Piezoelectrics with negative longitudinal piezoelectric coefficients will contract in the direction of an applied electric field. Such piezoelectrics are thought to be rare, but there is no fundamental physics preventing the realization of negative longitudinal piezoelectric effect in a single-phase material. Using first-principles calculations, we demonstrate that several hexagonal ABC ferroelectrics possess significant negative longitudinal piezoelectric effects. The data mining of a first-principles-based database of piezoelectrics reveals that this effect is a general phenomenon. The origin of this unusual piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces. Moreover, ferroelectrics with negative longitudinal piezoelectric coefficients show anomalous pressure-enhanced ferroelectricity. Our results offer design principles to aid the search for new piezoelectrics for novel electromechanical device applications.

The lattice parameter of magnesiowustite (Mg0.6Fe0.4)O has been measured up to a pressure of 30 GPa and a temperature of 800 K, using an external heated diamond anvil cell and diffraction using X-rays from a synchrotron source. The experiments were conducted under quasi-hydrostatic condition, using neon as a pressure transmitting medium. The experimental P-V-T data were fitted to a thermal-pressure model with the isothermal bulk modulus at room temperature K(T0) = 157 GPa, (partial derivative K(T0/partial derivative P)T = 4, (partial derivative K(T)/partial derivative T)p = -2.7(3) x 10(-2) GPa/K, (partial derivative K(T)/partial derivative T)v = -0.2(2) x 10(-2) GPa/K, and the Anderson-Gruneisen parameter delta(T) = 4.3(5) above the Debye temperature. The data were also fitted to the Mie-Gruneisen thermal equation of state. The least-squares fit yields the Debye temperature theta(D0) = 500(20) K, the Gruneisen parameter gamma-0 = 1.50(5), and the volume dependence q = 1.1 (5). Both thermal-pressure models give consistent P-V-T relations for magnesiowustite to 140 GPa and 4000 K. The P-V-T relations for magnesiowustite were also calculate by using a modified high-temperature Birch-Murnaghan equation of state with a delta(T) of 4.3. The results are consistent with those calculated by using the thermal-pressure model and the Mie-Gruneisen relation to 140 GPa and 3000 K.

Theoretical predictions of ZnO:MnO solid solutions (abbreviated here as ZMO) with the rocksalt-type structure suggest improved visible light absorption and suitable band edge positions for the overall water splitting reaction, but experimental efforts to produce such phases are limited by the low solubility of Zn within this structure type. Here, we produce solid solutions of ZnxMn1-xO with x = 0.5 and 0.3 in the metastable rocksalt phase, using high-pressure and high-temperature (HPHT) techniques. X-ray diffraction and electron microscopy methods were employed to determine the crystal structure, chemical composition, and homogeneity on the submicron scale. The solid solutions exhibit increased optical absorbance in the visible spectral range as compared to those of the parent oxides ZnO and MnO. Our theoretical calculations for ZnxMn1-xO with x = 0.5, 0.25 predict band gaps of 2.53 and 2.98 eV, respectively, with an unusually large band gap bowing. Our calculations also show small effective electron mass for these materials indicating their potential for solar energy applications. Initial photoelectrochemical tests reveal that ZMO solid solutions are suitable for water oxidation and warrant further experimental optimization.

The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth's earliest stages of formation(1-8). Xe isotopic anomalies have been linked to degassing during accretion(2-4), but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts(9,10) and their long-term preservation in the mantle. Here we describe measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated He-3/He-4 ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Our measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts(2-4,9,10). Portions of mantle involved in early high-pressure core formation would also be rich in FeO11,12, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs(13) with high He-3/He-4 ratios.

FeO is an insulator with anti-ferromagnetic (AFM) spin ordering at ambient pressure. At increased external pressure, the Neel temperature of FeO first increases at the pressure below 40 GPa. Experiments predict that the AFM ordering will collapse above 80 GPa, but the mechanism of the high pressure magnetic collapse is still unknown. Using the combination of density functional theory and dynamical mean-field theory (DFT+DMFT), the nature of the magnetic collapse of FeO is examined and its magnetic phase diagram up to 180 GPa is discussed.

Tetracyanomethane, C(CN)(4), is a tetrahedral molecule containing a central sp(3) carbon that is coordinated by reactive nitrile groups that could potentially transform to an extended CN network with a significant fraction of sp(3) carbon. High-purity C(CN)(4) was synthesized, and its physiochemical behavior was studied using in situ synchrotron angle-dispersive powder X-ray diffraction (PXRD) and Raman and infrared (IR) spectroscopies in a diamond anvil cell (DAC) up to 21 GPa. The pressure dependence of the fundamental vibrational modes associated with the molecular solid was determined, and some low-frequency Raman modes are reported for the first time. Crystalline molecular C(CN)(4) starts to polymerize above similar to 7 GPa and transforms into an interconnected disordered network, which is recoverable to ambient conditions. The results demonstrate feasibility for the pressure-induced polymerization of molecules with premeditated functionality.

The absence of low-thermal gradients in old metamorphic rocks (< 350 degrees C GPa(-1)) has been used to argue for a fundamental change in the style of plate tectonics during the Neoproterozoic Era. Here, we report data from an eclogite xenolith in Paleoproterozoic carbonatite in the North China craton that argues for cold subduction as early as 1.8 Ga. The carbonatite has a sediment-derived C isotope signature and enriched initial Sr-Nd isotope composition, indicative of ocean-crust components in the source. The eclogite records peak metamorphic pressures of 2.5-2.8 GPa at 650-670 degrees C, indicating a cold thermal gradient, 250(+/- 15)degrees C GPa(-1). Our data, combined with old low-temperature events in the West African and North American cratons, reveal a global pattern that modern-style subduction may have been established during the Paleoproterozoic Era. Paleoproterozoic carbonatites are closely associated with granulites and eclogites in orogens worldwide, playing a critical role in the Columbia supercontinent amalgamation and deep carbon cycle through time.

The rotational and translational dynamics of molecular hydrogen trapped within beta-hydroquinone clathrate (H-2@beta-HQ)-a practical example of a quantum particle trapped within an anisotropic confining potential-were investigated using inelastic neutron scattering and Raman spectroscopy. High-resolution vibrational spectra, including those collected from the VISION spectrometer at Oak Ridge National Laboratory, indicate relatively strong attractive interaction between guest and host with a strikingly large splitting of rotational energy levels compared with similar guest-host systems. Unlike related molecular systems in which confined H-2 exhibits nearly free rotation, the behavior of H-2@beta-HQ is explained using a two-dimensional (2D) hindered rotor model with barrier height more than 2 times the rotational constant (-16.2 meV).

Using the dynamic compression technique, the sound velocities of Fe-11.8wt%S were measured up to 211.4 (4.5)GPa and 6,150K. Discontinuities both in shock velocity and sound velocity indicate that Fe-11.8wt%S completely melts at a pressure of 111.3 (2.3)GPa. By the energy conservation law, the calculated liquidus temperature is about 2,500 (300)K. Extrapolated to the inner-core boundary based on the Lindeman law, the liquidus temperature of Fe-11.8wt%S is 4,300 (300)K. We developed a thermodynamic model fit to the experimental data, which allows calculation of the densities and sound velocities of liquid Fe-S under core conditions. For liquid Fe-11.8wt%S and Fe-10wt%S, good agreement was achieved between the extrapolations using our model and experimental measurements at very low pressure. Under the conditions of the outer core, the densities and bulk sound velocities of Fe-10wt%S provide a good fit to observed seismic profiles of Earth's core. Our results imply that an upper limit of 10wt% S content in Earth's core satisfies the geophysical constraints. Simultaneously considering other geochemical constraints, the outer core may contain about 6wt% sulfur and 4wt% silicon.