Three different sodium-silicon clathrate compounds-Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6-were obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900-1100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi6, which crystallizes in the orthorhombic Eu4Ga8Ge16 structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi6), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous reports of their synthesis may be viewed as nonequilibrium, precursor-based routes to high-pressure phases at low-pressure conditions.

Diamond nanocrystals were synthesized catalyst-free from nano-porous carbon at high pressure and high temperature (HPHT). The synthesized nanocrystals have tunable diameters between 50 and 200 nm. The nanocrystals are dispersible in organic solvents such as acetone and are isotropic in nature as seen by dynamic light scattering.

We predict a tetragonal ground state for perovskite-structured PbCrO3 from density functional theory (DFT) + U calculations, and explain its anomalously large volume. The predicted structure is stabilized due to orbital ordering of Cr d in the presence of a large tetragonal crystal field, mainly due to off-centering of the Pb atom. At higher pressures (smaller volumes) there is a first-order transition to a cubic phase where the Cr-d orbitals are orbitally liquid. This phase transition is accompanied by a similar to 11.5% volume collapse, one of the largest known for transition-metal oxides. The large ferroelasticity and its strong coupling to the orbital degrees of freedom could be exploited to form potentially useful magnetostrictive materials.

The high-pressure and high-temperature formation and stability of recently discovered magnesium carbide Mg2C were studied by in situ X-ray diffraction up to 20 GPa and 1550 K. The insights into the thermodynamics of Mg2C under extreme conditions, and its metastability at 0.1 MPa and 300 K, were provided by ab initio calculations of total energies and phonon density of states as a function of pressure. We illustrate how the compound found occasionally in high-pressure experiments could be systematically predicted and discovered in a time-saving way. Similar theoretical approaches can be useful for prediction of synthesis conditions and recovery of new solids.

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.