La Laguna, Spain
La Laguna, Spain

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Achary S.N.,Bhabha Atomic Research Center | Errandonea D.,MALTA Consolider Team | Munoz A.,University of La Laguna | Rodriguez-Hernandez P.,University of La Laguna | And 5 more authors.
Dalton Transactions | Year: 2013

In this work we report the metastability and the energetics of the phase transitions of three different polymorphs of BiPO4, namely trigonal (Phase-I, space group P3121), monoclinic monazite-type (Phase-II, space group P21/n) and SbPO4-type monoclinic (Phase-III, space group P21/m) from ambient and non-ambient temperature powder XRD and neutron diffraction studies as well as ab initio density functional theory (DFT) calculations. The symmetry ambiguity between P21 and P21/m of the high temperature polymorph of BiPO4 has been resolved by a neutron diffraction study. The structure and vibrational properties of these polymorphs of the three polymorphs have also been reported in detail. Total energy calculations have been used to understand the experimentally observed metastable behavior of trigonal and monazite-type BiPO4. Interestingly, all of the three phases were found to coexist after heating a single phasic trigonal BiPO4 to 773 K. The irreversible nature of these phase transitions has been explained by the concepts of the interplay of the structural distortion, molar volume and total energy. © 2013 The Royal Society of Chemistry.


Panchal V.,MALTA Consolider Team | Lopez-Moreno S.,University of La Laguna | Santamaria-Perez D.,Complutense University of Madrid | Errandonea D.,MALTA Consolider Team | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to 12 and 16 GPa, respectively. Experiments reveal at 5.3 GPa the onset of a pressure-induced irreversible phase transition from the zircon to the monazite structure. Beyond this pressure, diffraction peaks and Raman-active modes of the monazite phase are measured. The zircon-to-monazite transition in CeVO4 is distinctive among the other rare-earth orthovanadates. We also observed softening of external translational T(E g) and internal ν2(B2g) bending modes. We attribute it to mechanical instabilities of zircon phase against the pressure-induced distortion. We additionally report lattice-dynamical and total-energy calculations which are in agreement with the experimental results. Finally, the effect of nonhydrostatic stresses on the structural sequence is studied and the equations of state of different phases are reported. © 2011 American Physical Society.


Ruiz-Fuertes J.,MALTA Consolider Team | Errandonea D.,MALTA Consolider Team | Lopez-Moreno S.,University of La Laguna | Gonzalez J.,University of Cantabria | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Raman scattering measurements and lattice-dynamics calculations were performed on magnesium tungstate (MgWO4) under high pressure up to 41 GPa. Experiments were carried out on a selection of pressure media. The influence of nonhydrostaticity on the structural properties of MgWO4 and isomorphic compounds is examined. Under quasihydrostatic conditions, a phase transition was found at 26 GPa in MgWO4. The high-pressure phase is tentatively assigned to a triclinic structure similar to that of CuWO 4. We also report and discuss the Raman symmetries, frequencies, and pressure coefficients in the low- and high-pressure phases. In addition, the Raman frequencies for different wolframites are compared and the variation of the mode frequency with the reduced mass across the family is investigated. Finally, the accuracy of theoretical calculations is systematically discussed for MgWO4, MnWO4, FeWO4, CoWO4, NiWO4, ZnWO4, and CdWO4. © 2011 American Physical Society.


Errandonea D.,MALTA Consolider Team | Popescu C.,ALBA Synchrotron Light Facility | Achary S.N.,Bhabha Atomic Research Center | Tyagi A.K.,Bhabha Atomic Research Center | Bettinelli M.,University of Verona
Materials Research Bulletin | Year: 2014

Room-temperature angle-dispersive X-ray diffraction measurements on zircon-type NdVO4 and monazite-type LaVO4 were performed in a diamond-anvil cell up to 12 GPa. In NdVO4, we found evidence for a non-reversible pressure-induced structural phase transition from zircon to a monazite-type structure at 6.5 GPa. Monazite-type LaVO4 also exhibits a phase transition but at 8.6 GPa. In this case the transition is reversible and isomorphic. In both compounds the pressure induced transitions involve a large volume collapse. Finally, the equations of state and axial compressibilities for the low-pressure phases are also determined. © 2013 Elsevier Ltd. All rights reserved.


Manjon F.J.,University of Malta | Gomis O.,University of Malta | Rodriguez-Hernandez P.,University of La Laguna | Perez-Gonzalez E.,University of La Laguna | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

A strong nonlinear pressure dependence of the optical absorption edge has been measured in defect chalcopyrites CdGa2 Se4 and HgGa2 Se4. The behavior is due to the nonlinear pressure dependence of the direct band-gap energy in these compounds as confirmed by ab initio calculations. Our calculations for CdGa2 Se4, HgGa2 Se4 and monoclinic β -Ga2 Se3 provide evidence that the nonlinear pressure dependence of the direct band-gap energy is a general feature of adamantine ordered-vacancy compounds irrespective of their composition and crystalline structure. The nonlinear behavior is due to a conduction band anticrossing at the Γ point of the Brillouin zone caused by the presence of ordered vacancies in the unit cell of these tetrahedrally coordinated compounds. © 2010 The American Physical Society.


Lacomba-Perales R.,MALTA Consolider Team | Martinez-Garcia D.,MALTA Consolider Team | Errandonea D.,MALTA Consolider Team | Errandonea D.,University of Valencia | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

In this work we report high-pressure (HP) and high-temperature (HT) ex situ and in situ experiments in BaWO4. Starting from powder samples of BaWO4, scheelite structure (I 41 /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-T conditions we found either the scheelite or the monoclinic BaWO4-II (P21/n) structure. We also performed HP-HT in situ Raman measurements in a single crystal of BaWO4 using a resistive-heated diamond-anvil cell. The transition from scheelite to the BaWO4-II phase was observed at 5 GPa for T=621 K. Ab initio lattice-dynamics calculations have been performed in order to characterize the vibrations of the BaWO4-II phase. Finally we carried out in situ powder angle-dispersive x-ray diffraction synchrotron measurements on BaWO4 compound following different P-T paths, extending its measured phase diagram in the 2-6 GPa and 300-2000 K range. © 2010 The American Physical Society.


Errandonea D.,MALTA Consolider Team | Gracia L.,MALTA Consolider Team | Beltran A.,MALTA Consolider Team | Vegas A.,CSIC - Institute of Physical Chemistry "Rocasolano" | Meng Y.,Carnegie Institution of Washington
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

AgClO4 has been studied under compression by x-ray diffraction and density functional theory calculations. Experimental evidence of a structural phase transition from the tetragonal structure of AgClO4 to an orthorhombic barite-type structure has been found at 5.1 GPa. The transition is supported by total-energy calculations. In addition, a second transition to a monoclinic structure is theoretically proposed to take place beyond 17 GPa. The equation of state of the different phases is reported as well as the calculated Raman-active phonons and their pressure evolution. Finally, we provide a description of all the structures of AgClO4 and discuss their relationships. The structures are also compared with those of AgCl in order to explain the structural sequence determined for AgClO4. © 2011 American Physical Society.


Ruiz-Fuertes J.,MALTA Consolider Team | Ruiz-Fuertes J.,University of Valencia | Segura A.,MALTA Consolider Team | Segura A.,University of Valencia | And 6 more authors.
Physical Review Letters | Year: 2012

High-pressure optical-absorption measurements performed in CuWO4 up to 20 GPa provide experimental evidence of the persistence of the Jahn-Teller (JT) distortion in the whole pressure range both in the low-pressure triclinic and in the high-pressure monoclinic phase. The electron-lattice couplings associated with the eg(E - e) and t2g(T - e) orbitals of Cu2+ in CuWO4 are obtained from correlations between the JT distortion of the CuO6 octahedron and the associated structure of Cu2+ d-electronic levels. This distortion and its associated JT energy (EJT) decrease upon compression in both phases. However, both the distortion and associated EJT increase sharply at the phase-transition pressure (PPT=9.9GPa), and we estimate that the JT distortion persists for a wide pressure range not being suppressed up to 37 GPa. These results shed light on the transition mechanism of multiferroic CuWO4, suggesting that the pressure-induced structural phase transition is a way to minimize the distortive effects associated with the toughness of the JT distortion. © 2012 American Physical Society.


Errandonea D.,MALTA Consolider Team | Kumar R.S.,University of Nevada, Las Vegas
Materials Research Bulletin | Year: 2014

We report on high-pressure X-ray diffraction measurements up to 51.2 GPa in PbCrO4 at room temperature. Three high-pressure phases with structures different than the ambient-pressure monazite-type (P21/n) are reported. One phase transition was found at 3.8 GPa to an isomorphic structure to monazite. A second transition occurs at 11.1 GPa. After this transition, the coexistence of tetragonal (I41/a) and monoclinic (P21) structures is detected up to 21.1 GPa. Beyond this pressure and up to 51.2 GPa, only the high-pressure monoclinic phase is observed. Upon decompression all structural changes are reversible. Finally, the axial compressibilities for the different phases have been determined as well as the equations of state. © 2014 Elsevier Ltd.


Renero-Lecuna C.,University of Cantabria | Martin-Rodriguez R.,University of Cantabria | Gonzalez J.A.,MALTA CONSOLIDER Team | Gonzalez J.A.,University of Cantabria | And 7 more authors.
Chemistry of Materials | Year: 2014

This work investigates the electronic structure and photoluminescence properties of Co2+-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co2+ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co2+ (0.01 mol %) can be related to the 4T1(P) → 4A2(F) transition of CoO4 (Td), contrary to MgAl2O 4:Co2+ and ZnAl2O4:Co2+ spinels in which the red emission is ascribed to the 2E(G) → 4A2(F) transition. We show that the low-temperature emission band consists of a 4T1(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the 4T1(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co2+ coordination from 4-fold Td to 6-fold Oh. We also show that the optical properties of ZnO:Co2+ (Td) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co2+ enters into regular Zn2+ sites in low concentration systems (less than 5% of Co 2+), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy. © 2013 American Chemical Society.

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