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.

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.

Structural, vibrational and electronic properties of α'-Ga2S3 under compression

We report a joint experimental and theoretical study of the low-pressure phase of α'-Ga₂S₃ under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α'-Ga₂S₃ at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga-S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α'-Ga₂S₃. We found that the structural channels are responsible for the anisotropic properties of α'-Ga₂S₃ and the A'(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α'-Ga₂S₃ at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga₂Se₃. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α'-Ga₂S₃ is stressed.

Analytical Tools for Disease Diagnosis in Animals via Fecal Volatilome

Volatilome analysis is growing in attention for the diagnosis of diseases in animals and humans. In particular, volatilome analysis in fecal samples is starting to be proposed as a fast, easy and noninvasive method for disease diagnosis. Volatilome comprises volatile organic compounds (VOCs), which are produced during both physiological and patho-physiological processes. Thus, VOCs from a pathological condition often differ from those of a healthy state and therefore the VOCs profile can be used in the detection of some diseases. Due to their strengths and advantages, feces are currently being used to obtain information related to health status in animals. However, they are complex samples, that can present problems for some analytical techniques and require special consideration in their use and preparation before analysis. This situation demands an effort to clarify which analytic options are currently being used in the research context to analyze the possibilities these offer, with the final objectives of contributing to develop a standardized methodology and to exploit feces potential as a diagnostic matrix. The current work reviews the studies focused on the diagnosis of animal diseases through fecal volatilome in order to evaluate the analytical methods used and their advantages and limitations. The alternatives found in the literature for sampling, storage, sample pretreatment, measurement and data treatment have been summarized, considering all the steps involved in the analytical process.

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.

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. We also present the theoretical framework to accurately explain the observed experimental results.

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.

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.

Arsenolite: a quasi-hydrostatic solid pressure-transmitting medium

This study reports the experimental characterization of the hydrostatic properties of arsenolite (As4O6), a molecular solid which is one of the softest minerals in the absence of hydrogen bonding. The high compressibility of arsenolite and its stability up to 15 GPa have been proved by x-ray diffraction measurements, and the progressive loss of hydrostaticity with increasing pressure up to 20 GPa has been monitored by ruby photoluminescence. Arsenolite has been found to exhibit hydrostatic behavior up to 2.5 GPa and a quasi-hydrostatic behavior up to 10 GPa at room temperature. This result opens the way to explore other molecular solids as possible quasi-hydrostatic pressure-transmitting media. The validity of arsenolite as an insulating, stable, non-penetrating and quasi-hydrostatic medium is explored by the study of the x-ray diffraction of zeolite ITQ-29 at high pressure.

Experimental and theoretical study of α-Eu2(MoO4)3 under compression

The compression process in the α-phase of europium trimolybdate was revised employing several experimental techniques. X-ray diffraction (using synchrotron and laboratory radiation sources), Raman scattering and photoluminescence experiments were performed up to a maximum pressure of 21 GPa. In addition, the crystal structure and Raman mode frequencies have been studied by means of first-principles density functional based methods. Results suggest that the compression process of α-Eu2(MoO4)3 can be described by three stages. Below 8 GPa, the α-phase suffers an isotropic contraction of the crystal structure. Between 8 and 12 GPa, the compound undergoes an anisotropic compression due to distortion and rotation of the MoO4 tetrahedra. At pressures above 12 GPa, the amorphization process starts without any previous occurrence of a crystalline-crystalline phase transition in the whole range of pressure. This behavior clearly differs from the process of compression and amorphization in trimolybdates with [Formula: see text]-phase and tritungstates with α-phase.

Structural, elastic and vibrational properties of nanocrystalline lutetium gallium garnet under high pressure

An ab initio study of the structural, elastic and vibrational properties of the lutetium gallium garnet (Lu₃Ga₅O₁₂) under pressure has been performed in the framework of the density functional theory, up to 95 GPa.

High-pressure structural behaviour of HoVO4: combined XRD experiments and ab initio calculations

We report a high-pressure experimental and theoretical investigation of the structural properties of zircon-type HoVO4. Angle-dispersive x-ray diffraction measurements were carried out under quasi-hydrostatic and partial non-hydrostatic conditions up to 28 and 23.7 GPa, respectively. In the first case, an irreversible phase transition is found at 8.2 GPa. In the second case, the onset of the transition is detected at 4.5 GPa, a second (reversible) transition is found at 20.4 GPa, and a partial decomposition of HoVO4 was observed. The structures of the different phases have been assigned and their equations of state (EOS) determined. Experimental results have also been compared to theoretical calculations which fully agree with quasi-hydrostatic experiments. Theory also suggests the possibility of another phase transition at 32 GPa; i.e. beyond the pressure limit covered by present experiments. Furthermore, calculations show that deviatoric stresses could trigger the transition found at 20.4 GPa under non-hydrostatic conditions. The reliability of the present experimental and theoretical results is supported by the consistency between the values yielded for transition pressures and EOS parameters by the two methods.

Pressure effects on the vibrational properties of α-Bi(2)O(3): an experimental and theoretical study

We report an experimental and theoretical high-pressure study of the vibrational properties of synthetic monoclinic bismuth oxide (α-Bi(2)O(3): ), also known as mineral bismite. The comparison of Raman scattering measurements and theoretical lattice-dynamics ab initio calculations is key to understanding the complex vibrational properties of bismite. On one hand, calculations help in the symmetry assignment of phonons and to discover the phonon interactions taking place in this low-symmetry compound, which shows considerable phonon anticrossings; and, on the other hand, measurements help to validate the accuracy of first-principles calculations relating to this compound. We have also studied the pressure-induced amorphization (PIA) of synthetic bismite occurring around 20 GPa and showed that it is reversible below 25 GPa. Furthermore, a partial temperature-induced recrystallization (TIR) of the amorphous sample can be observed above 20 GPa upon heating to 200°C, thus evidencing that PIA at room temperature occurs because of the inability of the α phase to undergo a phase transition to a high-pressure phase. Raman scattering measurements of the TIR sample at room temperature during pressure release have been performed. The interpretation of these results in the light of ab initio calculations of the candidate phases at high pressures has allowed us to tentatively attribute the TIR phase to the recently found high-pressure hexagonal HPC phase and to discuss its lattice dynamics.

Thermally activated cation ordering in ZnGa2Se4 single crystals studied by Raman scattering, optical absorption, and ab initio calculations

Order-disorder phase transitions induced by thermal annealing have been studied in the ordered-vacancy compound ZnGa2Se4 by means of Raman scattering and optical absorption measurements. The partially disordered as-grown sample with tetragonal defect stannite (DS) structure and I4¯2m space group has been subjected to controlled heating and cooling cycles. In situ Raman scattering measurements carried out during the whole annealing cycle show that annealing the sample to 400 °C results in a cation ordering in the sample, leading to the crystallization of the ordered tetragonal defect chalcopyrite (DC) structure with I4¯ space group. On decreasing temperature the ordered cation scheme of the DC phase can be retained at ambient conditions. The symmetry of the Raman-active modes in both DS and DC phases is discussed and the similarities and differences between the Raman spectra of the two phases emphasized. The ordered structure of annealed samples is confirmed by optical absorption measurements and ab initio calculations, that show that the direct bandgap of DC-ZnGa2Se4 is larger than that of DS-ZnGa2Se4.