Pressure dependence of the lattice dynamics of ZnO: An ab initio approach (original) (raw)

Dependence of phonon widths on pressure and isotopic mass: ZnO

physica status solidi (b), 2003

Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two-phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E high 2 phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two-phonon DOS provide an explanation for this variation of the FWHM: the E high 2 frequency falls on a sharp ridge of the 2-DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E high 2 frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discussed for the E low 2 phonons. Applications of the isotope and pressure techniques to the elucidation of two-phonon spectra will be presented.

Structural and electronic properties of ZnO under high pressures

Solid State Communications, 2006

In this work, we use first-principles calculations based on density-functional theory within the local-density approximation (LDA) to investigate the structural and electronic properties of ZnO under high-pressure. We have calculated the ground-state energy, the lattice constant, the bulk modulus, and its pressure derivative of the B4 (wurtzie), B3 (zinc blende), B2 (CsCl) and B1 (rocksalt) phases of ZnO. Moreover, the electronic structure, density of states (DOS) of the B4 (wurtzite) and B1 (rocksalt) phases of ZnO have been calculated. We show that our calculated values compare acceptably well with values reported in the literature.

Effect of Pressure on the Stability and Electronic Structure of ZnO0.5S0.5 and ZnO0.5Se0.5

Journal of Electronic Materials, 2017

Structures and high-pressure phase transitions in ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 have been investigated using density functional theory calculations. The previously proposed structures of ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 which are chalcopyrite (I 42d), rocksalt (Fm3m), wurtzite (P6 3 mc) and CuAu-I (P 4m2) have been fully investigated. Stabilities of these materials have been systematically studied up to 40 GPa using various approaches. We have confirmed the stability of the chalcopyrite structure up to 30 GPa for which the CuAu-I structure has been previously proposed. However, our calculation revealed that CuAu-I is not a stable structure under 32 GPa and 33 GPa for both ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 , respectively, which could explain the failure in several attempts to fabricate these materials under such conditions. We have also examined the pressure-dependence of the bandgap and electronic structure up to 30 GPa. We can conclude from our PDOS analysis that the applied pressure does not change the atomic state characters of electronic states near the top of valence and the bottom of conduction bands, but mainly modifies the dominant Zn-3d atomic state of the deep Bloch state at À1 eV below Fermi level.

Hartree-Fock study of phase changes in ZnO at high pressure

Physical review. B, Condensed matter, 1993

The total energy of ZnO as a function of unit cell volume has been calculated for the zinc-blende, wurtzite, and rocksalt structures by the ab initio all-electron periodic Hartree-Fock linear-combinationof-atomic-orbitals method using a large Gaussian basis set that was variationally optimized for the solid state. Extensive convergence tests with respect to cutoffs of the real-space Coulomb and exchange series were carried out to ensure that the calculations were performed with sufficient precision. The calculated structural properties (equilibrium lattice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment, as is the transition pressure between them (8.57 GPa versus 9 -9.5 GPa experimentally), indicating that the method can reliably predict quite small energy

Band gap and partial density of states for ZnO: Under high pressure

Journal of Alloys and Compounds, 2015

In this work, the effect of pressure on the electronic band structure, partial density of states (PDOS), and the band gap of the four phases of ZnO, namely B4 (wurtzite), B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type), has been investigated using the plane-wave pseudo-potential code CASTEP with three different schemes: the generalized gradient approximation (GGA) in the latter approach Perdew-Burk-Ernzerhof (PBE) with and without spin-orbit (SO) coupling, the new hybrid exchange-correlation functional named B3LYP functional and Hartree-Fock + Local Density Approximation (HF + LDA). These schemes are employed in order to treat the exchange-correlation effects. In addition, we will illustrate how the orbital motion of crystal electrons is affected by spin-orbit (SO) coupling. To our knowledge, this is the first theoretical study reported on ZnO using the B3LYP method. Our investigation shows that the increase of the pressure causes the nature of the band gap to change from direct to indirect. The mechanism responsible for this change of band structure is analyzed. The wide band gap of the B4 (wurtzite) phase at p = 0 as determined by the precedent methods is $3.221 and 3.222 eV (PBE) with and without spin-orbit (SO) coupling, respectively, 9.186 eV (HF + LDA) and 2.451 eV (B3LYP). The first two approaches provide the best agreement with the experiments. The band gap of B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type) and the strong contribution of d orbitals of Zn atoms on the structure of the bands will be discussed. The SO coupling effect on the band structure for all phases is presented. This effect on the electronic properties of the various phases of ZnO in particularly for the B3 (zinc-blende), B1 (rocksalt) and B2 (CsCl-type) phases at high pressures are not well-known. The aim of this work is to improve the DFT band gap error by introducing different schemes and therefore this study is important for future experimental work on this potential semiconductor material. In this study we have shown the effect of SO coupling on the bands structure of ZnO as a function of pressure, and analyzed the mechanism of this effect. The understanding of spin-orbit coupling related phenomena is very important in both fundamental research and in applications of semiconductors systems.

High-pressure Raman spectroscopy study of wurtzite ZnO

Physical Review B, 2002

The high pressure behavior of optical phonons in wurtzite zinc oxide (w-ZnO) has been studied using room temperature Raman spectroscopy and ab-initio calculations based on a plane wave pseudopotential method within the density functional theory. The pressure dependence of the zonecenter phonons (E2, A1 and E1) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the E low 2 mode. E1(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge e * T is experimentally found to increase under compression like e * T (P) = 2.02 + 6.4 • 10 −3 .P whereas calculations give e * T (P) = 2.09−2.5•10 −3 .P (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of anomaly in the E2 and A1 modes behavior before the phase transition.

Structural and electronic properties of rock salt phase of ZnO under compression

Journal of Physics and Chemistry of Solids, 2008

Structural and electronic properties of rock salt phase of ZnO under high pressure have been reinvestigated in the light of some recent experimental results. Behavior of direct and indirect energy band gap under increasing pressure is analyzed on account of overlapping of p (O) and d (Zn) orbitals and the results are compared with other theoretical studies. An empirical relation involving elemental electronegativity is suggested to estimate the change in band gap under increasing pressure. Furthermore, phase transformation of ZnO into other possible structures is also discussed and their structural and electronic properties analyzed. r

Lattice dynamics of ZnAl2O4 and ZnGa2O4 under high pressure

Annalen der Physik, 2011

In this work we present a first-principles density functional study of the vibrational properties of ZnAl2O4 and ZnGa2O4 as function of hydrostatic pressure. Based on our previous structural characterization of these two compounds under pressure, herewith, we report the pressure dependence on both systems of the vibrational modes for the cubic spinel structure, for the CaFe2O4-type structure (Pnma)inZnAl2O4 and for marokite (Pbcm)Z n G a 2O4. Additionally we report a second order phase transition in ZnGa2O4 from the marokite towards the CaTi2O4-type structure (Cmcm), for which we also calculate the pressure dependence of the vibrational modes at the Γ point. Our calculations are complemented with Raman scattering measurements up to 12 GPa that show a good overall agreement between our calculated and measured mode frequencies.

Optical properties and electronic structure of rock-salt ZnO under pressure

Applied Physics Letters, 2003

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.