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We present a detailed investigation of observable properties associated with the relative stability of the rocksalt (B1) and cesium chloride (B2) phases in the AX (AϭLi, Na, K, Rb, Cs; XϭF, Cl, Br, I͒ crystal family. Thermodynamic B1→B2 transition pressures and ⌬Y ϭY (B2)ϪY (B1) 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 ͑i͒ the evaluation of the energy Hessian at the B1 and B2 points and ͑ii͒ the decomposition of the energy and pressure in anionic and cationic classical and quantum-mechanical contributions.

A first principles, general study of the thermodynamic and kinetic aspects of the B1-B2 phase transition in alkali halides is presented. Particular attention is paid to (a) how to construct models of increasing complexity to be used with generic quantum-chemistry techniques and {b) the topological and symmetry-dependent features of the energetic and the Gibbs potential surfaces analyzed. Our results indicate that the transition may be thought of as involving the simultaneous opening of the rhombohedral angle in the primitive 81 crystallographic cell while a contraction of the lattice parameter takes place.Transition paths depend strongly on pressure and show large and asymmetric Gibbs barriers that qualitatively account for many of the empirical facts around the phenomenon of hysteresis.

We present the results of a combined experimental and theoretical investigation aimed to determine structural and equation-of-state parameters and phase stability thermodynamic boundaries of ZrSiO 4 polymorphs. Experimental unit-cell data have been obtained for a powdered sample in a diamond-anvil cell using energydispersive synchrotron x-ray diffraction with emphasis on the pressure range 0 -15 GPa. Static total-energy calculations have been performed within the density functional theory at local density and generalized gradient approximation levels using a plane-wave pseudopotential scheme. Our quantum-mechanical simulations explore the pressure response of the two observed tetragonal structures ͑zircon-and scheelite-type reidite͒ as well as of other potential post-scheelite polymorphs up to about 60 GPa. We find very good agreement between our experimental and calculated pressure-volume values for the low-pressure phase of ZrSiO 4 . A microscopic analysis of the bulk compressibility of zircon and reidite in terms of polyhedral and atomic contributions is proposed to clarify some of the discrepancies found in recent theoretical and experimental studies. Our results show the relevant role played by the oxygen atoms in the description of this property. The zircon-reidite equilibrium phase transition pressure is computed around 5 GPa. No other post-scheelite phase is found stable above this pressure though a decomposition into ZrO 2 ͑cottunite͒ and SiO 2 ͑stishovite͒ is predicted at about 6 GPa. These two transition pressure values are well below the experimental ranges detected in the laboratory in concordance with the large hysteresis associated with these transformations.

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