Determination of thermophysical properties of solids. Analysis of lattice and excess contributions.

Abstract

The concepts of correlation between macroscopic and microscopic properties and the development of efficient methods for analysis of experimental heat-capacity data of solids are the corner stones of this work. The heat capacities of several scientifically interesting solid compounds were measured, over the temperature region 6 to 350 K, and these results were analyzed. These include the measurement and interpretation of the subambient heat capacities of the low dimensional solids ZrTe$\sb5$ and HfTe$\sb5$, of the perovskite high $T\sb{C}$ superconductor YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$, and of Sm$\sb2$S$\sb3$ and Yb$\sb2$S$\sb3$. Resolutions of the heat capacities of all known lanthanide sesquisulfides to their components are also presented. These results are summarized here. The subambient heat capacities of ZrTe$\sb5$ and HfTe$\sb5$ were measured. No anomalous behavior was identified over the temperature region 7 to 350 K. At low temperatures the samples exhibit behavior that is typical of low dimensional materials and the effective thermodynamic dimension is between 2 and 1. The heat capacities of the compounds were correlated by means of the Komada/Westrum approximation to lattice heat capacities. The differences between the heat capacities of these compounds can be accounted for solely on mass effects since their volumes are essentially identical. The high $T\sb{C}$ superconductor YBa$\sb2$Cu$\sb3$O$\sb{7-\delta}$ undergoes a second order phase transition at 91.4 K with $\Delta C\sb{p}$ = 0.393R. Close examination of the electronic heat capacity of the compound shows that two superconducting phases are present with two energy gaps of 36.16 and 220.24$R$ $\cdot$ K. The lattice heat capacity of the compound was calculated by means of the Komada/Westrum approximation with the incorporation of 36 vibration frequencies as weighted Einstein functions. The heat capacities of $\gamma$-phase Sm$\sb2$S$\sb3$ and $\epsilon$-phase Yb$\sb2$S$\sb3$ were measured from 7 to 350 K. The data are resolved into lattice and Schottky contributions. The experimental heat capacities of all known lanthanide sesquisulfides were analyzed. The system is composed of three structure types. Schottky and lattice contributions were evaluated. The Komada/Westrum approximation to lattice heat capacities was utilized in the calculations. Electronic Stark levels that contribute to the observed heat capacities were identified. The results were compared with those of spectroscopic measurements. It is concluded that correlations between macroscopic and microscopic properties, based on thermodynamic and structural information, are not only feasible; but that such correlations can serve as an important scientific tool.