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La Laguna, Spain

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.

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.

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.

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.

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