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The structural transformation of FePO4 from the trigonal berlinite phase to the orthorhombic CrVO4 phase has been studied using neutron diffraction at high pressure and high-temperature. The berlin...

[EN] Divalent metal carbonates play an important role in Earth's carbon cycle, but the effect of chemical substitution is still poorly known. In this work, we have studied the structural and vibrational properties of natural mineral gaspeite (Ni0.75Mg0.22Ca0.03CO3) under high pressure and temperature using in situ synchrotron X-ray diffraction and Raman spectroscopy in diamond-anvil cells. These experiments have been complemented by ab initio simulations. Synchrotron high-pressure XRD measurements at room temperature using He as the pressure transmitting medium have shown that the calcite-type structure is stable up to 23.3 GPa. A bulk modulus at zero pressure of B-0 = 105(2) GPa with B-0' = 7.4(3) has been obtained from the experimental equation of state. This result indicates that gaspeite is the most incompressible of all the divalent metal carbonates. The axial compressibilities in gaspeite have a high anisotropy, with the c axis about 3 times more compressible than the a axis. We have followed under pressure the Raman modes of gaspeite up to 19 GPa. Moreover, Ni0.75Mg0.22Ca0.03CO3 compressed at 5.5 GPa has been heated up to 840 K, revealing that this carbonate is stable in these conditions. The equation of state of magnesian gaspeite at this temperature has been determined. Our ab initio calculations on NiCO3 at zero pressure and 5.5 GPa have allowed us to estimate the thermal expansion coefficients. The obtained values show that the anisotropy in the thermal expansion is comparable to that found in axial compressibility., Authors thank the financial support from the Spanish Ministerio de Ciencia, Innovacion y Universidades (MICINN) under projects MALTA Consolider Ingenio 2010 network (MAT2015-71070-REDC) and PGC2018-097520-A-I00 (co-financed with EU FEDER funds) and from the Generalitat Valenciana under project PROMETEO/2018/123. D.S.-P. and A.O.R. acknowledge the financial support of the Spanish MINECO for the RyC-2014-15643 and RyC-2016-20301 Ramo ' n y Cajal Grants, respectively. C.P. and O.G. acknowledge the financial support from the Spanish Ministerio de Economia y Competitividad (MINECO projects FIS2017-83295-P and MAT2016-75586-C4-1-P/2-P, respectively). We thank David Vie for carrying out the GTA measurements. Authors also thank Dr. Nicolescu and the Mineralogy and Meteroritic Department of the Yale Peabody Museum of Natural History for providing the mineral samples and ALBA-CELLS synchrotron for providing beamtime under experiment 2018082948. A.O.R. thanks the MALTA Consolider Supercomputing Centre and Compute Canada for computational resources. British Crown Owned Copyright 2020/AWE. Published with permission of the Controller of Her Britannic Majesty's Stationery Office.

Physical review / B 105(17), 174104 (2022). doi:10.1103/PhysRevB.105.174104, Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal to a pressure of 205 GPa. Between 20 and 165 GPa only the oC4 ($α$-uranium) phase is observed, in agreement with previous studies. At 171(5) GPa we find a transition to a tetragonal tI2 phase which is isostructural with the high-pressure post-oC4 phase seen in the neighboring lanthanide cerium above 12 GPa, and with the high-pressure phase of the actinide thorium seen above 100 GPa. Electronic structure calculations determine the oC4 → tI2 transition to occur at 130 GPa at 0 K, but find another phase, with the hP1 (simple hexagonal) structure, to have a lower enthalpy than both the oC4 and tI2 structures above 20 GPa at 0 K., Published by Inst., Woodbury, NY

Gadolinium has long been believed to undergo a high-pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended $s$-orbital to the compact $f$-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the $f$-electrons [Fabbris et al., Phys. Rev. B 88, 245103 (2013)]. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high-pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using x-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes $hcp\ensuremath{\rightarrow}9R\ensuremath{\rightarrow}\mathit{dhcp}\ensuremath{\rightarrow}\mathit{fcc}\ensuremath{\rightarrow}\mathit{d}\ensuremath{-}\mathit{fcc}$ is reproduced. The so-called ``volume-collapsed'' high-pressure phase is shown to be an unusual stacking of close-packed planes, with $\mathit{Fddd}$ symmetry and a density change of less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the $f$-electrons. The key result is that there is no XRD evidence for volume collapse in gadolinium. The sequence of phase transitions is well described by standard density functional theory. There is no need for special treatment of the $f$-electrons or evidence of $f$-electron bonding. Noting that in previous spectroscopic evidence there is no change in the $f$-electrons we conclude that high-pressure gadolinium has no complicated $f$-electron physics such as Mott-Hubbard, Kondo, or valence transitions.

Pressure (P)–volume (V)–temperature (T) relations of huntite [Mg3Ca(CO3)4] have been determined in situ up to 5 GPa and 500 °C using a resistive-heated diamond-anvil cell and synchrotron X-ray diff...

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 E2g mode and the silent B1g mode of Ru to be determined.

Both the trigonal (Berlinite-type, phase-I), and orthorhombic (CrVO4-type, phase-II) forms of FePO4 have been studied at high-pressure using neutron powder diffraction. Phase-II was prepared by a high-pressure, high-temperature synthetic route, and recovered to ambient conditions. We report the first high-pressure structural study of this phase up to 8.4GPa at room temperature. It is shown that with increasing pressure, the FeO6 octahedra become more regular and decrease in volume, while the PO4 tetrahedra become less regulars and increase in volume. For phase-I, similar changes in volume are determined, though without changes in distortion. At 2GPa a signature of amorphisation has been observed for phase-I with the appearance of broad phase-II reflections. To support the results of the high-pressure study, Raman spectroscopic and SQUID magnetometry studies have been performed.

Physical review / B 103(13), 134117 (2021). doi:10.1103/PhysRevB.103.134117, Angle-dispersive x-ray powder diffraction experiments have been performed on neodymium metal to a pressure of 302 GPa. Up to 70 GPa we observe the $hP4 → cF4 → hR24 → oI16 → hP3$ transition sequence reported previously. At 71(2) GPa we find a transition to a phase which has an orthorhombic structure (oF8) with eight atoms in the unit cell, space group Fddd. This structure is the same as that recently observed in samarium above 93 GPa, and is isostructural with high-pressure structures found in the actinides Am, Cf, and Cm. We see a further phase transition at 98(1) GPa to a phase with the orthorhombic α-U (oC4) structure, which remains stable up to 302 GPa, the highest pressure reached in this study. Electronic structure calculations find the same structural sequence, with calculated transition pressures of 66 and 88 GPa, respectively, for the $hP3 → F8$ and $oF8 → oC4$ transitions. The calculations further predict that oC4-Nd loses its magnetism at 100 GPa, in agreement with previous experimental results, and it is the accompanying decrease in enthalpy and volume that results in the transition to this phase. Comparison calculations on the oF8 and oC4 phases of Sm show that they both retain their magnetism to at least 240 GPa, with the result that oC4-Sm is calculated to have the lowest enthalpy over a narrow pressure region near 200 GPa at 0 K., Published by Inst., Woodbury, NY

Physical review / B 103(22), 224103 (2021). doi:10.1103/PhysRevB.103.224103, The high pressure phases of Rb have previously been investigated to 101 GPa, above which Rb is predicted to adopt a double-hexagonal close-packed (dhcp, Pearson hP4) structure similar to that already observed in cesium at 72 GPa. Previous ab initio structure searches have indicated that the hP4 phase should become stable in rubidium at 143GPa. We present data from static compression experiments on Rb up to 264(8)GPa, showing the onset of the hP4 phase at 207(6)GPa. The V/V$_0$ of ���0.121 measured at 264GPa constitutes the highest compression ratio (more than eightfold) at which structural information has been obtained from a metal using x-ray diffraction methods and is second only to x-ray measurements performed on hydrogen at V/V$_0$���0.094 at 190GPa. At these extreme compression ratios, the compressive behavior of rubidium shifts from that of a free electron metal to that of a regular d-block metal., Published by Inst., Woodbury, NY

Physical review / B 103(13), 134103 (2021). doi:10.1103/PhysRevB.103.134103, Vanadium is reported to undergo a pressure-induced bcc-rhombohedral phase transition at 30–70 GPa, with a transition pressure that is sensitive to the hydrostaticity of the sample environment. However, the experimental evidence for the structure of the high-pressure phase being rhombohedral is surprisingly weak. We have restudied vanadium under pressure to 154 GPa using both polycrystalline and single-crystal samples, and a variety of different pressure transmitting media (PTM). We find that only when using single-crystal samples does one observe a rhombohedral high-pressure phase; the high-pressure diffraction profiles from the polycrystalline samples do not fit a rhombohedral lattice, irrespective of the PTM used. The single-crystal samples reveal two rhombohedral phases, with a continuous transition between them, and distortions from cubic symmetry are much smaller than previously calculated., Published by Inst., Woodbury, NY

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, or α phase. The effect of temperature on the volume expansion and compressibility of α–Ti-6Al-4V is modest. The martensitic α → ω (hexagonal) transition occurs at ∼30 GPa, with both phases coexisting until at ∼38–40 GPa the transition to the ω phase is completed. Between 300 K and 844 K the α → ω transition 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 the α and ω phases of Ti-6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in the ω–Ti-6Al-4V on decompression, with the hexagonal structure reverting back to the α phase 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-4V α–β–ω triple point to occur at ∼900 K and 30 GPa, in good agreement with our calculations.

Physical review / B 102(9), 094104 (2020). doi:10.1103/PhysRevB.102.094104, Angle-dispersive x-ray powder diffraction experiments have been performed on yttrium metal up to 183 GPa.We find that the recently discoveredoF16 structure observed in the high-Ztrivalent lanthanides is also adoptedby yttrium above 106 GPa, pressures where it has a superconducting temperature of∼20 K. We have also refinedboth tetragonal and rhombohedral structures against the diffraction data from the preceding “distorted-fcc” phaseand we are unable to state categorically which of these is the true structure of this phase. Finally, analysis ofyttrium’s equation of state reveals a marked change in the compressibility upon adoption of theoF16 structure,after which the compression is that of a “regular” metal. Electronic structure calculations ofoF16-Y confirm itsstability overoF8 structure seen in Nd and Sm, and provide insight into the nature of the shift of orbital characterfromstodunder compression., Published by Inst., Woodbury, NY

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 ${\overline{n}}_{\mathrm{Ga}}^{\mathrm{Ga}}$ from 10.4(2) at 0.1 GPa approaching $\ensuremath{\sim}12$ by 25 GPa. Topological cluster analysis of the simulation trajectories finds increasing fractions of fivefold symmetric and crystalline motifs at high $p\text{\ensuremath{-}}T$. Although the liquid progressively resembles a hard-sphere structure towards the melting curve, the deviation from this simple description remains large ($\ensuremath{\ge}40%$) across all $p\text{\ensuremath{-}}T$ space, with specific motifs of different geometries strongly correlating with low local two-body excess entropy at high $p\text{\ensuremath{-}}T$.

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.

Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40-60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given. The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase. Funding Agencies|Spanish Ministerio de Ciencia, Innovacion y Universidades [MAT2016-75586-C4-1-P, PGC2018-097520-A-100, RED2018-102612-T]; Generalitat ValencianaGeneralitat Valenciana [Prometeo/2018/123 EFIMAT]; Spanish governmentSpanish Government [RyC-2014-15643]; US Department of EnergyUnited States Department of Energy (DOE) [DE-AC52-07NA27344]

Melting curve and phase diagram of vanadium under high-pressure and high-temperature conditions We report a combined experimental and theoretical study of the melting curve and the structural behavior of vanadium under extreme pressure and temperature. We performed powder x-ray-diffraction experiments up to 120 GPa and 4000 K, determining the phase boundary of the body-centered cubic-to-rhombohedral transition and melting temperatures at different pressures. Melting temperatures have also been established from the observation of temperature plateaus during laser heating, and the results from the density-functional theory calculations. Results obtained from our experiments and calculations are fully consistent and lead to an accurate determination of the melting curve of vanadium. These results are discussed in comparison with previous studies. The melting temperatures determined in this study are higher than those previously obtained using the speckle method, but also considerably lower than those obtained from shockwave experiments and linear muffin-tin orbital calculations. Finally, a high-pressure, high-temperature equation of state up to 120 GPa and 2800 K has also been determined.

Using synchrotron x-ray diffraction, we show that the long-accepted monoclinic structure of the ``collapsed'' high-pressure phases reported in seven lanthanide elements [Nd, Tb, Gd, Dy, Ho, Er, and (probably) Tm] is incorrect. In Tb, Gd, Dy, Ho, Er, and Tm we show that the collapsed phases have a 16-atom orthorhombic structure ($oF16$) not previously seen in the elements, whereas in Nd we show that it has an eight-atom orthorhombic structure ($oF8$) previously reported in several actinide elements. $oF16$ and $oF8$ are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides, and group-III elements (Sc and Y) are much more related than previously imagined. Electronic structure calculations of Tb, combined with quantum many-body corrections, confirm the experimental observation, and calculate that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an antiferromagnetic state between 60 and 80 GPa. We find that the magnetic properties of Tb survive to the highest pressures obtained in our experiments (110 GPa). Further calculations of the collapsed phases of Gd and Dy, again using the correct crystal structure, show the former to be a type-A antiferromagnet, whereas the latter is ferromagnetic.

Scientific reports 9(1), 7898 (2019). doi:10.1038/s41598-019-44326-9, 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 Ca$_5$(Si$_2$O$_7$)(CO$_3$)$_2$ 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, Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Metal–organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure–temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure. The simultaneous high-pressure and high-temperature phase diagram of two MOFs, ZIF-4 and ZIF-62, is mapped. Crystalline, pressure- and temperature-amorphous, and liquid states are found, while melting temperature is found to decrease with pressure.

We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to ∼22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic oC4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the α-cF4–oC4–tI2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the oC4 phase, and 1–2 GPa lower than previously reported. We find the α-cF4 → tI2 phase boundary to have a positive gradient of 280 K (GPa)−1, less steep than the 670 K (GPa)−1 reported previously, and find the oC4 → tI2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2–3 GPa in the transition pressures at which the oC4 → tI2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the α-cF4 → tI2 is not second order at all temperatures up to 820 K.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Inorganic chemistry 57(21), 14005 - 14012 (2018). doi:10.1021/acs.inorgchem.8b01808, The zircon to scheelite phase boundary of ErVO$_4$ has been studied by high-pressure and high-temperature powder and single-crystal X-ray diffraction. This study has allowed us to delimit the best synthesis conditions of its scheelite-type phase, determine the ambient-temperature equation of state of the zircon and scheelite-type structures, and obtain the thermal equation of state of the zircon-type polymorph. The results obtained with powder samples indicate that zircon-type ErVO$_4$ transforms to scheelite at 8.2 GPa and 293 K and at 7.5 GPa and 693 K. The analyses yield bulk moduli K$_0$ of 158(13) GPa for the zircon phase and 158(17) GPa for the scheelite phase, with a temperature derivative of dK$_0$/dT = −[3.8(2)] × 10$^{–3}$ GPa K$^{–1}$ and a volumetric thermal expansion of $α_0$ = [0.9(2)] × 10$^{–5}$ K$^{–1}$ for the zircon phase according to the Berman model. The results are compared with those of other zircon-type vanadates, raising the need for careful experiments with highly crystalline scheelite to obtain reliable bulk moduli of this phase. Finally, we have performed single-crystal diffraction experiments from 110 to 395 K, and the obtained volumetric thermal expansion $(α_0 )$ for zircon-type ErVO$_4$ in the 300–395 K range is [1.4(2)] × 10$^{–5}$ K$^{–1}$, in good agreement with previous data and with our experimental value given from the thermal equation of state fit within the limits of uncertainty., Published by American Chemical Society, Washington, DC

The phase diagram of zinc (Zn) has been explored up to 140 GPa and 6000K, 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 300K up to the melting temperature. The pressure at which c/a reaches root 3 (approximate to 10GPa) 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: T-m = 690 K(1 + P/10.5 GPa)(0.76), 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., The authors are thankful for the financial support to this research from the Spanish Ministerio de Economía y Competitividad, the Spanish Research Agency, and the European Fund for Regional Development under Grant Nos. MAT2016- 75586-C4-1-P, and MAT2015-71070-REDC (MALTA Consolider). British Crown Owned Copyright 2018/AWE.

Metal-organic frameworks (MOFs) are microporous materials with huge potential as host structures for chemical processes, including retention, catalytic reaction, or separation of guest molecules. Structural collapse at high-pressure, and unusual behaviours at elevated temperatures, such as melting and transitions to liquid states, have recently been observed in the family. Here, we show that the effect of the application of simultaneous high-pressure and -temperature on a MOF can be understood in terms of silicate analogues, with crystalline, amorphous and liquid states occurring across the pressure - temperature phase diagram. The response of ZIF-62, the MOF on which we focus, to simultaneous pressure and temperature reveals a complex behaviour with distinct high- and low- density amorphous phases occurring over different regions of the pressure-temperature space. In-situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF-state expands significantly towards lower temperatures at intermediate, industrially achievable pressures. Our results imply a novel route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition upon heating at ambient pressure.

X-ray powder diffraction experiments using resistively heated diamond anvil cells have been conducted in order to establish the phase behavior of antimony up to 31 GPa and 835 K. The dip in the melting curve at 5.7 GPa and 840 K is identified as the triple point between the Sb-I, incommensurate Sb-II, and liquid phases. No evidence of the previously reported simple cubic phase was observed. Determination of the phase boundary between Sb-II and Sb-III suggests the existence of a second triple point in the region of 13 GPa and 1200 K. The incommensurate composite structure of Sb-II was found to remain ordered to the highest temperatures studies-no evidence of disordering of the guest-atom chains was observed. Indeed, the modulation reflections that arise from interactions between the host and guest subsystems were found to be present to the highest temperatures, suggesting such interactions remain relatively strong in Sb even in the presence of increased thermal motion. Finally, we show that the incommensurately modulated structure recently reported as giving an improved fit to diffraction data from incommensurate Ba-IV can be rejected as the structure of Sb-II using a simple density argument.

Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model., Part of the research was supported by the Spanish Government MINECO under Grants No. MAT2016-75586-C4-1/4P and No. MAT2015-71070-REDC.

[EN] The crystal structure of CO2-filled pure-SiO2 LTA zeolite has been studied at high pressures and temperatures using synchrotron-based X-ray powder diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them accommodated in the large alpha-cages and one in the beta-cages, giving a SiO2/CO2 stoichiometric ratio smaller than 2. The structure remains stable under pressure up to 20 GPa with a slight pressure-dependent rhombohedral distortion, indicating that pressure-induced amorphization is prevented by the insertion of guest species in this open framework. The ambient temperature lattice compressibility has been determined. In situ high-pressure resistive-heating experiments up to 750 K allow us to estimate the thermal expansivity at P approximate to 5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal expansion of the empty zeolite structure. No evidence of any chemical reaction was observed. The possibility of synthesizing a silicon carbonate at high temperatures and higher pressures is discussed in terms of the evolution of C-O and Si-O distances between molecular and framework atoms., The authors thank the financial support of the Spanish Ministerio de Economia y Competitividad (MINECO), the Spanish Research Agency (AEI), and the European Fund for Regional Development (FEDER) under Grant Nos. MAT2016-75586-C4-1-P, MAT2015-71842-P, Severo Ochoa SEV-2012-0267, and No.MAT2015-71070-REDC (MALTA Consolider). D.S.-P. and J.R.-F. acknowledge MINECO for a Ramon y Cajal and a Juan de la Cierva contract, respectively. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-1128799) and Department of Energy- GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR 11-57758. CO2 gas was also loaded at Diamond Light Source. Authors thank synchrotron ALBA-CELLS for beamtime allocation at MSPD line. British Crown Owned Copyright 2017/AWE. Published with permission of the Controller of Her Britannic Majesty's Stationery Office.

Using x-ray diffraction at the Linac Coherent Light Source x-ray free-electron laser, we have determined simultaneously and self-consistently the phase transitions and equation of state (EOS) of the lightest transition metal, scandium, under shock compression. On compression scandium undergoes a structural phase transition between 32 and 35 GPa to the same bcc structure seen at high temperatures at ambient pressures, and then a further transition at 46 GPa to the incommensurate host-guest polymorph found above 21 GPa in static compression at room temperature. Shock melting of the host-guest phase is observed between 53 and 72 GPa with the disappearance of Bragg scattering and the growth of a broad asymmetric diffraction peak from the high-density liquid.

The pressure-induced structural transformations in metasilicate MSiO3 glass (M = Mg or Ca) on cold-compression from ambient pressure to 17.5 GPa were investigated by neutron diffraction. The structure of the glass recovered to ambient conditions from a pressure of 8.2 or 17.5 GPa was also investigated by neutron or X-ray diffraction. The experimental work was complemented by molecular dynamics simulations using a newly-developed aspherical ion model. The results show network structures based predominantly on corner-sharing tetrahedral SiO4 units. At pressures up to ~8 GPa, there is little change to the network connectivity as described by the Qn speciation, where n denotes the number of bridging oxygen (BO) atoms per SiO4 tetrahedron. On compression of the glass to 17.5 GPa, the Mg–O coordination number increases from 4.5(1) to 6.2(1), and the Ca–O coordination number increases from 6.15(17) to 7.41(7). In both cases, the increased M-O coordination numbers are accompanied by an increased fraction of M-BO versus M-NBO connections, where NBO denotes a non-bridging oxygen atom. The results give the fraction of triple-bridging oxygen atoms as ~0.5% at 17.5 GPa, which does not support the formation of a substantial fraction of oxygen triclusters in either glass. The M-O coordination number of the recovered glass is larger than for the uncompressed material, which originates from an increased fraction of M-BO connections, and increases with the pressure from which the glass is recovered. The results suggest that the measured decrease in viscosity of molten MSiO3 on pressure increasing from ambient to ~8 GPa is not related to a large change in network polymerization, but to the appearance of higher-coordinated M-centred polyhedra that contain a larger fraction of weaker M-BO bonds. Keywords: Glass structure, Pressure, Neutron diffraction, X-ray diffraction, Molecular dynamics

The phase diagram of magnesium has been investigated to 211 GPa at 300 K, and to 105 GPa at 4500 K, by using a combination of x-ray diffraction and resistive and laser heating. The ambient pressure hcp structure is found to start transforming to the bcc structure at ∼45 GPa, with a large region of phase-coexistence that becomes smaller at higher temperatures. The bcc phase is stable to the highest pressures reached. The hcp-bcc phase boundary has been studied on both compression and decompression, and its slope is found to be negative and steeper than calculations have previously predicted. The laser-heating studies extend the melting curve of magnesium to 105 GPa and suggest that, at the highest pressures, the melting temperature increases more rapidly with pressure than previously reported. Finally, we observe some evidence of a new phase in the region of 10 GPa and 1200 K, where previous studies have reported a double-hexagonal-close-packed (dhcp) phase. However, the additional diffraction peaks we observe cannot be accounted for by the dhcp phase alone.

We present a combined theoretical and experimental study of the high-pressure behavior of thallium. X-ray diffraction experiments have been carried out at room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the pressure range of previous experiments. We have confirmed the hcp-fcc transition at 3.5 GPa and determined that the fcc structure remains stable up to the highest pressure attained in the experiments. In addition, HP-HT experiments have been performed up to 8 GPa and 700 K by using a combination of x-ray diffraction and a resistively heated diamond-anvil cell. Information on the phase boundaries is obtained, as well as crystallographic information on the HT bcc phase. The equation of state for different phases is reported. Ab initio calculations have also been carried out considering several potential high-pressure structures. They are consistent with the experimental results and predict that, among the structures considered in the calculations, the fcc structure of thallium is stable up to 4.3 TPa. Calculations also predict the post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa., Comment: 29 pages, 14 figures

We report results from an experimental and theoretical study of the ternary alloy Ti-6Al-4V to 221 GPa. We observe a phase transition to the hexagonal $\ensuremath{\omega}$ phase at approximately 30 GPa, and then a further transition to the cubic $\ensuremath{\beta}$ phase starting at 94--99 GPa. We do not observe the orthorhombic $\ensuremath{\gamma}$ and $\ensuremath{\delta}$ phases reported previously in pure Ti. Computational studies show that this sequence is possible only if there is significant local atomic ordering during the compression process, yet insufficient atomic diffusion to reach the phase-separated thermodynamic equilibrium state.

The electronic structures of the early transition metals are characterised by the relationship that exists between the occupied narrow d bands and the broad sp bands. Under pressure, the sp bands rise faster in energy, causing electrons to be transferred to the d bands (Gupta et al., 2008). This process is known as the s-d transition and it governs the structural properties of the transition metals. At ambient conditions, pure Ti crystallizes in the 2-atom hcp, or  phase crystal structure (space group P63/mmc) and has an axial ratio (c/a) ~ 1.58. Under pressure, the  phase undergoes a martensitic transformation at room temperature (RT) into the 3-atom hexagonal, or  phase structure (space group P6/mmm). The appearance of the ω phase at high pressure raises a number of scientific and engineering issues mainly because the  phase appears to be fairly brittle compared with the  phase, and this may significantly limit the use of Ti in high pressure applications. Furthermore, after pressure treatment the ω phase appears to be fully, or at least, partially recoverable at ambient conditions, thus raising questions as to which is the lowest thermodynamically stable crystallographic phase of Ti at RT and pressure.

We report results from an experimental and theoretical study of the room temperature (RT) compression of the ternary alloy Ti-6Al-4V. In this work, we have extended knowledge of the equation of state (EOS) from 40 GPa to 221 GPa, and observed a different sequence of phase transitions to that reported previously for pure Ti.

We report on a method of light–shift engineering where an additional laser is used to tune the energy of an excited atomic state. We show that the technique can be used to enhance the loading of a deep optical lattice or selectively load specific sites in a lattice.

We report on a method of light-shift engineering where an auxiliary laser is used to tune the atomic transition frequency. The technique is used to selectively load a specific region of an optical lattice. The results are explained by calculating the differential light-shift of each hyperfine state. We conclude that the remarkable spatial selectivity of light-shift engineering using an auxiliary laser provides a powerful technique to prepare ultra-cold trapped atoms for experiments on quantum gases and quantum information processing., 4 pages, 5 figures