Thallium under extreme compression

On the Electride Nature of Na-hP4

Early quantum mechanical models suggested that pressure drives solids towards free-electron metal behavior where the ions are locked into simple close-packed structures. The prediction and subsequent discovery of high-pressure electrides (HPEs), compounds assuming open structures where the valence electrons are localized in interstitial voids, required a paradigm shift. Our quantum chemical calculations on the iconic insulating Na-hP4 HPE show that increasing density causes a 3s→3pd electronic transition due to Pauli repulsion between the 1s2s and 3s states, and orthogonality of the 3pd states to the core. The large lobes of the resulting Na-pd hybrid orbitals point towards the center of an 11-membered penta-capped trigonal prism and overlap constructively, forming multicentered bonds, which are responsible for the emergence of the interstitial charge localization in Na-hP4. These multicentered bonds facilitate the increased density of this phase, which is key for its stabilization under pressure.

Melting line of calcium characterized by in situ LH-DAC XRD and first-principles calculations

In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.

Powder conductor for pressure calibration applied to large volume press under high pressure

Pressure is the core of high-pressure science and technology, and the accuracy of pressure calibration is of much importance for high-pressure experiments and production. Although the pressure limit of the large volume press (LVP) continues to increase, there are no well solutions for in situ pressure calibration. In this study, using in situ high-pressure electrical performance measurement technology, two ideal calibration standard materials in powder conductors, cadmium phosphide (Cd₃P₂) and zinc telluride (ZnTe) with stable physical and chemical properties and obvious resistance change, are applied to pressure calibration in the LVP. In situ high-pressure synchrotron radiation x-ray diffraction was used to verify the phase transition pressure point of Cd₃P₂. The introduction of powder conductors for pressure calibration commits to establish a pressure system, which is safer, more stable to operate, and more accurate in experimental measurements for the LVP.

Pressure-induced electrides and metallic phases in the Y-Cl system

Pressure can profoundly change the electronic structure, leading to the formation of new phases and materials with exotic properties. Herein, using evolutionary algorithms and density functional theory, we systematically investigate the behaviour of materials in the yttrium-chlorine binary system under pressure. Electrons are found to be spatially confined at low pressures in yttrium chlorides and tend to form new electrides. In particular, a novel yttrium chloride, Y₃Cl₂, is predicted to be thermodynamically and lattice dynamically stable at approximately 10 GPa. Further analyses of the electron localization function and partial charge density identify trigonal Y₃Cl₂as a new 2D high-pressure electride with partially localized electrons contributing to the conduction. By further increasing the pressure, electrons in the yttrium-chlorine binary system tend to delocalize with the electrides decomposing into two new compounds (Y₂Cl and YCl₂) and a new YCl phase (space groupP6₃/mmc) above 20 GPa. These newly discovered phases are all metallic in their stable pressure range according to band structure simulations without interstitial electron localization. The discovery of these unconventional yttrium chlorides may inspire strategies to search for low-pressure electrides in other rare-earth halogenide systems.

The phase diagram of Ti-6Al-4V at high-pressures and high-temperatures

We report results from a series of diamond-anvil-cell synchrotron x-ray diffraction and large-volume-press experiments, and calculations, to investigate the phase diagram of commercial polycrystalline high-strength Ti-6Al-4V alloy in pressure-temperature space. Up to ∼30 GPa and 886 K, Ti-6Al-4V is found to be stable in the hexagonal-close-packed, orαphase. The effect of temperature on the volume expansion and compressibility ofα-Ti-6Al-4V is modest. The martensiticα→ω(hexagonal) transition occurs at ∼30 GPa, with both phases coexisting until at ∼38-40 GPa the transition to theωphase is completed. Between 300 K and 844 K theα→ωtransition appears to be independent of temperature.ω-Ti-6Al-4V is stable to ∼91 GPa and 844 K, the highest combined pressure and temperature reached in these experiments. Pressure-volume-temperature equations-of-state for theαandωphases of Ti-6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in theω-Ti-6Al-4V on decompression, with the hexagonal structure reverting back to theαphase at pressures below ∼9 GPa at room temperature, and at a higher pressure at elevated temperatures. Based on our data, we estimate the Ti-6Al-4Vα-β-ωtriple point to occur at ∼900 K and 30 GPa, in good agreement with our calculations.

Structural Ordering in Liquid Gallium under Extreme Conditions

The atomic-scale structure, melting curve, and equation of state of liquid gallium has been measured to high pressure (p) and high temperature (T) up to 26 GPa and 900 K by in situ synchrotron x-ray diffraction. Ab initio molecular dynamics simulations up to 33.4 GPa and 1000 K are in excellent agreement with the experimental measurements, providing detailed insight at the level of pair distribution functions. The results reveal an absence of dimeric bonding in the liquid state and a continuous increase in average coordination number n[over ¯]_{Ga}^{Ga} from 10.4(2) at 0.1 GPa approaching ∼12 by 25 GPa. Topological cluster analysis of the simulation trajectories finds increasing fractions of fivefold symmetric and crystalline motifs at high p-T. Although the liquid progressively resembles a hard-sphere structure towards the melting curve, the deviation from this simple description remains large (≥40%) across all p-T space, with specific motifs of different geometries strongly correlating with low local two-body excess entropy at high p-T.

The HXD95: a modified Bassett-type hydrothermal diamond-anvil cell for in situ XRD experiments up to 5 GPa and 1300 K

A new diamond-anvil cell apparatus for in situ synchrotron X-ray diffraction measurements of liquids and glasses, at pressures from ambient to 5 GPa and temperatures from ambient to 1300 K, is reported. This portable setup enables in situ monitoring of the melting of complex compounds and the determination of the structure and properties of melts under moderately high pressure and high temperature conditions relevant to industrial processes and magmatic processes in the Earth's crust and shallow mantle. The device was constructed according to a modified Bassett-type hydrothermal diamond-anvil cell design with a large angular opening (θ = 95°). This paper reports the successful application of this device to record in situ synchrotron X-ray diffraction of liquid Ga and synthetic PbSiO3 glass to 1100 K and 3 GPa.

A Short Review of Current Computational Concepts for High-Pressure Phase Transition Studies in Molecular Crystals

High-pressure chemistry of organic compounds is a hot topic of modern chemistry. In this work, basic computational concepts for high-pressure phase transition studies in molecular crystals are described, showing their advantages and disadvantages. The interconnection of experimental and computational methods is highlighted, showing the importance of energy calculations in this field. Based on our deep understanding of methods’ limitations, we suggested the most convenient scheme for the computational study of high-pressure crystal structure changes. Finally, challenges and possible ways for progress in high-pressure phase transitions research of organic compounds are briefly discussed.

Thermal equation of state of ruthenium characterized by resistively heated diamond anvil cell

The high-pressure and high-temperature structural and chemical stability of ruthenium has been investigated via synchrotron X-ray diffraction using a resistively heated diamond anvil cell. In the present experiment, ruthenium remains stable in the hcp phase up to 150 GPa and 960 K. The thermal equation of state has been determined based upon the data collected following four different isotherms. A quasi-hydrostatic equation of state at ambient temperature has also been characterized up to 150 GPa. The measured equation of state and structural parameters have been compared to the results of ab initio simulations performed with several exchange-correlation functionals. The agreement between theory and experiments is generally quite good. Phonon calculations were also carried out to show that hcp ruthenium is not only structurally but also dynamically stable up to extreme pressures. These calculations also allow the pressure dependence of the Raman-active E_2g mode and the silent B_1g mode of Ru to be determined.

Post-tilleyite, a dense calcium silicate-carbonate phase

Calcium carbonate is a relevant constituent of the Earth's crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca5(Si2O7)(CO3)2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6-9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties

We have investigated the high-pressure behavior of PbCrO₄. In particular, we have probed the existence of structural transitions under high pressure (at 4.5 GPa) by single-crystal X-ray diffraction and density functional theory calculations. The structural sequence of PbCrO₄ is different than previously determined. Specifically, we have established that PbCrO₄, under pressure, displays a monoclinic-tetragonal phase transition, with no intermediate phases between the low-pressure monoclinic monazite structure (space group P2₁/ n) and the high-pressure tetragonal structure. The crystal structure of the high-pressure polymorph is, for the first time, undoubtedly determined to a tetragonal scheelite-type structure (space group I4₁/ a) with unit-cell parameters a = 5.1102(3) Å and c = 12.213(3) Å. These findings have been used for a reinterpretation of previously published Raman and optical-absorption results. Information of calculated infrared-active phonons will be also provided. In addition, the pressure dependence of the unit-cell parameters, atomic positions, bond distances, and polyhedral coordination are discussed. The softest and stiffest direction of compression for monazite-type PbCrO₄ are also reported. Finally, the theoretical pressure dependence of infrared-active modes is given, for the first time, for both polymorphs.

High-pressure/high-temperature phase diagram of zinc

The phase diagram of zinc (Zn) has been explored up to 140 GPa and 6000 K, by combining optical observations, x-ray diffraction, and ab initio calculations. In the pressure range covered by this study, Zn is found to retain a hexagonal close-packed (hcp) crystal symmetry up to the melting temperature. The known decrease of the axial ratio (c/a) of the hcp phase of Zn under compression is observed in x-ray diffraction experiments from 300 K up to the melting temperature. The pressure at which c/a reaches [Formula: see text] (≈10 GPa) is slightly affected by temperature. When this axial ratio is reached, we observed that single crystals of Zn, formed at high temperature, break into multiple poly-crystals. In addition, a noticeable change in the pressure dependence of c/a takes place at the same pressure. Both phenomena could be caused by an isomorphic second-order phase transition induced by pressure in Zn. The reported melt curve extends previous results from 24 to 135 GPa. The pressure dependence obtained for the melting temperature is accurately described up to 135 GPa by using a Simon-Glatzel equation: [Formula: see text], where P is the pressure in GPa. The determined melt curve agrees with previous low-pressure studies and with shock-wave experiments, with a melting temperature of 5060(30) K at 135 GPa. Finally, a thermal equation of state is reported, which at room-temperature agrees with the literature.

Structural Behavior of Natural Silicate-Carbonate Spurrite Mineral, Ca5(SiO4)2(CO3), under High-Pressure, High-Temperature Conditions

We report on high-pressure and high-temperature angle-dispersive synchrotron X-ray diffraction and high-pressure Raman data up to 27 GPa and 700 K for natural silicate carbonate Ca₅(SiO₄)₂(CO₃) spurrite mineral. No phase transition was found in the studied P-T range. The room-temperature bulk modulus of spurrite using Ne as the pressure-transmitting medium is B₀ = 77(1) GPa with a first-pressure derivative of B₀' = 5.9(2). The structure compression is highly anisotropic, the b axis being approximately 30% more compressible than the a and c axes. The volumetric thermal expansivity value around 8 GPa was estimated to be 4.1(3) × 10^-5 K^-1. A comparison with intimately related minerals CaCO₃ calcite and aragonite and β-Ca₂SiO₄ larnite shows that, as the composition and structural features of spurrite suggest, its compressibility and thermal expansivity lie between those of the silicate and carbonate end members. The crystal chemistry and thermodynamic properties of spurrite are discussed.

Role of relativity in high-pressure phase transitions of thallium

We demonstrate the relativistic effects in high-pressure phase transitions of heavy element thallium. The known first phase transition from h.c.p. to f.c.c. is initially investigated by various relativistic levels and exchange-correlation functionals as implemented in FPLO method, as well as scalar relativistic scheme within PAW formalism. The electronic structure calculations are interpreted from the perspective of energetic stability and electronic density of states. The full relativistic scheme (FR) within L(S)DA performs to be the scheme that resembles mostly with experimental results with a transition pressure of 3 GPa. The s-p hybridization and the valence-core overlapping of 6s and 5d states are the primary reasons behind the f.c.c. phase occurrence. A recent proposed phase, i.e., a body-centered tetragonal (b.c.t.) phase, is confirmed with a small distortion from the f.c.c. phase. We have also predicted a reversible b.c.t. → f.c.c. phase transition at 800 GPa. This finding has been suggested that almost all the III-A elements (Ga, In and Tl) exhibit the b.c.t. → f.c.c. phase transition at extremely high pressure.