Atomic force constants and the heat capacities of materials at elevated temperatures

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Abstract
Atomic force constants for C, Si, and Ge in diamond, graphite, and crystalline silicon and germanium are determined by the analysis of the heat capacities of these materials. These force constants are then compared with those found in simple molecular analogs of these crystals. The dependence of the atomic force constants upon crystal and molecular structure is analyzed in terms of an ``atoms in molecules'' model. Using this model and approximate electron densities, determined by either Slater‐type atomic orbitals or the Thomas‐Fermi theory, the magnitudes of the atomic force constants were estimated and compared with their experimental values.