Local compressibilities in crystals

An application of the atoms in molecules theory to the partitioning of stat ic thermodynamic properties in condensed systems is presented. Attention is focused on the definition and the behavior of atomic compressibilities. In verses of bulk moduli are found to be simple weighted averages of atomic co mpressibilities. Two kinds of systems are investigated as examples: four re lated oxide spinels and the alkali halide family. Our analyses show that th e puzzling constancy of the bulk moduli of these spinels is a consequence o f the value of the compressibility of an oxide ion. A functional dependence between ionic bulk moduli and ionic volume is also proposed.