Room-temperature Equations Of State Research Articles

Identification of the high-pressure phases of α-SnWO4 combining x-ray diffraction and crystal structure prediction

We have characterized the high-pressure behavior of -SnWO4. The compound has been studied up to 30 GPa using a diamond-anvil cell and synchrotron powder X-ray diffraction. We report evidence of two structural phase transitions in the pressure range covered in our study, and we propose a crystal structure for the two high-pressure phases. The first one, observed around 12.9 GPa, has been obtained combining indexation using DICVOL and density-functional theory calculations. The second high-pressure phase, observed around 17.5 GPa, has been determined by using the CALYPSO code, the prediction of which was supported by a Le Bail fit to the experimental X-ray diffraction patterns. The proposed structural sequence involves two successive collapses of the unit-cell volume and an increase in the coordination number of Sn and W atoms. The room-temperature equations of state, the principal axes of compression and their compressibility, the elastic constants, and the elastic moduli are reported for -SnWO4 and for the two high-pressure phases.

Palladium at high pressure and high temperature: A combined experimental and theoretical study

Palladium is one of the most important technological materials, yet its phase diagram remains poorly understood. At ambient conditions, its solid phase is face-centered cubic (fcc). However, another solid phase of Pd, body-centered cubic (bcc), was very recently predicted in two independent theoretical studies to occur at high pressures and temperatures. In this work, we report an experimental study on the room-temperature equation of state (EOS) of Pd to a pressure of 80 GPa, as well as a theoretical study on the phase diagram of Pd including both fcc-Pd and bcc-Pd. Our theoretical approach consists in ab initio quantum 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 transition boundaries. We obtain the melting curves of both fcc-Pd and bcc-Pd and an equation for the fcc–bcc solid–solid phase transition boundary as well as the thermal EOS of Pd which is in agreement with experimental data and QMD simulations. We uncover the presence of another solid phase of Pd on its phase diagram, namely, random hexagonal close-packed (rhcp), and estimate the location of the rhcp-bcc solid–solid phase transition boundary and the rhcp–fcc–bcc triple point. We also discuss the topological similarity of the phase diagrams of palladium and silver, the neighbor of Pd in the periodic table. We argue that Pd is a reliable standard for shock-compression studies and present the analytic model of its principal Hugoniot in a wide pressure range.

Structural instability in LaNbO4 under compression

In this work, we report a high-pressure study on fergusonite-type LaNbO4. Powder x-ray diffraction and Raman spectroscopic experiments support the occurrence of a phase transition between 11 and 14 GPa. The transition takes place from a monoclinic fergusonite-type structure (space group I2/a) to another monoclinic structure (space group P21/c). The phase transition is reversible, and the high-pressure phase is isomorphic to the high-pressure phase of HoNbO4. The high-pressure phase remains stable up to 33.3 GPa, the highest pressure reached in the present measurements. Density-functional theory calculations found that in the pressure range of the studies; the high-pressure phase has a higher enthalpy than the low-pressure fergusonite phase. We propose that the high-pressure phase is metastable and it is observed because of non-hydrostatic conditions in the experiments. The pressure dependence of unit-cell parameters of the low-pressure phase and the room-temperature equation of state are reported. The pressure dependence of various Raman and IR frequencies as obtained from experiment and theory is also reported. For the fergusonite phase, we have also obtained the isothermal compressibility tensor, elastics constants, and elastic moduli.

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 group . 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.

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 and ab initio density-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 to ab initio calculations, the reported phase transition is related to a mechanical instability and a phonon softening.

Comparative study of the high-pressure behavior of ZnV2O6, Zn2V2O7, and Zn3V2O8

Abstract We report a study of the high-pressure structural behavior of ZnV2O6, Zn2V2O7, and Zn3V2O8, which has been explored by means of synchrotron powder x-ray diffraction. We found that ZnV2O6 and Zn3V2O8 remain in the ambient-pressure structure up to 15 GPa. In contrast, in the same pressure range, Zn2V2O7 undergoes three phase transitions at 0.7, 3.0, and 10.8 GPa, respectively. Possible crystal structures for the first and second high-pressure phases are proposed. Reasons for the distinctive behavior of Zn2V2O7 are discussed. The compressibility of the different polymorphs has been determined. The response to pressure is found to be anisotropic in all the considered compounds and the room-temperature equations of state have been determined. The bulk moduli of ZnV2O6 (129(2) GPa) and Zn3V2O8 (120(2) GPa) are consistent with a structural framework composed of compressible ZnO6 octahedra and uncompressible VO4 tetrahedra. In contrast, Zn2V2O7 is highly compressible with a bulk modulus of 58(9) GPa, which is almost half of the bulk modulus of the other two vanadates. The large compressibility of Zn2V2O7 and its sequence of structural transitions are related to the fact that this material is less dense than the other zinc vanadates and to the penta-coordination of Zn atoms by oxygen atoms in Zn2V2O7. A comparison to the high-pressure behavior of related compounds is presented.

Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO4.

We have studied the high-pressure behavior of FeVO4 by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO4 is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO4-I'), which was not detected by previous powder XRD studies. We have determined that FeVO4, under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO4-I) to a second previously unknown triclinic structure (FeVO4-I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO4-II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO4 polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO4-I and FeVO4-I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.

High-pressure behavior ofCaMoO4

We report a high-pressure study of tetragonal scheelite-type CaMoO4 up to 29 GPa. In order to characterize its high-pressure behavior, we have combined Raman and optical-absorption measurements with density-functional theory calculations. We have found evidence of a pressure-induced phase transition near 15 GPa. Experiments and calculations agree in assigning the high-pressure phase to a monoclinic fergusonite-type structure. The reported results are consistent with previous powder x-ray-diffraction experiments, but are in contradiction with the conclusions obtained from earlier Raman measurements, which support the existence of more than one phase transition in the pressure range covered by our studies. The observed scheelite-fergusonite transition induces significant changes in the electronic band gap and phonon spectrum of CaMoO4. We have determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependences of the Raman-active and infrared-active modes. In addition, based upon calculations of the phonon dispersion of the scheelite phase, carried out at a pressure higher than the transition pressure, we propose a possible mechanism for the reported phase transition. Furthermore, from the calculations we determined the pressure dependence of the unit-cell parameters and atomic positions of the different phases and their room-temperature equations of state. These results are compared with previous experiments showing a very good agreement. Finally, information on bond compressibility is reported and correlated with the macroscopic compressibility of CaMoO4. The reported results are of interest for the many technological applications of this oxide.

Compressibility and structural behavior of pure and Fe-doped SnO2 nanocrystals

We have performed high-pressure synchrotron X-ray diffraction experiments on nanoparticles of pure tin dioxide (particle size ∼30 nm) and 10 mol % Fe-doped tin dioxide (particle size ∼18 nm). The structural behavior of undoped tin dioxide nanoparticles has been studied up to 32 GPa, while the Fe-doped tin dioxide nanoparticles have been studied only up to 19 GPa. We have found that both samples present at ∼13 GPa a second-order structural phase transition from the ambient pressure tetragonal rutile-type structure (P42/mnm) to an orthorhombic CaCl2-type structure (space group Pnnm). No phase coexistence was observed for this transition. Additionally, pure SnO2 presents a phase transition to a cubic structure at ∼24 GPa. The evolution of the lattice parameters with pressure and the room-temperature equations of state are reported for the different phases. The reported results suggest that the partial substitution of Sn by Fe induces an enhancement of the bulk modulus of SnO2. Results are compared with previous studies on bulk and nanocrystalline SnO2. The effects of pressure on Sn-O bonds are also analyzed.

Phase Stability of Lanthanum Orthovanadate at High Pressure

When monoclinic monazite-type LaVO4 (space group P21/n) is squeezed up to 12 GPa at room temperature, a phase transition to another monoclinic phase has been found. The structure of the high-pressure phase of LaVO4 is indexed with the same space group (P21/n), but with a larger unit-cell in which the number of atoms is doubled. The transition leads to an 8% increase in the density of LaVO4. The occurrence of such a transition has been determined by x-ray diffraction, Raman spectroscopy, and ab initio calculations. The combination of the three techniques allows us to also characterize accurately the pressure evolution of unit-cell parameters and the Raman (and IR)-active phonons of the low- and high-pressure phase. In particular, room-temperature equations of state have been determined. The changes driven by pressure in the crystal structure induce sharp modifications in the color of LaVO4 crystals, suggesting that behind the monoclinic-to-monoclinic transition there are important changes of the electronic properties of LaVO4.

Experimental and theoretical investigation ofThGeO4at high pressure

We report here the combined results of angle-dispersive x-ray diffraction experiments performed on ThGeO4 up to 40 GPa and total-energy density-functional theory calculations. Zircon-type ThGeO4 is found to undergo a pressure-driven phase transition at 11 GPa to the tetragonal scheelite structure. A second phase transition to a monoclinic M-fergusonite type is found beyond 26 GPa. The same transition has been observed in samples that crystallize in the scheelite phase at ambient pressure. No additional phase transition or evidence of decomposition of ThGeO4 has been detected up to 40 GPa. The unit-cell parameters of the monoclinic high-pressure phase are a = 4.98(2) A, b = 11.08(4) A, c = 4.87(2) A, and beta = 90.1(1), Z = 4 at 28.8 GPa. The scheelite-fergusonite transition is reversible and the zircon-scheelite transition non-reversible. From the experiments and the calculations, the room temperature equation of state for the different phases is also obtained. The anisotropic compressibility of the studied crystal is discussed in terms of the differential compressibility of the Th-O and Ge-O bonds.

Direct measurement of the liquid 4:1 methanol–ethanol equation of state up to 5 GPa

Despite the widespread use of the 4:1 methanol–ethanol mixture as a hydrostatic medium by the high-pressure community, its equation of state (EOS) is usually substituted by that of pure methanol. Here we report accurate direct volumetric measurements of the room temperature EOS for this mixture up to 5 GPa. A brief description of the optical technique (imaging+interferometric) specifically developed for this purpose is given, as well as the general characteristics of the required miniature diamond anvil cell. Our results are compared with the available data of similar pure fluids’ EOS in the same pressure range. We find that there exist noticeable differences between the EOS of the mixture and that of pure methanol in terms of molar quantities.

Melting line of Krypton in extreme thermodynamic regimes

We have performed extensive computer simulations of the thermodynamic and structural properties of the krypton rare gas modeled by the modified Buckingham exponential-6 interatomic potential. Using a new set of potential parameters, we have found a good agreement with the room temperature equation of state at very high pressure obtained by diamond anvil cell experiments. Moreover, the melting line of the model has been estimated through the Lindemann criterion; the agreement with the low-pressure experiments is excellent, whereas at higher pressure, the model poorly reproduces the typical softening of the experimental melting curve.

Structural studies of gadolinium at high pressure and temperature

High pressure and high temperature angle-dispersive x-ray diffraction measurements on gadolinium were performed up to $93\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and nearly $2500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ using argon as pressure medium. Our room temperature measurements confirmed the transition pressures reported in a previous study carried out using nitrogen as pressure medium, solving former discrepancies found in the literature. A new phase was observed above $1200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ between $39\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $52\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Additionally, the structure of the previously observed dfcc and post-dfcc phases was determined, being the space group symmetry of the post-dfcc phase $(C2∕m)$, different from what was predicted before. Finally, the room temperature equation of state was determined for Gd and its pressure-temperature phase diagram redefined.

Effect of Ca2+ and Fe2+ on the equation of state of MgSiO3 orthopyroxene

Room temperature equations of state (EOSs) have been determined for synthetic and natural orthopyroxenes on and near the (Mg,Fe)SiO3 join by single‐crystal X‐ray diffraction to a maximum pressure of 11 GPa. The values of the room pressure bulk moduli, K0,T, and its pressure derivative, K0′, are shown to be independent of the form of the equation of state used to fit the data. Variation in the Fe2+ content of the orthopyroxene has very little effect on either K0,T or K0′, with the EOS parameters of synthetic FeSiO3 orthoferrosilite being K0,T=95.1 (3.0) GPa, and K0′=10.6 (1.6), both within one combined experimental estimated standard deviation of those previously determined for synthetic MgSiO3 orthoenstatite. However, substitution of even very small amounts of Ca2+ (and Al3+) to the orthopyroxene structure causes an increase in K0,T by some 14%. The density difference between the synthetic MgSiO3 and natural orthoenstatites (calculated from the equations of state at 10 GPa) is of the order of 3.4%, with a corresponding unit‐cell volume difference of the order of ∼0.3%. It is concluded that pure MgSiO3 orthoenstatite is not a good representation of the pyroxene component of the Earth's upper mantle.

Molecular-dynamics simulations of solid C 60 and C 70 through a spherical two-body potential

Molecular dynamics calculations of bulk properties of solid C 60 and C 70 , simulated in terms of a simple two-body potential, are presented. The bulk moduli and the room temperature equations of state are determined and compared with the available experimental data. The change in the lattice parameters as a function of pressure and temperature is also investigated.

The equation of state of polyamorphic germania glass: A two-domain description of the viscoelastic response

We have measured the quasistatic room temperature equation of state of GeO2 glass under hydrostatic conditions to 7.1 GPa. From ambient pressure to 4 GPa the compression displays completely reversible elastic behavior. Above 4 GPa the glass becomes anelastic and exhibits a dramatic increase in the static compressibility. This change in elastic response is concomitant with the onset of the previously reported pressure-induced germanium coordination change. The equation of state data can be quantitatively described by a two-domain model composed of four- and six-coordinated germanium clusters. The model accurately reproduces the previously measured change in the average Ge–O bond length of germania with pressure and rationalizes the different pressure dependent compressional behavior observed in quasistatic and ultrasonic measurements. We further conjecture that the vitreous polyamorphism exhibited by germania glass at high pressures, and the pressure-induced crystal-to-amorphous transition of quartz-isotypic GeO2, both result from similar underlying coordination instabilities in the germania tetrahedral framework.

Pressure-volume relations for Zn, Cd, Ga, In and TI at room temperature to 30 GPa and above

Abstract The elemental metals Zn, Cd, Ga, In and TI are studied by energy dispersive X-ray diffraction under pressures up to 30 GPa and above. Room temperature equation of state (EOS) data are derived and compared with results of earlier static and dynamic measurements at lower and higher pressures, respectively.

Solid krypton: Equation of state and elastic properties.

The properties of solid krypton have been measured as a function of pressure in a diamond-anvil cell using energy-dispersive x-ray diffraction and Brillouin scattering. A room-temperature equation of state and a complete set of elastic constants are deduced. The analysis of these results indicates a possible phase transition above 30 GPa.