We investigate the lattice dynamics and thermodynamics of nonmagnetic bcc vanadium as a function of temperature and pressure, using the first principles linear response linear-muffin-tin-orbital method. The calculated phonon density of states (DOS) show strong temperature dependence, different from inelastic neutron scattering measurements where the phonon DOS show little change from room temperature up to 1273 K. We obtain the Helmholtz free energy including both electronic and phonon contributions and calculate various. equation of state properties such as the bulk modulus and the thermal expansion coefficient. A detailed comparison has been made with available experimental measurements.

We use density functional theory to study the effect of Fe and Al on properties of MgSiO3 perovskite and post-perovskite. The addition of Fe increases the compressibility and density of MgSiO3 and considerably decreases the transition pressure between the two phases. MgSiO3 perovskite transforms to post-perovskite at about 112 GPa. FeSiO3 is stable as post-perovskite at all pressures relative to perovskite. We find ferrous iron to be in a high spin state over the whole mantle pressure range, and it partitions preferentially into the post-perovskite structure. Ferrous iron in MgSiO3 decreases the seismic wave velocities and slightly decreases the seismic anisotropy. At 120 GPa, FeSiO3 post-perovskite has Vp = 12.4 km/s and Vs = 6.3 km/s and MgSiO3 post-perovskite has Vp = 14.2 km/s and Vs = 7.9 km/s. The seismic anisotropy of post-perovskite MgSiO3 is 15% for Vp and 26% for Vs. Aluminum slightly decreases the density and increases the transition pressure. Pure alumina transforms from perovskite to post-perovskite at 120 GPa. Al2O3 also increases the compressibility of perovskite and decreases that of post-perovskite. Al decreases the seismic wave velocities and considerably increases the seismic anisotropy of post-perovslcite. At 120 GPa, post-perovslcite Al2O3 has Vp = 13.8 km/s and Vs = 7.4 km/s and seismic anisotropy of 18% for Vp and 43% for Vs. For proposed mantle compositions such as pyrolite the changes in seismic wave velocities due to the transition from perovskite to post-perovskite, that is positive jumps in both Vp and Vs, can explain those observed at the top of the D '' layer.

We report the complete set of elastic constants and the bulk modulus for single-crystal Pb(Mg1/3Nb2/3)O-3 (PMN) at room temperature obtained from Brillouin spectroscopy and molecular-dynamics (MD) simulations. The bulk modulus from Brillouin spectroscopy is found to be 103 GPa, in good agreement with earlier x-ray studies. We also derived the refractive index along all principal axes and found PMN to be optically isotropic, with a refractive index value of 2.52 +/- 0.02. PMN shows elastic anisotropy with A=1.7. The MD simulations of PMN using the random site model overestimate the elastic constants by 20-50 GPa and the bulk modulus of 148 GPa, but the elastic anisotropy matches the Brillouin results of A=1.7. We also determined the elastic constants for various models of PMN and we predict variation in the elastic constants based on chemical ordering.

Using a materials by design approach, the authors find a class of ordered oxynitride piezoelectrics with perovskite structure. They predict that ordered YSiO2N and YGeO2N are characterized by large nonlinear optic responses and by some of the largest polarizations known to date.

We search for the existence of the post-perovskite structure in several nonmagnetic M2O3 sesquioxides using density-functional calculations. For each material we consider the corundum, Rh2O3-type II, perovskite, and post-perovskite structures. The perovskite structure is unstable with respect to at least one of the other structures at all pressures for all materials. The post-perovskite structure is stable above 120 GPa in Al2O3, above 344 GPa in Rh2O3, above 136 GPa in Ga2O3, and above 47 GPa in In2O3.

A piezoelectric material is one that generates a voltage in response to a mechanical strain ( and vice versa). The most useful piezoelectric materials display a transition region in their composition phase diagrams, known as a morphotropic phase boundary(1,2), where the crystal structure changes abruptly and the electromechanical properties are maximal. As a result, modern piezoelectric materials for technological applications are usually complex, engineered, solid solutions, which complicates their manufacture as well as introducing complexity in the study of the microscopic origins of their properties. Here we show that even a pure compound, in this case lead titanate, can display a morphotropic phase boundary under pressure. The results are consistent with first-principles theoretical predictions(3), but show a richer phase diagram than anticipated; moreover, the predicted electromechanical coupling at the transition is larger than any known. Our results show that the high electromechanical coupling in solid solutions with lead titanate is due to tuning of the high- pressure morphotropic phase boundary in pure lead titanate to ambient pressure. We also find that complex microstructures or compositions are not necessary to obtain strong piezoelectricity. This opens the door to the possible discovery of high- performance, pure-compound electromechanical materials, which could greatly decrease costs and expand the utility of piezoelectric materials.

We investigate by first-principles calculations the effect of ferrous iron, Fe2+, on the structure and the equation of state Of MgSiO3 post-perovskite. We find that ferrous iron is high-spin over the pressure range of the mantle assuming a ferromagnetic structure. The bulk modulus and the specific volume increase with the addition of ferrous iron to MgSiO3. We find that Fe partitions preferentially to post-perovskite and broadens the two-phase pressure range. (C) 2008 Elsevier B.V. All rights reserved.

In a recent paper [Z. Wu and R. E. Cohen, Phys. Rev. B 73, 235116 (2006)], we proposed an exchange functional model that better describes crystal structures than that of the Perdew-Burke-Ernzerhof ansatz. In this reply we address the issue raised by Zhao and Truhlar in their comment by emphasizing the rationale of the model.

Recent theoretical simulations using density functional theory (DFT) and novel low temperature high energy x-ray diffraction experiments clearly show the existence of a high pressure morphotropic phase boundary (MPB) in pure PbTiO3. The experiments show a richer phase diagram than the simulations, with multiple monoclinic phases (Pm and Cm) in the MPB region. In this paper we examine the MPB region in more detail using high precision DFT calculations within the local-density approximation (LDA) and the Wu-Cohen generalized gradient approximation. Our results support the polarization rotation theory and open up fresh possibilities for applying chemical pressure to engineer novel electromechanical materials. We also explain why the zone-boundary mode is more likely to be stable only at higher pressures above similar to 25 GPa and not at moderate pressures of similar to 10 GPa, using the LDA.