We use molecular dynamics with a first-principles-based shell model potential to study pyroelectricity in lithium niobate. We find that the primary pyroelectric effect is dominant, and pyroelectricity can be understood simply from the anharmonic change in crystal structure with temperature and the Born effective charges on the ions. This opens an experimental route to study pyroelectricity, as candidate pyroelectric materials can be studied with x-ray diffraction as a function of temperature in conjunction with theoretical effective charges. We also predict an appreciable pressure effect on pyroelectricity, so that chemical pressure, i.e., doping, could enhance the pyroelectric and electrocaloric effects.

Subduction zone plays a significant role in deep carbon cycle. Solid-state reaction "dolomite (ankerite) = magnesite (siderite) + aragonite" has been first identified in two carbonated eclogites from southwestern Tianshan orogenic belt, China, which is the largest known ultra-high-pressure (UHP) oceanic subduction zone so far. In comparison to some other carbonates from high-pressure metamorphic rocks, carbonates from southwestern Tianshan carbonated eclogites and metapelites have high Fe content. In order to understand the effect of Fe on the stability of dolomite at high pressure, several experiments have been carried out using multi-anvil apparatus at pressures up to 8 GPa and temperature between 600 and 1200 degrees C. Both petrological observation and experimental study indicate that the stability of dolomite dramatically deceases with increasing Fe content in the solid solution of dolomite and ankerite at high pressure, and the peak pressure and temperature for the studied eclogites from southwestern Tianshan, China, are 2.5 GPa and 550 degrees C. The P-T conditions estimated according to dolomite decomposition in previous petrologic works should be revised by taking into account of the effect of Fe on the stability of dolomite at high pressure. More attention should also be paid to the effect of Fe on the stability of Fe-bearing carbonate during the decarbonation of the deep carbon cycle in the subduction zone. (C) 2014 Elsevier Ltd. All rights reserved.

The molecular interactions and structural behavior of a previously unexplored clathrate system, hydrogen-loaded beta-hydroquinone (beta-HQ+H-2), were investigated under high pressure with synchrotron X-ray diffraction and Raman/infrared spectroscopies. The beta-HQ+H-2 system exhibits coupling of two independently rare phenomena: multiple occupancy and negative compressibility. The number of H-2 molecules per cavity increases from one to three, causing unit cell volume increase by way of unique crystallographic interstitial guest positioning. We anticipate these occupancy-derived trends may be general to a range of inclusion compounds and may aid the chemical and crystallographic design of both high-occupancy hydrogen storage clathrates and novel, variable-composition materials with tunable mechanical properties.

We combine high-pressure x-ray diffraction, high-pressure Raman scattering, and optical microscopic methods to investigate a series of PMN-xPT solid solutions (x=0.2, 0.3, 0.33, 0.35,0.37,0.4) in diamond anvil cells up to 20 GPa at 300 K. The Raman spectra show a new peak centered at 380 cm(-1) above 6 GPa for all samples, consistent with previous observations. X-ray diffraction measurements are consistent with this spectral change indicating a structural phase transition. For example, we find that the triplet at the pseudocubic [220] Bragg peak merges into a doublet above 6 GPa. The analyzed results indicate that the morphotropic phase boundary (x= 0.33 to 0.37) with monoclinic symmetry persists up to 7 GPa. The pressure dependence of ferroelectric domains in PMN-0.32PT single crystals was observed with a polarizing optical microscope. The domain wall density decreases with pressure and the domains disappears at modest pressure of 3 GPa. This indicates that the high pressure phase of PMN-PT is not a macroscopic polar state. We suggest a phase diagram for the PMN-xPT solid solutions.

We have conducted a series of melting experiments in the Fe-C system at pressures up to 25 GPa in the temperature range of 1473-2073 K. The results define the phase relations at several pressures, including the eutectic temperature and composition as a function of pressure, carbon partitioning between solid iron and liquid, and change of melting relations involving iron carbides. In order to interpolate and extrapolate the phase relations over a wide pressure and temperature range, we have established a comprehensive thermodynamic model in the Fe-C binary system. The calculated phase diagrams at pressures of 5, 10, and 20 GPa reproduce the experimental data, including the solubility of carbon in solid iron and the effect of pressure on the eutectic temperature and composition. The formation of Fe7C3 at pressures above 5 GPa is correctly modeled and the change of phase relations in the Fe-C system between 5 and 10 GPa is captured in the model. The model provides predictions of the phase relations at 136 GPa and 330 GPa, based on existing knowledge of the thermochemistry of the system at lower pressure. The calculated phase relations can be used to understand the role of carbon during inner core crystallization, predicting carbon distribution between the inner and outer cores and mineralogy of the solid inner core. (C) 2014 Elsevier B.V. All rights reserved.

Based on a shell model potential obtained from first principles calculations, we performed molecular dynamics simulations to investigate the electromechanical response of a ferroelectric perovskite under finite temperature and electric field. We characterize the switching paths by which a homogeneous polarization reorientation process would take place in the prototypical ferroelectric PbTiO(3). We observe the hysteresis loop and butterfly electric-strain curve and obtain finite temperature piezoelectric coefficients in good agreement with experiments. (C) 2011 American Institute of Physics. [doi:10.1063/1.3646377]

A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (alpha-Mg2C3), the new monoclinic C2/m structure (beta-Mg2C3) contains linear C-3(4-) chains that are isoelectronic with CO2. Unlike alpha-Mg2C3, which contains alternating layers of C-3(4-) chains oriented in opposite directions, all C-3(4-) chains within beta-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of beta-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C=C stretching near 1200 cm(-1) and C-13 resonances confirming the presence of the rare allylenide anion.

The oxygen fugacity (fO(2)) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 degrees C using iron iridium and iron platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite magnetite quartz oxygen buffer decrease with temperature from similar to-0.8 to similar to-1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized 102 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from similar to-1.5 log units at 1,100 degrees C to similar to-2.4 log units at 1,300 degrees C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mossbauer spectroscopy. A preliminary model was derived for determining the fO(2) of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/Sigma Fe ratio of garnet, using the equilibrium,

Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies. Here, we report the formation of a new orthorhombic allotrope of silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure; second, sodium was removed from the precursor by a thermal 'degassing' process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp(3) silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

We have performed molecular dynamics simulations using a shell model potential developed by fitting first-principles results to describe the behavior of the relaxor-ferroelectric (1 - x)PbMg1/3Nb2/3O3-xPbTiO(3) (PMN-xPT) as a function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-M-B-O-M-C-T. The high-temperature phase is cubic at all compositions. Our simulations give the slopes of the morphotropic phase boundaries, crucial for high-temperature applications. We find that the phase diagram of PMN-xPT can be understood within the random site model.