This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45Ϯ0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed ͑4.6 eV at 10 GPa͒, with a positive pressure coefficient ͑around 40 Ϯ3 meV/GPa between 5 and 19 GPa͒. These results are interpreted on the basis of first-principles electronic band structure calculations.

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ABSTRACT:We used an ab initio total energy pseudopotential technique within the density functional theory in the local-density approximation to determine the full set of first-order elastic constants of BAs, which have not been established experimentally. We also calculated the bulk modulus, the optical phonon frequency at the ⌫ point, and we present a study of the electronic band structure. Finally we also determined the pressure dependence of the elastic constants C ij. Due to the absence of the experimental data our results can be considered as reliable predictions of the elastic moduli and the pressure derivatives.

We use an ab initio total energy pseudopotential technique within the Density Functional Theory in the local-density approximation to determine the vibrational normal modes (phonons) defined within the first Brillouin Zone of BAs. We also study the effects of hydrostatic pressure on these phonons and the mode Grüneisen parameters using the response-function formalism for the computation. There is a very good agreement between the transversal optical phonon frequency at the Γ point calculated on this way and that with the frozen phonon technique. We also study the electronic band structure, the band-gap pressure dependence and the generalized stability criteria.1 Introduction A wide variety of physical properties of solids depends on their lattice-dynamical behaviour. Their understanding in terms of phonons is considered to be one of the most convincing pieces of evidence that our current quantum picture of solids is correct. Theoretical studies of structural and vibrational properties of materials are now routinely obtained by means of first principles calculations. The achievement of Density Functional Theory (DFT) and the development of a very efficient perturbative approach, the Density Functional Perturbation Theory (DFPT), even using the Local Density Approximation (LDA), make possible to obtain accurate phonon dispersions on a fine grid of wave vectors covering the entire Brillouin Zone.Boron compounds have attracted increasing research interest in the last few years. These materials are wide band gap semiconductors of technological interest, for potential optical and high temperature applications. The group of the zinc-blende boron compounds displays a peculiar behaviour when comparing with other semiconductors of the III-V family. Apparently their unusual properties are related with the absence of the p electrons in the core and the small core size of the B atom as shown by several studies [1 -4]. According to the Phillips van Vechten [5] scale BAs is the least ionic compound of the III -V family, f i = 0.002, so BAs is a boundary compound in the region that separates the ionic and covalent compounds.In a previous work [6], we had obtained the full set of first-order elastic constants of BAs, the bulk modulus, the optical phonon frequency at the Γ point, and the electronic band structure. In this work, we determine the pressure dependence of the electronic structure of BAs studying the main gaps behaviour under pressure. It is well known that DFT -LDA does not accurately describes the band gap, due to the well known underestimations of gaps. However, the pressure derivatives of conduction band states and their relative positions are given reasonably well within the DFT -LDA. By the other hand, we are also