Heat capacity of PbS: Isotope effects

In recent years, the availability of highly pure stable isotopes has made possible the investigation of the dependence of the physical properties of crystals, in particular, semiconductors, on their isotopic composition. Following the investigation of the specific heat (C-p,C-v) of monatomic crystals such as diamond, silicon, and germanium, similar investigations have been undertaken for the tetrahedral diatomic systems ZnO and GaN (wurtzite structure), for which the effect of the mass of the cation differs from that of the anion. In this paper, we present measurements for a semiconductor with rocksalt structure, namely, lead sulfide. Because of the large difference in the atomic mass of both constituents (M-Pb=207.21 amu and M-S=32.06 amu, for the natural isotopic abundance), the effects of varying the cation and that of the anion mass are very different for this canonical semiconductor. We compare the measured temperature dependence of C-p approximate to C-v, and the corresponding derivatives with respect to M-Pb and M-S, with ab initio calculations based on the lattice dynamics obtained from the local density approximation electronic band structure. Quantitative deviations between theory and experiment are attributed to the absence of spin-orbit interaction in the ABINIT program used for the electronic band structure calculations.