Electronic and electrocatalytic properties of PbTiO3: unveiling the effect of strain and oxygen vacancy.

First-principles calculations based on density-functional theory have been used to investigate the effect of biaxial strain and oxygen vacancy on the electronic, photocatalytic, and electrocatalytic properties of PbTiO3 oxide. Our results show that PbTiO3 has a high exciton binding energy and a band gap that can be easily moderated with different strain regimes. From a reactivity viewpoint, the highly exothermic adsorption of hydrogen atoms in both pristine and strained PbTiO3 structures does not make it a potential electrocatalyst for the hydrogen evolution reaction. Fortunately, the presence of oxygen vacancies on the PbTiO3 surface induces moderate adsorption energies, making the reduced PbTiO3 suitable for hydrogen evolution reaction processes.

Monoclinic-triclinic phase transition induced by pressure in fergusonite-type YbNbO4.

We have carried out a high-pressure study on monoclinic fergusonite-type YbNbO4. Synchrotron powder x-ray diffraction experiments and density-functional theory simulations have been performed. We found a gradual increase of symmetry under compression, with calculations predicting a second-order monoclinic-tetragonal transition at 15 GPa. However, experiments provided evidence of a transition at 11.6 GPa to a triclinic structure, described by space groupP1¯. The appearance of the triclinic phase, which according to calculations is dynamically unstable under hydrostatic conditions, seems to be related to the presence of non-hydrostatic stresses. The triclinic high-pressure phase remains stable up to 31.9 GPa and the phase transition is not reversible. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the high-pressure studies, we report ambient-pressure Raman and infrared spectroscopy measurements which have been compared with density-functional theory calculations.

Lattice dynamics of zircon-type NdVO4and scheelite-type PrVO4under high-pressure.

Zircon-type NdVO4and scheelite-type PrVO4have been studied by means of Raman spectroscopy up to approximately 20 GPa. In the first compound, zircon-scheelite and scheelite-fergusonite phase transitions are reported at 6.4(3) and 19.6(4) GPa, respectively. In the case of scheelite-type PrVO4, a reversible phase transition to a PbWO4-III structure is observed at 16.8(5) GPa. In both cases, a scheelite-type structure is recovered in a metastable state at low pressures. The pressure evolution of the Raman modes is also reported. Our experimental findings are supported byab initiocalculations, which allowed us to discuss the role of mechanic and dynamical instabilities in the phase transition mechanisms.

Evolution of Structural and Electronic Properties of TiSe2 under High Pressure.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3̅m1) to another trigonal structure (S-G-P3̅c1). Additionally, at 16 GPa, the P3̅c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3̅m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature (Tc) of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides.

Ab initiophase diagram of silver.

Silver has been considered as one of the simple one-phase materials that do not exhibit high pressure or high temperature polymorphism. The solid phase of Ag at ambient conditions is face-centered cubic (fcc) one. However, very recently another solid phase of silver, body-centered cubic (bcc) one, was detected in shock-wave (SW) experiments, and a more sophisticated phase diagram of Ag with the two solid phases was published by Smirnov. In this work, using a suite ofab initioquantum molecular dynamics (QMD) simulations based on the Z methodology which combines both direct Z method for the simulation of melting curves and inverse Z method for the calculation of solid-solid phase boundaries, we refine the phase diagram of Smirnov. We calculate the melting curves of both fcc-Ag and bcc-Ag and obtain an equation for the fcc-bcc solid-solid phase transition boundary. We also obtain the thermal equation of state of Ag which is in agreement with experimental data and QMD simulations. We argue that, despite being a polymorphic rather than a simple one-phase material, silver can be considered as an SW standard.

High-pressure monoclinic-monoclinic transition in fergusonite-type HoNbO4.

In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO4. Powder x-ray diffraction experiments andab initiodensity-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P21/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO4, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO4. According toab initiocalculations, the reported phase transition is related to a mechanical instability and a phonon softening.

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-ß-ωtriple point to occur at ∼900 K and 30 GPa, in good agreement with our calculations.

High-pressure characterization of multifunctional CrVO4.

The structural stability and physical properties of CrVO4 under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

The high-pressure, high-temperature phase diagram of cerium.

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.

Monoclinic-tetragonal-monoclinic phase transitions in Eu0.1Bi0.9VO4 under pressure.

The promising technological material Eu0.1Bi0.9VO4, has been studied for the first time at room-temperature under high-pressure, up to 24.9 GPa, by means of in situ angle dispersive powder x-ray diffraction (XRD). The compound undergoes two phase transitions at 1.9 and 16.1 GPa. The first transition is from the monoclinic fergusonite-type structure (space group I2/a) to a tetragonal scheelite-type structure (space group I41/a), being a ferroelastic-paraelastic transformation similar to that previously reported for isomorphic pristine BiVO4. The second phase transition is first-order in nature. The scheelite-type and the second high-pressure phase coexist in a wide pressure range. A monoclinic structure (space group P21/n) is proposed for the second high-pressure phase. Both transitions are reversible upon decompression. Details of the different crystal structures are reported. All the three observed structures are composed of network of VO4 tetrahedra and BiO8 (or EuO8 due to the substitution of Bi by Eu) dodecahedra. The room-temperature P-V equation of state and axial anisotropic compressibilities of the fergusonite and scheelite polymorphs are also given. In particular, the isothermal compressibility tensor for the monoclinic fergusonite phase has been calculated.

Phase stability and electronic structure of iridium metal at the megabar range.

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.

High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study.

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO4 are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

Experimental and theoretical study on the optical properties of LaVO4 crystals under pressure.

We report optical absorption and luminescence measurements in pure and trivalent neodymium (Nd3+) doped LaVO4 crystals up to 25 GPa. Nd3+ luminescence has been employed as a tool to follow the structural changes in the crystal. We also present band-structure and crystal-field calculations that provide the theoretical framework to accurately explain the observed experimental results. In particular, both optical absorption and luminescence measurements evidence that a phase transition takes place close to 12 GPa. They also provide information on the pressure dependence of the band-gap as well as the emission lines under compression. We found drastic changes in the optical properties of LaVO4 when the phase transition to a BaWO4-II structure occurs, which can be related to changes in the coordination number of vanadium ions and in the local sites of Nd3+. Reported results are analyzed in comparison with those of previous X-ray diffraction and Raman experiments, as well as with the features of related compounds. For the first time, a consistent picture is reported explaining the behavior of the optical and electronic properties of LaVO4 at high-pressures.

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.

High-pressure structural and vibrational properties of monazite-type BiPO4, LaPO4, CePO4, and PrPO4.

Monazite-type BiPO4, LaPO4, CePO4, and PrPO4 have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us with information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

New pressure-induced polymorphic transitions of anhydrous magnesium sulfate.

The effects of pressure on the crystal structure of the three known polymorphs of magnesium sulfate (α-MgSO4, ß-MgSO4, and γ-MgSO4) have been theoretically studied by means of density-functional theory calculations up to 45 GPa. We determined that under ambient conditions γ-MgSO4 is an unstable polymorph, which decomposes into MgO + SO3, and that the response of the other two polymorphs to hydrostatic pressure is non-isotropic. Additionally, we found that at all pressures ß-MgSO4 has a larger enthalpy than α-MgSO4. This indicates that ß-MgSO4 is thermodynamically unstable versus α-MgSO4 and predicts the occurrence of a ß-α phase transition under moderate compression. Our calculations also predict the existence under pressure of additional phase transitions to two new polymorphs of MgSO4, which we named δ-MgSO4 and ε-MgSO4. The α-δ transition is predicted to occur at 17.5 GPa, and the δ-ε transition at 35 GPa, pressures that nowadays can be experimentally easily achieved. All the predicted structural transformations are characterized as first-order transitions. This suggests that they can be non-reversible, and therefore the new polymorphs could be recovered as metastable polymorphs under ambient conditions. The crystal structure of the two new polymorphs is reported. In them, the coordination number of sulfur is four as in the previously known polymorphs, but the coordination number of magnesium is eight instead of six. In this article we will report the axial and bond compressibility for the four polymorphs of MgSO4. The pressure-volume equation of state of each phase is also given, which is described by a third-order Birch-Murnaghan equation. The values obtained for the bulk modulus are 62 GPa, 57 GPa, 102 GPa, and 119 GPa for α-MgSO4, ß-MgSO4, δ-MgSO4, and ε-MgSO4, respectively. Finally, the electronic band structure of these four polymorphs of MgSO4 has been calculated for the first time. The obtained results will be presented and discussed.

Structural and vibrational properties of corundum-type In2O3 nanocrystals under compression.

This work reports the structural and vibrational properties of nanocrystals of corundum-type In2O3 (rh-In2O3) at high pressures by using angle-dispersive x-ray diffraction and Raman scattering measurements up to 30 GPa. The equation of state and the pressure dependence of the Raman-active modes of the corundum phase in nanocrystals are in good agreement with previous studies on bulk material and theoretical simulations on bulk rh-In2O3. Nanocrystalline rh-In2O3 showed stability under compression at least up to 20 GPa, unlike bulk rh-In2O3 which gradually transforms to the orthorhombic Pbca (Rh2O3-III-type) structure above 12-14 GPa. The different stability range found in nanocrystalline and bulk corundum-type In2O3 is discussed.

On the high-pressure phase stability and elastic properties of ß-titanium alloys.

We have studied the compressibility and stability of different ß-titanium alloys at high pressure, including binary Ti-Mo, Ti-24Nb-4Zr-8Sn (Ti2448) and Ti-36Nb-2Ta-0.3O (gum metal). We observed stability of the ß phase in these alloys to 40 GPa, well into the ω phase region in the P-T diagram of pure titanium. Gum metal was pressurised above 70 GPa and forms a phase with a crystal structure similar to the η phase of pure Ti. The bulk moduli determined for the different alloys range from 97 ± 3 GPa (Ti2448) to 124 ± 6 GPa (Ti-16.8Mo-0.13O).

High-pressure structural, elastic, and thermodynamic properties of zircon-type HoPO4 and TmPO4.

Zircon-type holmium phosphate (HoPO4) and thulium phosphate (TmPO4) have been studied by single-crystal x-ray diffraction and ab initio calculations. We report on the influence of pressure on the crystal structure, and on the elastic and thermodynamic properties. The equation of state for both compounds is accurately determined. We have also obtained information on the polyhedral compressibility which is used to explain the anisotropic axial compressibility and the bulk compressibility. Both compounds are ductile and more resistive to volume compression than to shear deformation at all pressures. Furthermore, the elastic anisotropy is enhanced upon compression. Finally, the calculations indicate that the possible causes that make the zircon structure unstable are mechanical instabilities and the softening of a silent B 1u mode.

ScVO4 under non-hydrostatic compression: a new metastable polymorph.

The high-pressure (HP) behaviour of scandium vanadate (ScVO4) is investigated under non-hydrostatic compression. The compound is studied by means of synchrotron-based powder x-ray diffraction (XRD) and optical-absorption techniques. The occurrence of a non-reversible phase transition is detected. The transition is from the zircon structure to the fergusonite-type structure and takes place around 6 GPa with nearly 10% volume discontinuity. XRD measurements on the pressure cycled sample confirm for the first time that the fergusonite-type ScVO4 can be recovered as the metastable phase at ambient conditions. Raman spectroscopic measurements verify the metastable phase to be of a fergusonite-type phase. Theoretical calculations also corroborate the experimental findings. The fergusonite phase is found to be stiffer than the ambient-pressure zircon phase, as indicated by the observed experimental and theoretical bulk moduli. The optical properties and lattice-dynamics calculation of the fergusonite ScVO4 are discussed. At ambient pressure the band gap of the zircon (fergusonite)-type ScVO4 is 2.75 eV (2.3 eV). This fact suggests that the novel metastable polymorph of ScVO4 can have applications in green technologies; for instance, it can be used as photocatalytic material for hydrogen production by water splitting.