Zhongqing Wu

Pressure‐volume‐temperature relations in MgO: An ultrahigh pressure‐temperature scale for planetary sciences applications

[1] In situ crystallography based on diamond anvil cells have been extended to the multimegabar regime. Temperatures in these experiments have crossed the 2500 K mark. Yet, current high pressure-temperature (PT) standards of calibration produce uncertainties that inhibit clear conclusions about phenomena of importance to planetary processes, e.g., the postperovskite transition in Earth’s mantle. We introduce a new thermal equation of state (EOS) of MgO which appears to be predictive up to the multimegabar and thousands of kelvin range. It is obtained by combining first principles local density approximation quasi-harmonic (QHA) calculations with experimental lowpressure data. This EOS agrees exceptionally well with shock compression data. The postspinel and postperovskite phase boundaries recalculated using our EOS match seismic observations. The latter, in particular, supports the idea that postperovskite transforms back to perovskite before the core-mantle boundary. The recalculated experimental Clapeyron slope of the postperovskite transition is also more consistent with those obtained by first principles calculations. Citation: Wu, Z., R. M. Wentzcovitch, K. Umemoto, B. Li, K. Hirose, and J.-C. Zheng (2008), Pressure-volume-temperature relations in MgO: An ultrahigh pressure-temperature scale for planetary sciences applications, J. Geophys. Res., 113, B06204,

Spin crossover in ferropericlase and velocity heterogeneities in the lower mantle.

Significance Seismic tomography reveals Earth’s internal structure in great detail. Lateral variations of seismic wave velocities expose enigmatic mantle structures that, to be deciphered, require independent knowledge of acoustic velocities in minerals. Using density functional theory-based computational methods we show that a spin state change in iron in ferropericlase, the second major phase of the Earth’s lower mantle, produces seismic velocity anomalies that can be misinterpreted as compositional heterogeneity. This spin change reduces thermally induced longitudinal velocity variations between ∼1,500- and 2,000-km depths. This phenomenon is observed in P velocity tomography models and has been thought to be related to a chemical transition in the mantle. The spin change in iron in ferropericlase may offer an alternative interpretation of this phenomenon. Deciphering the origin of seismic velocity heterogeneities in the mantle is crucial to understanding internal structures and processes at work in the Earth. The spin crossover in iron in ferropericlase (Fp), the second most abundant phase in the lower mantle, introduces unfamiliar effects on seismic velocities. First-principles calculations indicate that anticorrelation between shear velocity (VS) and bulk sound velocity (Vφ) in the mantle, usually interpreted as compositional heterogeneity, can also be produced in homogeneous aggregates containing Fp. The spin crossover also suppresses thermally induced heterogeneity in longitudinal velocity (VP) at certain depths but not in VS. This effect is observed in tomography models at conditions where the spin crossover in Fp is expected in the lower mantle. In addition, the one-of-a-kind signature of this spin crossover in the RS/P (∂⁡ln⁡VS/∂⁡ln⁡VP) heterogeneity ratio might be a useful fingerprint to detect the presence of Fp in the lower mantle.

Density functional study of 1,3,5-trinitro-1,3,5-triazine molecular crystal with van der Waals interactions

Volume dependence of the total energy and vibrational properties of crystalline l,3,5-trinitro-l,3,5-triazine (RDX) are calculated using the density functional theory (DFT). For this molecular crystal, properties calculated with a generalized gradient approximation to the exchange-correlation energy differ drastically from experimental values. This discrepancy arises from the inadequacy in treating weak van der Waals (vdW) interactions between molecules in the crystal, and an empirical vdW correction to DFT (DFT-D approach by Grimme) is shown to account for the dispersion effects accurately for the RDX crystal, while incurring little computational overhead. The nonempirical van der Waals density-functional (vdW-DF) method also provides an accurate description of the vdW corrections but with orders-of-magnitude more computation. We find that the vibrational properties of RDX are affected in a nontrivial manner by the vdW correction due to its dual role--reduction of the equilibrium volume and additional atomic forces.

Thermoelasticity of Fe2+-bearing bridgmanite

We present local density approximation augmented by the Hubbard-type correction calculations of high-temperature elastic properties of bridgmanite with composition (Mg(1-x)Fex2+)SiO3 for 0 <= x <= 0.125. Results of elastic moduli and acoustic velocities for the Mg end-member (x=0) agree very well with the latest high-pressure and high-temperature experimental measurements. In the iron-bearing system, we focus particularly on the change in thermoelastic parameters across the state change that occurs in ferrous iron above similar to 30GPa, often attributed to a high-spin (HS) to intermediate-spin (IS) crossover but explained by first-principles calculations as a lateral displacement of substitutional iron in the perovskite cage. We show that the measured effect of this change on the equation of state of this system can be explained by the lateral displacement of substitutional iron and not by the HS to IS crossover. The calculated elastic properties of (Mg0.875Fe0.1252+)SiO3 along an adiabatic mantle geotherm somewhat overestimate longitudinal velocities but produce densities and shear velocities quite consistent with the Preliminary Reference Earth Model data throughout most of the lower mantle.

Lattice dynamics and thermal equation of state of platinum

Platinum is widely used as a pressure calibration standard. However, the established thermal equation of state EOS has uncertainties especially in the high P-T range. We use density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local-density approximation LDA, Perdew-BurkeErnzerhof, and the recently proposed Wu-Cohen functional. The electronic thermal free energy was evaluated using the Mermin functional. The vibrational part was computed within the quasiharmonic approximation using density-functional perturbation theory and pseudopotentials. Special attention was paid to the influence of the electronic temperature on the phonon frequencies. We find that, in overall, LDA results agree best with the experiments. Based on the density-functional theory calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.

Vibrational and thermodynamic properties of wadsleyite: A density functional study

[i] The vibrational properties of Mg 2 SiO 4 wadsleyite have been calculated over a wide pressure range using density functional perturbation theory (DFPT). Both the normal mode frequencies and their volume dependences are consistent with the available Raman and infrared data. We provide detailed information about vibrational properties that are still not experimentally available. The vibrational density of states (vDOS) is used to calculate the Helmholtz free energy within the quasi-harmonic approximation (QHA) and other thermodynamic quantities without further approximations. The extensive and successful comparisons with experiments demonstrate once more that the QHA combined with first principles vDOSs can provide accurate thermodynamic properties of minerals over the large pressure-temperature regime relevant for the Earth.

Huge enhancement of electromechanical responses in compositionally modulated Pb(Zr(1-x )Tix)O3.

Monte Carlo simulations based on a first-principles-derived Hamiltonian are conducted to study the properties of Pb(Zr1-xTix)O3 alloys compositionally modulated along the [100] pseudocubic direction near the morphotropic phase boundary. It is shown that compositional modulation causes the polarization to continuously rotate away from the modulation direction, resulting in the unexpected triclinic and C-type monoclinic ground states and huge enhancement of electromechanical responses (the peak of piezoelectric coefficient is as high as 30,000 pC/N). The orientation dependence of dipole-dipole interaction in modulated structure is revealed as the microscopic mechanism to be responsible for these anomalies.

Vibrational and thermodynamic properties of β-HMX: A first-principles investigation

Thermodynamic properties of β-HMX crystal are investigated using the quasi-harmonic approximation and density functional theory within the local density approximation (LDA), generalized gradient approximation (GGA), and GGA + empirical van der Waals (vdW) correction. It is found that GGA well describes the thermal expansion coefficient and heat capacity but fails to produce correct bulk modulus and equilibrium volume. The vdW correction improves the bulk modulus and volume, but worsens the thermal expansion coefficient and heat capacity. In contrast, LDA describes all thermodynamic properties with reasonable accuracy, and overall is a good exchange-correlation functional for β-HMX molecular crystal. The results also demonstrate significant contributions of phonons to the equation of state. The static calculation of equilibrium volume for β-HMX differs from the room-temperature value incorporating lattice vibrations by over 5%. Therefore, for molecular crystals, it is essential to include phonon contributions when calculated equation of state is compared with experimental data at ambient condition.

Unusual vortex structure in ultrathin Pb(Zr0.5Ti0.5)O3 films

Using a first-principles-based approach, we determine the ferroelectric pattern in PbZr0.5Ti0.5O3 ultrathin film. It is found that vortex stripes are formed in the system. The relation between the vortex stripes and the 180° domains is discussed. When a local external field is exerted, the vortex stripe transforms into the vortex loop structure, which leads to the formation of a smaller domain with the polarization antiparallel to the field in the center of the field region. This may provide a convenient way to manipulate nanodomains in thin films.

Ferroelectricity in Pb ( Zr 0.5 Ti 0.5 ) O 3 thin films: Critical thickness and 180° stripe domains

The ferroelectric properties of disorder