| Abstract Scope |
Entropy is the underlying concept of thermodynamics. For materials, entropy comes from the degrees of freedom of atoms and electrons. Today ab initio computations, inelastic scattering, and calorimetry can assess the entropy and free energy of materials with sufficient accuracy to predict phase stability and transformations. Other thermophysical properties can be predicted; for example, thermal expansion can be obtained from the pressure dependence of entropy. Most of the entropy of materials is from the vibrations of atoms. At lower temperatures, vibrations are understood by quantizing the independent normal modes of a crystal. However, the quanta of vibrations, i.e., phonons, lose their independence at high temperatures because of interactions between themselves, and interactions between phonons and electrons or spins. I will describe how these interactions are sorted out, how they alter the free energy, and how they contribute to positive or negative thermal expansion. Some of the behavior is nonintuitive. |