Yasuhisa Yamamura

Heat capacity anomaly due to the α-to-β structural phase transition in ZrW2O8

Abstract Heat capacity of ZrW 2 O 8 has been measured as a function of temperature from 340 to 550xa0K. An anomaly in heat capacity corresponding to the α-to-β structural phase transition was clearly detected for the first time around 431xa0K. The transition temperature was determined to be 431±1 K , and the minimum bound of the transition entropy as 2.1xa0Jxa0K −1 xa0mol −1 . Although this value is smaller than the theoretical one of R xa0lnxa02 (=5.8xa0Jxa0K −1 xa0mol −1 ), the phase transition is considered to be of the order–disorder type.

Heat capacity measurements and phase transition of crystalline 4,4″-difluoro-p-terphenyl

Abstract The heat capacity of 4,4″-difluoro- p -terphenyl was measured between 8 and 303 K by adiabatic calorimetry. A phase transition associated with molecular twisting was observed as a sharp peak at (127.05 ± 0.05) K. The enthalpy and the entropy of transition were determined as (180 ± 20) J mol −1 and (1.41 ± 0.15) J K −1 mol −1 , respectively. It is pointed out that this phase transition lies in the border region between an order-disorder and displacive-type transition under normal pressure. The apparatus used for calorimetry is described.

Synthesis and physical properties of negative thermal expansion materials Zr1−xMxW2O8−y (M=Sc, In, Y) substituted for Zr(IV) sites by M(III) ions

Abstract Zr1−xMxW2O8−y (M=Sc, In and Y) solid solutions substituted up to x=0.04 for Zr(IV) sites by M(III) ions were synthesized by a solid-state reaction. X-ray diffraction experiments from 90 to 560 K revealed that all solid solutions had a cubic crystal structure and showed negative thermal expansion coefficients. The lattice parameters of Zr1−xMxW2O8−y were smaller than that of ZrW2O8 probably due to oxygen defects, though the ionic radii of substituted M3+ ions were larger than that of Zr4+. Order–disorder phase transition temperatures of the substituted samples drastically decreased in the order of Y, In and Sc compared to the percolation theory, and decreased with increasing M content.

Structural and thermal properties of LiMn2O4 substituted for manganese by iron

Abstract Structural and thermal properties of Fe-doped lithium manganese oxides, Li 0.9 Mn 2− x Fe x O 4 and LiMn 2− x Fe x O 4 , were studied by means of X-ray diffraction (XRD), Mossbauer spectroscopy, DSC and TGA methods. Mossbauer spectroscopy measurements revealed that the iron in Fe-doped samples existed as high spin Fe 3+ (main component) and Fe 4+ (minor component) in noncubic octahedral site (16d). Lattice constants of Fe-doped samples increased with increasing doped iron contents, probably due to the substitution of Fe 4+ with large ionic radius for Mn 4+ with small ionic radius. Disappearance of endothermic peak due to Jahn–Teller effect of Mn 3+ and Fe 4+ around room temperature for Li 0.9 Mn 2− x Fe x O 4 with higher iron contents reflects mainly the substitution of Fe 3+ for Mn 3+ . Starting temperature of weight loss for LiMn 2− x Fe x O 4 determined by TGA measurements increased with increasing iron contents and weight loss at constant temperature decreased. Doped iron of the samples seems to play an important role in the thermal stability of the spinel structure at high and low temperatures.

Evidence of charge stripes, charge-spin-orbital coupling and phase transition in a simple copper oxide CuO

There is a great current interest in understanding the role of charge stripes in high- T c superconductivity. We found that static charge ordering and alignment of charge-ordered domains analogous ...

Glass transition due to freezing of intramolecular motion: Crystalline trans-azobenzene and trans-stilbene

Abstract The heat capacities of crystalline trans-azobenzene and trans-stilbene were measured by adiabatic calorimetry. No anomalies were detected in both compounds at any temperature regions where possible phase transitions have been previously suggested. Anomalies due to glass transitions were observed at about 170 and 110 K for trans-stilbene and trans-azobenzene, respectively. The magnitude of the heat capacity jump at the transition is analysed and the transitions are attributed to the freezing of the intramolecular crankshaft motion. Thermodynamic functions are tabulated for trans-azobenzene.

Effect of ionic radius on electrical conductivity of doped SmAlO3 perovskite oxide

Abstract Electrical conductivity of SmAlO 3 substituted by Ni, Mg or Zn ions for Al site was measured as a function of temperature. The electrical conductivity of doped SmAl 0.95 M 0.05 O 3− x (M=Ni, Mg and Zn) was about three to four orders of magnitude larger than that of undoped SmAlO 3 and increased in the order of Ni, Mg and Zn ions. Effect of ionic radius of doped ions on the electrical conductivity of SmAl 0.95 M 0.05 O 3− x (M=Ni, Mg and Zn) was discussed. It has been found from the oxygen partial pressure dependence of electrical conductivity that SmAl 0.95 Zn 0.05 O 3− x was a mixed conductor of oxide ion and hole from 973 to 1273 K. Transport number of oxide ion in SmAl 0.95 Zn 0.05 O 3− x increased with decreasing temperature. Oxide ion conductivity of SmAl 0.95 Zn 0.05 O 3− x was compared with those for typical oxide ion conductors and was discussed from the point of view of a tolerance factor.

Phase transition in crystalline p-polyphenyls : heat capacity of 4,4'''-difluoro-p-quaterphenyl

Abstract The heat capacity of crystalline 4,4‴-difluoro- p -quaterphenyl was measured from 8 K to 303 K by adiabatic calorimetry. A broad anomaly was detected between 140 and 250 K and attributed to a structural phase transition associated with molecular twisting. The temperature, the enthalpy and the entropy of transition were determined as (200.5 ± 0.5) K, (322 ± 20) J·mol −1 and (1.65 ± 0.10) J·K −1 ·mol −1 , respectively. The properties of the transition are compared with those of unsubstituted and fluorine-substituted p -polyphenyls (of two to four benzene rings in a molecule) in the light of a unified model of displacive and order-disorder types of structural phase transition.

Heat capacity and Grüneisen function of negative thermal expansion compound HfW2O8

Abstract Heat capacity of HfW2O8 was measured from 1.8 to 330xa0K. Gruneisen function of HfW2O8 was obtained between 80 and 330xa0K from the heat capacity and other thermal data. Effective phonon density of states was determined through an analysis of the heat capacity of HfW2O8 assuming one Debye, two Einstein and two rectangular type functions. The mode Gruneisen parameters were estimated from these phonon distribution. It is found that two low-energy Einstein modes at 3–6xa0meV have large negative model Gruneisen parameters, resulting in the negative thermal expansion.

Phase transition associated with molecular twisting in crystalline 4,4″-difluoro-p-terphenyl displacive or order-disorder transition?

Abstract The heat capacity of crystalline 4,4″-difluoro- p -terphenyl was measured from 8 K to 303 K by adiabatic calorimetry. The anomaly with a peak at (127.05 ± 0.05) K due to a structural phase transition was detected between 100 and 160 K. The enthalpy and the entropy of transition were determined as (180 ± 20) J.mol −1 and (1.41 ± 0.15) J.K −1 .mol −1 , respectively. The properties of the transition are compared with those of p -terphenyl. The possibility is pointed out that an transition in the present compound lies in the “transition” region from order-disorder to a displacive type of structural phase transition.