We have performed first-principles calculations of the electronic structure of ZnO, and applied them to the determination of structural and lattice-dynamical properties and their dependence on pressure. The dynamical matrices have been obtained for the wurtzite, zinc-blende, and rocksalt modifications with several lattice parameters optimized for pressures up to 12 GPa. These matrices are employed to calculate the one-phonon densities of states ͑DOS͒ and the two-phonon DOS associated with either sums or differences of phonons. These results provide the essential tools to analyze the effect of isotope-induced mass disorder and anharmonicity on phonon linewidths, which we discuss here and compare with experimental data from Raman spectroscopy, including first-and second-order spectra. Agreement of calculated properties with experimental results improves considerably when the renormalization due to anharmonicity is subtracted from the experimental data.

The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering. DOI: 10.1103/PhysRevLett.98.095503 PACS numbers: 63.20.Dj, 61.10.Eq, 63.10.+a, 71.15.Mb Besides their intermediate structure between twodimensional sheets and 3D crystals, layered compounds are relevant materials for storage of other compounds [1] and as potential building blocks for nanotubes [2]. Among these materials, graphite and hexagonal boron nitride (h-BN) have drawn most of the attention due to their simple hexagonal structure, their fascinating properties, and the successful realization of carbon-[3] and more recently BN [4 -6] nanotubes. A wide band gap semiconductor, h-BN, has been grown successfully very recently as a single crystal. It exhibits a lasing behavior at 5.8 eV that makes it attractive for optoelectronic applications in the ultraviolet energy range [7].Despite the tremendous effort devoted to the characterization of physical properties of h-BN, the lattice dynamics, responsible for the elastic and thermodynamic properties such as the heat capacity and the thermal expansion, are still under debate. First principles calculations are generally accepted as the most accurate theoretical description of the lattice dynamics [8]. Recently, for h-BN, the validity of state of the art ab initio calculations was put in doubt by two experiments: Rokuta et al. [9] reported the only experimental data available for the phonon dispersion relations by electron-energy loss spectroscopy (EELS) performed on a monolayer of h-BN deposited on a Ni(111) substrate. Among several deviations from ab initio calculations of the lattice dynamics both for a monolayer [10,11] and for bulk h-BN [12,13], the EELS data show a degeneracy at the K point between acoustic (ZA) and optical (ZO) modes polarized along the c axis. The origin of this degeneracy has not been clarified yet. More recently [14], second-order Raman spectra of single crystalline h-BN revealed two features that contested the predictions based on a simple doubling of the energy scale of ab initio calculations for the one-phonon density of states, suggesting deviations up to 40% for the calculated energy of the TA branch in the K-M region. Since h-BN is a relevant material for UV lasing, it is imperative to resolve the existing controversies and critically assess the quality of the ab initio calculations. Furthermore, we need exact information on the lattice dynamics in order to properly understand the contributions due to electron-phonon coupling observed in the cathodoluminescence and absorption spectra of h-BN [7] and BN nanotubes [15,16].In this Letter we address the above discrepancies in the vibrational properties by reporting inelastic x-ray scattering (IXS) measurements...

Anomalous Raman modes have been reported in several recent papers dealing with doped-and undoped-ZnO layers grown by different methods. Most of these anomalous Raman modes have been attributed to local vibrational modes of impurities or defects. However, we will show that most of the observed modes correspond to wurtzite-ZnO silent modes allowed by the breakdown of the translational crystal symmetry induced by defects and impurities.

No abstract

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.

The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct assignment of transverse and longitudinal phonon branches at K. We observe an almost degeneracy of the three TO-, LA-, and LO-derived phonon branches and a strong phonon trigonal warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which exhibits a trigonal warping effect opposite to that of the electronic band structure. We determined the electron-phonon coupling constant to be 166 ͑eV/ Å͒ 2 in excellent agreement to GW calculations. These results are fundamental for understanding angle-resolved photoemission, doubleresonance Raman and transport measurements of graphene-based systems.

We present the results of a first-principles theoretical study of the relative stability of several structural phases of the group-III nitrides AlN, GaN, and InN that complements the picture of the behavior under pressure of these technologically important materials. Along with structures which have been previously considered in other theoretical studies of these materials ͑comprising those of the observed phases: wurtzite, zinc-blende, and rocksalt; and the d-␤-Sn, NiAs, and CsCl structures͒ we have also assessed the stability of several novel structures, viz., the cinnabar structure, the Cmcm structure, and the sc16 structure, which have been recently observed in high-pressure experiments on various related compounds ͑e.g., GaAs͒ and have also been reported to be either stable or close to stable in a certain range of pressures in other III-V and II-VI compounds on the basis of first-principles calculations. Our results indicate, however, that in AlN, GaN, and InN, the high-pressure rocksalt phase remains stable with respect to any other phase considered in this study up to the highest pressures investigated of ϳ200 GPa, which agrees with the available experimental data. We have further considered the effect of the semicore d orbitals of GaN and InN on the phase diagram of these compounds.

We have investigated the lattice dynamics of a wurtzite GaN single crystal by inelastic x-ray scattering. Several dispersion branches and phonons at high-symmetry points have been measured, including the two zone-center Raman-and infrared-inactive silent modes. The experiments have been complemented by ab initio calculations. They are in very good agreement with our measurements, not only for phonon energies, but also for scattering intensities, thus validating the correctness of the eigenvectors. Other phenomenological and ab initio theories exhibit significant differences.