We report first principles density functional calculations of the Born effective charges and electronic dielectric tensors for the relaxor PMN (PbMg1/3Nb2/3O3). Visualization of the Born charge tensors as "charge ellipsoids" have provided microscopic insights on the factors governing piezoelectric enhancements with polarization rotation. Several 15 and 30-atom ferroelectric and antiferroelectric supercells of PMN involving 1:2 and 1:1 chemical ordering have been studied. A cascading set of ferroelectric phonon instabilities lead to several low symmetry monoclinic structures. We find a ground state with a 15-atom unit cell with 1:2 chemical ordering along [111] with a monoclinic C2 structure. (c) 2006 Elsevier B.V. All rights reserved.

We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron at high pressures using both first-principles linear response quasiharmonic calculations based on the full potential linear-muffin-tin-orbital (LMTO) method and the particle-in-cell (PIC) model for the vibrational partition function using a tight-binding total-energy method. The tight-binding model shows excellent agreement with the all-electron LMTO method. When the hcp structure is stable, the calculated geometric-mean frequency and the Helmholtz free energy of epsilon-Fe from PIC and the linear response lattice dynamics agree very well, as does the axial ratio as a function of temperature and pressure. On-site anharmonicity proves to be small up to the melting temperature, and PIC gives a good estimate of its sign and magnitude. At low pressures, epsilon-Fe becomes dynamically unstable at large c/a ratios, and the PIC model might fail where the structure approaches lattice instability. The PIC approximation describes well the vibrational behavior away from the instability, and thus is a reasonable approach to compute high temperature properties of materials. Our results show significant differences from earlier PIC studies, which gave much larger axial ratio increases with increasing temperature, or reported large differences between PIC and lattice dynamics results.

We investigate the elastic and isotropic aggregate properties of ferromagnetic bcc iron as a function of temperature and pressure by computing the Helmholtz free energies for the volume-conserving strained structures using the first-principles linear response linear-muffin-tin-orbital method and the generalized-gradient approximation. We include the electronic excitation contributions to the free energy from the band structures, and phonon contributions from quasiharmonic lattice dynamics. We make detailed comparisons between our calculated elastic moduli and their temperature and pressure dependences with available experimental and theoretical data. The isotropic aggregate sound velocities obtained based on the calculated elastic moduli agree with available ultrasonic and diamond-anvil-cell data. Birch's law, which assumes a linear increase in sound velocity with increasing atomic density, fails for bcc Fe under extreme conditions. First-principles linear-response lattice dynamics is shown to provide a tractable approach to examine the elasticity of transition metals at high pressures and high temperatures.

A complete set of elastic and piezoelectric constants for single-domain rhombohedral Pb(Zn1/3Nb2/3)O-3-4.5%PbTiO3 is obtained using Brillouin scattering. The bulk modulus and elastic constants agree with the values obtained from ultrasonic methods, but the piezoelectric constants are smaller. Differences in piezoelectric constants from different techniques are due to frequency dispersion and the contributions of domain boundaries. The pressure dependence of the Brillouin shifts of amorphous BeH2 was measured from ambient pressure to 17 GPa. The equation of state is deduced from the pressure dependence of the sound velocity; the bulk modulus is 14.2 (3.0) GPa and its pressure derivative is 5.3 (0.5). (c) 2006 Elsevier B.V. All rights reserved.

The All Sky Extrasolar Planet Survey (ASEPS) would use the Sloan 2.5-m wide field telescope and new generation multiple object high throughput Doppler instruments to undertake a large-scale visible and near-IR band Doppler survey of up to similar to 250,000 relatively bright stars (generally V up to < 13 for the visible and J < 11 for the near IR) for extrasolar planets between 2008-2013. An extended survey continuing until similar to 2020 could survey an additional similar to 250,000 stars and obtain information on long-period planets from the earlier detected planet sample, possibly detecting many solar analogs. ASEPS aims to increase the number of extrasolar planets by nearly two orders of magnitude (up to similar to 10,000 planets in the 12-year survey using all clear nights). This dramatic increase in the number of known planets would allow astronomers to study correlations among the diverse properties of extrasolar planets much more effectively than at present. Additionally, the large number of planet discoveries will enable the detection of rare planets that may have eluded previous planet searches, as well as transiting planets, and interacting multiple planet systems. In March-June 2006, a single full-scale multi-object W.M. Keck Exoplanet Tracker (Keck ET) with 60 object capability was commissioned and a trial planet survey of similar to 420 V=8-12 solar type stars has been conducted at Sloan telescope. Since the 2006 August engineering run, the instrument performance (throughput, image quality, and Doppler precision) has been substantially improved. Additional stars are being searched for planets.

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.