| Research Abstract |
Thermodynamic studies have been made on superionic conductors which have long been attracting much attention because of its high potentiality in a variety of applications such as fuel cells, gas sensors and so on. However, the mechanism of high ionic conductivity in the solids is still an open question.
At first, the structural and thermodynamic properies have been studied on AgCrS_2, CuCrS_2, NaCrS_2.Rb_4Cu_<16>I_7Cl_<13>, Ag_8GeTe_6, Cu_8GeTe_6, Cu_3BiS_3, and PbSnF_4 on which the experiments and measurements have been made by X-ray diffractometry, impedance measurements, neutron diffractometry and precise heat capacity measurements. For the low temperature calorimetry down to 0.5 k, a small size calorimeter was newly designed and constructed using ^3He-evaporation type refrigerator.
Fine powders of Y_2O_3-ZrO_2 containing different amounts of Y_2O_3 were prepared by a hydrolysis techniques using ZrOCl_2・8H_2O and YCl_3. The elemental analysis gave the values of x=0,0.0200,0.0396,0.0776,0.0970 for the five samples represented as (ZrO_2) _<1-x> (Y_2O_3) _x. The samples were characterized by SEM,ICP analysis, X-ray diffractometry and Raman scattering, and then precise heat capacity measurements were made on the fine powder and sintered specimens. Yttria doping softens the crystal lattice and causes increasing of the heat capacity value, and thus the heat capacity of the sample of x=0.0970 is larger than that of x=0.0776 at high temperature region where almost all the lattice vibrations are fully excited and the number of vacancies corresponds to the heat capacity. In the lowest temperature region, however, the distribution of the vacancies plays an important role as well as the number of vacancies over the crystal lattice sites, because only low-frequency vibrations are excited in the low temperature region and such low-energy modes are strongly influenced by the distributin of the vacancies.
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