Search results for "scheelite"
showing 10 items of 27 documents
A comparative study of photocatalytically active nanocrystalline tetragonal T zyrcon- type and monoclinic scheelite-type bismuth vanadate
2018
The authors from Vinča Institute of Nuclear Sciences acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project no: 172056 ). The work of K. Smits was supported by Latvian National Research Program IMIS2 (Grant no. 302/2012 ).
Experimental and theoretical investigation of the stability of the monoclinicBaWO4-II phase at high pressure and high temperature
2010
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…
Comment on “Molten salt synthesis of barium molybdate and tungstate microcrystals”
2009
Abstract In this comment we discuss recent results presented by P. Afanasiev on the optical properties of microcrystalline BaMoO 4 and BaWO 4 [Materials Letters 61 (2007) 4622]. Its aim is to show that the band-gap energy reported in that work for BaWO 4 is not reliable and largely underestimate the correct value. As a consequence of it, the challenge made in Ref. [P. Afanasiev, Materials Letters 61 (2007) 4622] to the previous understanding of the electronic structure of scheelite-type compounds is not valid.
Molecular dynamics and reverse Monte Carlo modeling of scheelite-type AWO4(A = Ca, Sr, Ba) WL3-edge EXAFS spectra
2016
Classical molecular dynamics (MD) and reverse Monte Carlo methods coupled with ab initio multiple-scattering extended x-ray absorption fine structure (EXAFS) calculations were used for modeling of scheelite-type AWO4 (A = Ca, Sr, Ba) W L 3-edge EXAFS spectra. The two theoretical approaches are complementary and allowed us to perform analysis of full EXAFS spectra. Both methods reproduce well the structure and dynamics of tungstates in the outer coordination shells, however the classical MD simulations underestimate the W–O bond MSRD due to a neglect of quantum zero-point-motion. The thermal vibration amplitudes, correlation effects and anisotropy of the tungstate structure were also estimat…
Characterisation of scheelite LaW0.16Nb0.84O4.08 ion conductor by combined synchrotron techniques: Structure, W oxidation state and interdiffusion
2021
Abstract Scheelite-type materials such as LaNbO4 are increasingly attracting attention as a possible alternative to the most common fluorite and perovskite structure as ion conductors. However, they are much less used and investigated. The introduction of tungsten in lanthanum orthoniobate leads to conduction properties that are compatible with oxygen ion conductivity. In this paper, we studied the effect of the introduction of tungsten in the LaNbO4 structure. High resolution X-ray diffraction showed that in LaNb1-xWxO4+x/2 with x = 0.16 the monoclinic distortion is largely suppressed and the tetragonal phase is predominant at room temperature. By XANES/EXAFS we proved that tungsten is in …
Lattice dynamics study of scheelite tungstates under high pressure I.BaWO4
2006
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…
First-principles study of elastic and thermal properties of scheelite-type molybdates and tungstates
2020
Abstract First-principles calculations are carried out to study the physical properties of scheelite-type AMoO4 molybdates and AWO4 tungstates (A = Ca, Sr, Ba, and Pb). We consider two flavors for the exchange-correlation functional, the local-density approximation (LDA) and the generalized gradient approximation (GGA). The second-order elastic constants were determined, and we found that c11 is larger than c33 for the eight investigated compounds. This fact is consistent with the well-known anisotropic compressibility of scheelite-type molybdates and tungstates. The calculated elastic constants are used to determine macroscopic properties which are relevant for applications, such as the bu…
Ab initio study of the mechanical and electronic properties of scheelite-type XWO4(X = Ca, Sr, Ba) compounds
2017
The structural, mechanical, and electronic properties of scheelite-type CaWO4, SrWO4, and BaWO4 have been investigated using density-functional theory (DFT) within the generalized-gradient approximation (GGA). In particular, we have studied the effect of pressure in the crystal structure, elastic constants [Formula: see text], elastic moduli ([Formula: see text], [Formula: see text] and [Formula: see text]), and elastic anisotropy. We have also investigated the band structure of the three studied compounds and the effect of pressure in their electronic bandgap. The obtained results compare well with experimental results regarding the high-pressure (HP) behavior of the crystal structure. Th…
Effect of pressure on the luminescence properties of Nd3+ doped SrWO4 laser crystal
2008
Abstract The luminescence spectra of the 4 F 3/2 → 4 I 9/2 transition of Nd 3+ ions in a SrWO 4 crystal have been analyzed as a function of pressure at room temperature. Experiments have been performed in a diamond-anvil cell up to 13 GPa. At around 10 GPa some changes in the emission spectra have been observed which are attributed to a structural phase transition of the SrWO 4 matrix. These results are in good agreement with a previous paper, in which in a pure SrWO 4 matrix a scheelite to fergusonite phase transition is found around 10.5 GPa. Moreover, with increasing pressure, the decay curves from the 4 F 3/2 are nonexponential and faster indicating that the energy transfer processes …
On the ferroelastic nature of the scheelite-to-fergusonite phase transition in orthotungstates and orthomolybdates
2009
Abstract New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO 4 orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Raman spectroscopy data. In the studied scheelite-type compounds, one external translational mode of B g symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard A g mode in the fergusonite high-pressure phase. However, other scheelite-type compounds not undergoing the scheelite-to-fergusonite transition do not show softening of the B g mode. The reported results have allowed us to establish a relationship betwee…