We present a detailed investigation of observable properties associated with the relative stability of the rocksalt $\mathrm{}\left(B1\mathrm{}\right)\mathrm{}$ and cesium chloride $\mathrm{}\left(B2\mathrm{}\right)\mathrm{}$ phases in the $\mathrm{AX}$ $(A=\mathrm{Li},$ Na, K, Rb, Cs; $X=\mathrm{F},$ Cl, Br, I) crystal family. Thermodynamic $B\overrightarrow{1}B2\mathrm{}$ transition pressures and $\Delta Y=Y\left(B2\mathrm{}\right)\mathrm{}-Y\left(B1\mathrm{}\right)\mathrm{}$ differences in total energies, volumes, and bulk moduli at zero and transition pressures are computed following a localized Hartree-Fock method. The arrangement of the data in clear trends is shown to be mainly dominated by the cation atomic number. This behavior is well interpreted in terms of a variety of microscopic arguments that emerge from (i) the evaluation of the energy Hessian at the $B1\mathrm{}$ and $B2\mathrm{}$ points and (ii) the decomposition of the energy and pressure in anionic and cationic classical and quantum-mechanical contributions.

We report the results of a combined experimental and theoretical investigation on the stability and the volume behavior under hydrostatic pressure of the rocksalt $\mathrm{}\left(B1\mathrm{}\right)\mathrm{}$ phase of ZnO. Synchrotron-radiation x-ray powder-diffraction data are obtained from 0 to 30 GPa. Static simulations of the $\mathrm{ZnO}\hspace{0.5em}B1\mathrm{}$ phase are performed using the ab initio perturbed ion method and the local and nonlocal approximations to the density-functional theory. After the pressure induced transition from the wurtzite phase, we have found that a large fraction of the $B1\mathrm{}$ high-pressure phase is retained when pressure is released. The metastability of this ZnO polymorph is confirmed through the theoretical evaluation of the Hessian eigenvalues of a nine-parameter potential energy surface. This allows us to treat the experimental and theoretical pressure-volume data on an equal basis. In both cases, we have obtained values of the bulk modulus in the range of 160–194 GPa. For its zero-pressure first derivative, the experimental and theoretical data yield a value of $4.4\pm \mathrm{1.0.}$ Overall, our results show that the $\mathrm{ZnO}\hspace{0.5em}B1\mathrm{}$ phase is slightly more compressible than previously reported.

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