess heat capacity showed a maximum at 10 mol%, which coincided with the experimental results.

Specific Heat and Phase Transition Phenomena in (CH3NH3)5Bi2Cl11.

Specific heat of (CH 3 NH 3 ) 5 Bi 2 Cl 11 was measured by adiabatic calorimetry between 13 K and 325 K. A typical second-order type of anomaly was observed at 307.4 K. The enthalpy and entropy of transition were determined as 3.06 kJ mol -1 and 10.6 J K -1 mol -1 , respectively. A broad hump in the curve of specific heat was found around 160 K. Its contribution was estimated as the excess specific heat extending from about 100 K to 200 K over an assumed normal base line, which results in the values of enthalpy and entropy as 1.57 kJ mol -1 and 9.85 J K -1 mol -1 , respectively. The phase transition in the crystal was discussed in connection with such specific heat anomalies.

Low temperature heat capacity and thermodynamic functions of LiCoO2

The sample of LiCoO2 was synthesized, and the heat capacity was measured by adiabatic calorimetry between 13 and 300 K. The smoothed values of the heat capacity were calculated from the data. The thermodynamic functions, standard enthalpy, entropy and Gibbs energy, of LiCoO2 were calculated from the heat capacity and the numerical values are tabulated at selected temperatures from 15 to 300 K. The heat capacity, enthalpy, entropy, and Gibbs energy at T=298.15 K are 71.57 J K–1mol–1, 9.853 kJ mol–1, 52.45 J K–1 mol–1, –5.786 kJ mol–1, respectively.

Dielectric anomaly involving magnetic phase transition in a hexagonal antiferromagnet RbCoBr3

Abstract The dielectric and magnetic phase transitions in a triangular lattice antiferromagnet RbCoBr3 were studied by means of some dielectric, magnetic and calorimetric measurements. In this compound, which is a typical Ising-like antiferromagnet with antiferromagnetic moment along the hexagonal c-axis, a ferroelectric phase transition involving magnetic ordering with weak ferromagnetic moments confined in the c-plane was found to occur at T N(=37.OK). Those results imply that the magnetic ordering of this system is assisted by a polar lattice distortion, which can release partially the spin frustration due to the antiferromagnetic spin arrangement on a triangular lattice. This co-ordering is certainly ascribed to a probable competition between lattice-system and spin-system. Experimental data on this interesting phenomenon are presented and a possible mechanism is proposed.

Low temperature heat capacities of zirconia and yttria-doped zirconia (ZrO2)1−x(Y2O3)x (x = 0, 0.0200, 0.0396)

Abstract Heat capacity measurements were performed on zirconia and yttria-doped zirconia, (ZrO 2 ) 1− x (Y 2 O 3 ) x ( x = 0, 0.0200, 0.0396), between 13 and 300 K by adiabatic calorimetry. Powder X-ray diffraction and Raman spectra showed that the structure of the samples with x = 0 and 0.0396 was monoclinic and tetragonal, respectively, while the sample with x = 0.0200 was a mixture of the two phases. The heat capacity of the sample of x = 0.0200 was not estimated on the basis of the additivity rule from the values of the other two samples ( x = 0, 0.0396). A broad hump was found around 20 K in the heat capacity curve of C p T −3 versus T of the sample of x = 0.0396.

Low-Temperature Phase Transitions of an Organic Ferroelectrics, Phenazine–Chloranilic Acid

The heat capacity of a ferroelectrics consisting of two organic components having a centrosymmetric molecular shape, phenazine–chloranilic acid, was precisely measured by adiabatic calorimetry below room temperature. The paraelectric–ferroelectric phase transition was detected at 248 K as a heat capacity anomaly. The excess entropy involved is consistent with the previously assumed displacive nature. In addition to the ferroelectric phase transition, two phase transitions were observed in heat capacity and dielectric constant at 136 and 146 K.

Low-temperature heat capacity of layer structure lithium nickel oxide

Abstract Heat capacity of lithium nickel oxide of nearly stoichiometric composition (Li 0.99 Ni 1.01 O 2 ) was measured using an adiabatic calorimeter between 13 and 300 K. Abnormally large heat capacity was observed below 20 K, and it seems be related to the spin glass transition observed in the magnetic susceptibility measurements of nearly stoichiometric samples of Li 1− x Ni 1+ x O 2 . The heat capacity value above 100 K was compared with LiCoO 2 , and it was found that the heat capacity of Li 0.99 Ni 1.01 O 2 is much larger than that of LiCoO 2 in spite of almost the same atomic weight of Ni and Co. The heat capacity difference was discussed in terms of the lattice vibrations and bond strength in the crystals.

Structures and Phase Transitions in Rb2MoO4 and Rb2WO4

The structural phase transitions of Rb2MoO4 and Rb2WO4 have been reinvestigated with the use of heat capacity measurement and synchrotron X-ray and neutron scattering techniques. In Rb2MoO4, the existence of polymorphous structures has been confirmed. These crystal structures at 290 K are a monoclinic β-K2MoO4 type one and an orthorhombic β-K2SO4 type one. In an annealed sample, two typical first-order-type anomalies were observed at 783 and 752 K. Furthermore, new λ-type anomaly equivalent to the second normal-incommensurate phase transition in A2BO4-type ferroelectrics was observed at 223 K. On the other hand, the crystal structure at 290 K of Rb2WO4 was a monoclinic β-K2MoO4 type one and no phase transition was detected down to 8 K.