0000000000048560
AUTHOR
Javier López-solano
Experimental and theoretical investigation of the stability of the monoclinicBaWO4-II phase at high pressure and high temperature
In this work we report high-pressure (HP) and high-temperature (HT) ex situ and in situ experiments in ${\text{BaWO}}_{4}$. Starting from powder samples of ${\text{BaWO}}_{4}$, scheelite structure $(I{4}_{1}/a)$, we reached conditions of 2.5--5.5 GPa and 400--1100 K using a Paris-Edinburgh press. The quenched samples were characterized by x-ray diffraction and Raman measurements at ambient conditions. Depending upon the final $P\text{\ensuremath{-}}T$ conditions we found either the scheelite or the monoclinic ${\text{BaWO}}_{4}$-II $(P{2}_{1}/n)$ structure. We also performed HP-HT in situ Raman measurements in a single crystal of ${\text{BaWO}}_{4}$ using a resistive-heated diamond-anvil ce…
High-pressure structural and lattice dynamical study ofHgWO4
We have synthesized monoclinic mercury tungstate $({\text{HgWO}}_{4})$ and characterized its structural and vibrational properties at room conditions. Additionally, we report the structural and lattice dynamical behavior of ${\text{HgWO}}_{4}$ under high pressure studied by means of x-ray diffraction and Raman-scattering measurements up to 16 GPa and 25 GPa, respectively. The pressure dependence of the structural parameters and Raman-active first-order phonons of monoclinic $C2/c$ ${\text{HgWO}}_{4}$ are discussed in the light of our theoretical first-principles total-energy and lattice dynamics calculations. Our measurements show that the monoclinic phase of ${\text{HgWO}}_{4}$ is stable u…
High-pressure study of ScVO4by Raman scattering andab initiocalculations
We report results of experimental and theoretical lattice-dynamics studies on scandium orthovanadate up to 35 GPa. Raman-active modes of the low-pressure zircon phase are measured up to 8.2 GPa, where the onset of an irreversible zircon-to-scheelite phase transition is detected. Raman-active modes in the scheelite structure are observed up to 16.5 GPa. Beyond 18.2 GPa we detected a gradual splitting of the ${E}_{g}$ modes of the scheelite phase, indicating the onset of a second phase transition. Raman symmetries, frequencies, and pressure coefficients in the three phases of ScVO${}_{4}$ are discussed in the light of ab initio lattice-dynamics calculations that support the experimental resul…
High-pressure x-ray diffraction andab initiostudy ofNi2Mo3N,Pd2Mo3N,Pt2Mo3N,Co3Mo3N, andFe3Mo3N: Two families of ultra-incompressible bimetallic interstitial nitrides
We have studied by means of high-pressure x-ray diffraction the structural stability of ${\text{Ni}}_{2}{\text{Mo}}_{3}\text{N}$, ${\text{Co}}_{3}{\text{Mo}}_{3}\text{N}$, and ${\text{Fe}}_{3}{\text{Mo}}_{3}\text{N}$. We also report ab initio computing modeling of the high-pressure properties of these compounds, ${\text{Pd}}_{2}{\text{Mo}}_{3}\text{N}$ and ${\text{Pt}}_{2}{\text{Mo}}_{3}\text{N}$. We have found that the nitrides remain stable in the ambient-pressure cubic structure at least up to 50 GPa and determined their equation of state. All of them have a bulk modulus larger than 300 GPa. Single-crystal elastic constants have been calculated in order to quantify the stiffness of the i…
Lattice dynamics study of scheelite tungstates under high pressure I.BaWO4
Room-temperature Raman scattering has been measured in lead tungstate up to 17 GPa. We report the pressure dependence of all the Raman modes of the tetragonal scheelite phase PbWO4-I or stolzite, space group I41 /a, which is stable at ambient conditions. Upon compression the Raman spectrum undergoes significant changes around 6.2 GPa due to the onset of a partial structural phase transition to the monoclinic PbWO4-III phase space group P21 /n. Further changes in the spectrum occur at 7.9 GPa, related to a scheelite-to-fergusonite transition. This transition is observed due to the sluggishness and kinetic hindrance of the I → III transition. Consequently, we found the coexistence of the sche…
Theoretical and experimental study of CaWO4 and SrWO4 under pressure
Abstract In this paper, we combine a theoretical study of the structural phases of CaWO 4 and SrWO 4 under high pressure along with the results of angle-dispersive X-ray diffraction (ADXRD) and X-ray absorption near-edge structure (XANES) measurements of both tungstates up to approximately 20 GPa. The theoretical study was performed within the ab initio framework of the density functional theory (DFT) using a plane-wave basis set and the pseudopotential scheme, with the generalized gradient approximation (GGA) for the exchange and correlation contribution to the energy. Under normal conditions, CaWO 4 and SrWO 4 crystallize in the scheelite structure. Our results show that in a hydrostatic …
High-pressure theoretical and experimental study of HgWO4
HgWO 4 at ambient pressure is characterized using a combination of ab initio calculations, X-ray diffraction and Raman scattering measurements. The effect of low pressure and temperature on the structural stability is analysed. Extending our ab initio study to the range of higher pressures, a sequence of stable phases up to 30GPa is proposed. © 2011 Taylor & Francis.
HgGa2 Se4 under high pressure: An optical absorption study
High-pressure optical absorption measurements have been performed in defect chalcopyrite HgGa2Se4 to investigate the influence of pressure on the bandgap energy and its relation with the pressure-induced order–disorder processes that occur in this ordered-vacancy compound. Two different experiments have been carried out in which the sample undergoes either a partial or a total pressure-induced disorder process at 15.4 and 30.8 GPa, respectively. It has been found that the direct bandgap energies of the recovered samples at 1 GPa were around 0.15 and 0.23 eV smaller than that of the original sample, respectively, and that both recovered samples have different pressure coefficients of the dir…
Electronic structure of p-type ultraviolet-transparent conducting CuScO2 films
Abstract We investigate the electronic structure of CuScO 2 thin films grown on sapphire and mica substrates by pulsed laser deposition. X-ray diffraction and microanalysis confirm that the films have the expected delafossite crystal structure and stoichiometric proportions. The electronic structure is investigated by means of X-ray and ultraviolet photoelectron spectroscopy. Electronic states in the range 0–1350 eV are identified, making reference to theoretical density-of-states calculations up to 80 eV. Photoelectron spectra near the Fermi energy confirm the p-character of the films. Optical absorption spectroscopy shows that the films are transparent up to 3.7 eV and exhibit an intense …
Determination of the high-pressure crystal structure ofBaWO4andPbWO4
We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ and $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at pressures of up to $56\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $24\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, respectively. $\mathrm{Ba}\mathrm{W}{\mathrm{O}}_{4}$ is found to undergo a pressure-driven phase transition at $7.1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in $\mathrm{Pb}\mathrm{W}{\mathrm{O}}_{4}$ at $9\phantom{\rule{0…
Theoretical and experimental study of the structural stability ofTbPO4at high pressures
We have performed a theoretical and experimental study of the structural stability of terbium phosphate at high pressures. Theoretical ab initio total-energy and lattice-dynamics calculations together with x-ray diffraction experiments have allowed us to completely characterize a phase transition at {approx}9.8 GPa from the zircon to the monazite structure. Furthermore, total-energy calculations have been performed to check the relative stability of 17 candidate structures at different pressures and allow us to propose the zircon {yields} monazite {yields} scheelite {yields} SrUO{sub 4}-type sequence of stable structures with increasing pressure. In this sequence, sixfold P coordination is …
Crystal structure of HgGa2Se4 under compression
We report on high-pressure x-ray diffraction measurements up to 17.2 GPa in mercury digallium selenide (HgGa2Se4). The equation of state and the axial compressibilities for the low-pressure tetragonal phase have been determined and compared to related compounds. HgGa2Se4 exhibits a phase transition on upstroke toward a disordered rock-salt structure beyond 17 GPa, while on downstroke it undergoes a phase transition below 2.1 GPa to a phase that could be assigned to a metastable zinc-blende structure with a total cation-vacancy disorder. Thermal annealing at low- and high-pressure shows that kinetics plays an important role on pressure-driven transitions.
Negative pressures in CaWO4 nanocrystals
Tetragonal scheelite-type CaWO4 nanocrystals recently prepared by a hydrothermal method show an enhancement of its structural symmetry with the decrease in nanocrystal size. The analysis of the volume dependence of the structural parameters in CaWO4 nanocrystals with the help of ab initio total-energy calculations shows that the enhancement of the symmetry in the scheelite-type nanocrystals is a consequence of the negative pressure exerted on the nanocrystals; i.e., the nanocrystals are under tension. Besides, the behavior of the structural parameters in CaWO4 nanocrystals for sizes below 10 nm suggests an onset of a scheelite-to-zircon phase transformation in good agreement with the predic…
Pressure effects on the electronic and optical properties ofAWO4wolframites (A =Cd, Mg, Mn, and Zn): The distinctive behavior of multiferroic MnWO4
The electronic band-structure and band-gap dependence on the $d$ character of ${A}^{2+}$ cation in $A$WO${}_{4}$ wolframite-type oxides is investigated for different compounds ($A$ $=$ Mg, Zn, Cd, and Mn) by means of optical-absorption spectroscopy and first-principles density-functional calculations. High pressure is used to tune their properties up to 10 GPa by changing the bonding distances establishing electronic to structural correlations. The effect of unfilled $d$ levels is found to produce changes in the nature of the band gap as well as its pressure dependence without structural changes. Thus, whereas Mg, Zn, and Cd, with empty or filled $d$ electron shells, give rise to direct and…
Lattice Dynamics Study of HgGa2Se4 at High Pressures
We report on Raman scattering measurements in mercury digallium selenide (HgGa2Se4) up to 25 GPa. We also performed, for the low-pressure defect-chalcopyrite structure, lattice-dynamics ab initio calculations at high pressures which agree with experiments. Measurements evidence that the semiconductor HgGa2Se4 exhibits a pressure-induced phase transition above 19 GPa to a previously undetected structure. This transition is followed by a transformation to a Raman-inactive phase above 23.4 GPa. On downstroke from 25 GPa until 2.5 GPa, a broad Raman spectrum was observed, which has been attributed to a fourth phase, and whose pressure dependence was followed during a second upstroke. Candidate …
High-pressure structural study of the scheelite tungstatesCaWO4andSrWO4
Angle-dispersive x-ray-diffraction and x-ray-absorption near-edge structure measurements have been performed on ${\mathrm{CaWO}}_{4}$ and ${\mathrm{SrWO}}_{4}$ up to pressures of approximately 20 GPa. Both materials display similar behavior in the range of pressures investigated in our experiments. As in the previously reported case of ${\mathrm{CaWO}}_{4}$, under hydrostatic conditions ${\mathrm{SrWO}}_{4}$ undergoes a pressure-induced scheelite-to-fergusonite transition around 10 GPa. Our experimental results are compared to those found in the literature and are further supported by ab initio total-energy calculations, from which we also predict the instability at larger pressures of the …