0000000000012659
AUTHOR
R. Lacomba-perales
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…
Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius
We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn). From the experimental results their band-gap energy has been determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV (CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The results are discussed in terms of the electronic structure of the studied tungstates. It has been found that those compounds where only the s electron states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4) have larger band-gap energies than those where also p, d, and f states of the A2+ cation contribu…
In-situ high-pressure Raman scattering studies in PbWO4 up to 48 GPa
The effect of pressure on the Raman spectrum of PbWO4 has been investigated up to 48 GPa in a diamond-anvil cell using neon as pressure-transmitting medium. Changes are detected in the Raman spectrum at 6.8 GPa as a consequence of a structural phase transition from the tetragonal scheelite structure to the monoclinic PbWO4-III structure. Two additional phase transitions are detected at 15.5 and 21.2 GPa to the previously unknown crystalline phases IV and V. The last one remains stable up to 43.3 GPa. At 47.7 GPa all Raman modes disappear, which could be caused by a pressure-induced amorphization. All structural changes are reversible, being the scheelite phase recovered at ambient pressure.…
High-pressure electrical transport measurements on p-type GaSe and InSe
We performed high-pressure Hall effect and resistivity measurements in p-type GaSe and InSe up to 12 GPa. The pressure behaviour of the transport parameters shows dramatic differences between both materials. In GaSe, the hole concentration and mobility increase moderately and continuously. In InSe, the hole mobility raises rapidly and the hole concentration increases abruptly near 0.8 GPa. The observed results are attributed to the different pressure evolution of the valence-band structure in each material. In InSe a carrier-type inversion is also detected near 4.5 GPa.
High-pressure structural phase transitions in CuWO4
We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experim…
High-pressure phase transitions and compressibility of wolframite-type tungstates
This paper reports an investigation on the phase diagram and compressibility of wolframite-type tungstates by means of x-ray powder diffraction and absorption in a diamond-anvil cell and ab initio calculations. The diffraction experiments show that monoclinic wolframite-type MgWO4 suffers at least two phase transitions, the first one being to a triclinic polymorph with a structure similar to that of CuWO4 and FeMoO4-II. The onset of each transition is detected at 17.1 and 31 GPa. In ZnWO4 the onset of the monoclinic-triclinic transition has been also found at 15.1 GPa. These findings are supported by density-functional theory calculations, which predict the occurrence of additional transiti…
Polymorphism in Strontium Tungstate SrWO 4 under Quasi-Hydrostatic Compression
The structural and vibrational properties of SrWO4 have been studied experimentally up to 27 and 46 GPa, respectively, by angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy measurements as well as using ab initio calculations. The existence of four polymorphs upon quasi-hydrostatic compression is reported. The three phase transitions were found at 11.5, 19.0, and 39.5 GPa. The ambient-pressure SrWO4 tetragonal scheelite-type structure (S.G. I41/a) undergoes a transition to a monoclinic fergusonite-type structure (S.G. I2/a) at 11.5 GPa with a 1.5% volume decrease. Subsequently, at 19.0 GPa, another structural transformation takes place. Our calculations indicate two possi…
Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa
We have studied CaWO4 under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO4 beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of th…
Phase transitions in wolframite-typeCdWO4at high pressure studied by Raman spectroscopy and density-functional theory
Room-temperature Raman scattering was measured in ${\text{CdWO}}_{4}$ up to 43 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases changes in the Raman spectra are detected at 20 and 35 GPa due to the onset of reversible structural phase transitions. We also report ab initio total-energy and lattice-dynamics calculations for the different phases of ${\text{CdWO}}_{4}$. They helped us determine the crystalline structure of the high-pressure phases. Experimental and theoretical results suggest the coexistence of two structures from 20 to 35 GPa: one with tetragonal symmetry and another with triclinic symmetry. Beyo…
High-pressure stability and compressibility ofAPO4(A=La, Nd, Eu, Gd, Er, and Y) orthophosphates: An x-ray diffraction study using synchrotron radiation
Room-temperature angle-dispersive x-ray diffraction measurements on zircon-type ${\text{YPO}}_{4}$ and ${\text{ErPO}}_{4}$, and monazite-type ${\text{GdPO}}_{4}$, ${\text{EuPO}}_{4}$, ${\text{NdPO}}_{4}$, and ${\text{LaPO}}_{4}$ were performed in a diamond-anvil cell up to 30 GPa using neon as pressure-transmitting medium. In the zircon-structured oxides we found evidence of a reversible pressure-induced structural phase transformation from zircon to a monazite-type structure. The onset of the transition is at 19.7 GPa in ${\text{YPO}}_{4}$ and 17.3 GPa in ${\text{ErPO}}_{4}$. In ${\text{LaPO}}_{4}$ a nonreversible transition is found at 26.1 GPa and a barite-type structure is proposed for …
High-pressure structural investigation of several zircon-type orthovanadates
Room temperature angle-dispersive x-ray diffraction measurements on zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low- and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO4 is less compressible than EuVO4 and LuVO4, b…
Effects of high pressure on the optical absorption spectrum of scintillating PbWO4 crystals
The pressure behavior of the absorption edge of PbWO4 was studied up to 15.3 GPa. It red-shifts at -71 meV/GPa below 6.1 GPa, but at 6.3 GPa the band-gap collapses from 3.5 eV to 2.75 eV. From 6.3 GPa to 11.1 GPa, the absorption edge moves with a pressure coefficient of -98 meV/GPa, undergoing additional changes at 12.2 GPa. The results are discussed in terms of the electronic structure of PbWO4 which attribute the behavior of the band-gap to changes in the local atomic structure. The changes observed at 6.3 GPa and 12.2 GPa are attributed to phase transitions.
High-pressure and high-temperature X-ray diffraction studies of scheelite BaWO4
International audience; We carried out high-pressure (HP) and high-temperature (HT) in situ ADXRD synchrotron measurements in barium tungstate (BaWO4 ) up to 7.5 GPa and 800 K. Coexistence of the scheelite and fergusonite structures was found beyond 7 GPa, both at room temperature and HT, suggesting a polymorphism zone in the P –T phase diagram. The experiments are complemented by thermodynamic calculations within the quasi-harmonic approximation. At ambient pressure, a volume thermal expansivity of 9.5 × 10− 6 K−1 was obtained for scheelite BaWO4 . At HP, the thermal expansivity of the fergusonite doubles that of scheelite. Theoretical equation of state curves at HP and HT are also present…
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…
High-Pressure Raman Scattering of CaWO4 Up to 46.3 GPa: Evidence of a New High-Pressure Phase
International audience; The high-pressure behavior of CaWO4 wasanalyzed at room temperature by Raman spectroscopy.Pressure was generated using a diamond-anvil cell and Ne aspressure-transmitting medium. The pressure range of previousstudies has been extended from 23.4 to 46.3 GPa. Theexperiments reveal the existence of two reversible phasetransitions. The first one occurs from the tetragonal scheelitestructure to the monoclinic fergusonite structure and isobserved at 10 GPa. The onset of a previously unknownsecond transition is found at 33.4 GPa. The two high-pressurephases coexist up to 39.4 GPa. The Raman spectra measuredfor the low-pressure phase and the first high-pressure phase arecons…
Complex high-pressure polymorphism of barium tungstate
We have studied BaWO 4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as a pressure-transmitting medium (quasihydrostatic conditions), we found that BaWO 4 transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO 4-II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure- transmitting medium has resulted in a phase transition to a completely different structure, suggesting nonhydrostaticity may…