High-pressure structural study of the scheelite tungstates CaWO[subscript 4] and SrWO[subscript 4 (original) (raw)
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High-pressure structural study of the scheelite tungstates CaWO4 and SrWO4
Physical Review B, 2005
Angle-dispersive x-ray diffraction (ADXRD) and x-ray absorption near edge structure (XANES) measurements have been performed in the AWO4 tungstates CaWO4 and SrWO4 under high pressure up to approximately 20 GPa. Similar phase transitions and phase transition pressures have been observed for both tungstates using the two techniques in the studied pressure range. Both materials are found to undergo a pressure-induced scheelite-to-fergusonite phase transition under sufficiently hydrostatic conditions. Our results are compared to those found previously in the literature and supported by ab initio total energy calculations. From the total energy calculations we have also predicted a second phase transition from the fergusonite structure to a new structure identified as Cmca. Finally, a linear relationship between the charge density in the AO8 polyhedra of ABO4 scheelite-related structures and the bulk modulus is discussed and used to predict the bulk modulus of other materials, like zircon.
Lattice dynamics study of scheelite tungstates under high pressure I. BaWO4
Physical Review B, 2006
Room-temperature Raman scattering has been measured in barium tungstate ͑BaWO 4 ͒ up to 16 GPa. We report the pressure dependence of all the Raman active first-order phonons of the tetragonal scheelite phase ͑BaWO 4 -I, space group I4 1 / a͒, which is stable at normal conditions. As pressure increases the Raman spectrum undergoes significant changes around 6.9 GPa due to the onset of the structural phase transition to the monoclinic BaWO 4 -II phase ͑space group P2 1 / n͒. This transition is only completed above 9.5 GPa. A further change in the spectrum is observed at 7.5 GPa related to a scheelite-to-fergusonite transition. The scheelite, BaWO 4 -II, and fergusonite phases coexist up to 9.0 GPa due to the sluggishness of the I → II phase transition. Further to the experimental study, we have performed ab initio lattice dynamics calculations that have greatly helped us in assigning and discussing the pressure behavior of the observed Raman modes of the three phases.
Theoretical and experimental study of CaWO 4 and SrWO 4 under pressure
Journal of Physics and Chemistry of Solids, 2006
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 environment, both compounds undergo a scheelite-to-fergusonite phase transition with increasing pressure. We present a comparison of the evolution of the structural parameters, equation of state, and of the features of the transition, finding an overall excellent agreement between the experimental and theoretical results. r
Physical Review Materials
X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire α-RE 2 (WO 4) 3 (RE = La-Ho) family with modulated scheelite structure (α phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters a, c, and β was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter a decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter LaNd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the a axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the WO 2− 4 tetrahedrons. These changes correlate with those in the alternating RE …RE …RE chains and blocks of cationic vacancies arranged along the a axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having "hinge frames."
Physica Status Solidi B-basic Solid State Physics, 2007
Recent experimental measurements and ab initio calculations in scheelite CaWO4, SrWO4, BaWO4, PbWO4, EuWO4 and YLiF4 crystals reveal the existence of complex high-pressure phase diagrams, which present striking differences but also relevant similarities. In this work we show that the high-pressure structural sequence in the studied scheelites can be understood on the basis of the positions of the different ABX4 compounds in Fukunaga and Yamaoka's diagram and in Bastide's diagram. Our study can help to understand the phase diagrams and high-pressure phase transitions occurring in ABX4 compounds with scheelite, wolframite, fergusonite, zircon, or pseudoscheelite structures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi B-basic Solid State Physics, 2007
The structural properties of CaWO4, SrWO4, BaWO4, PbWO4, and EuWO4 scintillating crystals under pressure have been studied by X-ray powder diffraction, X-ray absorption near-edge structure measurements, Raman spectroscopy, and ab initio density functional theory calculations. The results obtained from these studies will be reviewed here and their differences and similitudes discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physical Review B, 2011
In this paper we report a density functional study of the structural, electronic, and vibrational properties of the EuWO 4 compound in the scheelite structure. We use Raman spectroscopy to complement the study for this phase at ambient pressure. The first part of the paper is devoted to analyzing the results obtained with the Perdew-Burke-Ernzerhof for solids exchange-correlation functional within the GGA + U approximation and compare those with our experimental results and reported available data. We also present the evolution of these properties, for the same crystal structure, up to 8 GPa. The second part of the paper is devoted to discussing our study on the high-pressure phase transitions of EuWO 4 , for which we follow the evolution of the structural properties as a function of pressure. The results show that the high-pressure behavior of EuWO 4 is very similar to that of CaWO 4 , SrWO 4 , BaWO 4 , and PbWO 4 , for which the scheelite structure undergoes a phase transition to the fergusonite structure, an observation also supported by our experimental data. Additionally, we can also conclude that at higher pressures, EuWO 4 evolves to the BaWO 4 -II and Cmca crystal phases.