Entropy Terms in Statistical Thermodynamic Analysis Formula for Non-stoichiometric Interstitial Compounds

摘要

A series of statistical thermodynamic analyses were made since 1974 for different types of non-stoichiometric interstitial compounds MXx under simplifying a priori assumption of constant interaction energy E(X-X) between nearest neighbour interstitial atoms X within a homogeneity composition range of MX_(x) at arbitrary temperature T [K]. Mode of distribution of X atoms in interstitial sites in MX_(x) lattice is represented by number θ of available interstitial sites for occupation by X atoms per M atom and the value of θ is determined to fulfil the a priori assumption. Mode of atomic configuration would yield major contribution to entropy term ?S that appears in conventional thermodynamic expression of Gibbs free energy of formation, ?G, in form of T?S. In the statistical thermodynamic formulation, contribution of tightly bound electron appearing in form of RT ln f_(X) where f_(X) refers to atomic partition function of X atom in the MX_(x) lattice and R the universal gas constant. Judging from this mathematical form of the term, R ln f_(X) is considered to represent entropic contribution from tightly bound electron to X atom in MX_(x) lattice. In the published series of works on statistical thermodynamic analysis for non-stoichiometric interstitial compounds, calculated values for R ln f_(X) were reported but they were not reviewed with serious attention because R ln f_(X) was considered merely as a secondary factor compared to principal factor E(X-M) referring to interaction energy between X and M in MX_(x) lattice that represents enthalpy ?H in conventional thermodynamic term. In this review article, consideration is given exclusively to the factor R ln f_(X) evaluated in statistical thermodynamic approach to non-stoichiometric interstitial compounds.