126 research outputs found
High-pressure x-ray diffraction study of bulk and nanocrystalline PbMoO4
We studied the effects of high-pressure on the crystalline structure of bulk
and nanocrystalline scheelite-type PbMoO4. We found that in both cases the
compressibility of the materials is highly non-isotropic, being the c-axis the
most compressible one. We also observed that the volume compressibility of
nanocrystals becomes higher that the bulk one at 5 GPa. In addition, at 10.7(8)
GPa we observed the onset of an structural phase transition in bulk PbMoO4. The
high-pressure phase has a monoclinic structure similar to M-fergusonite. The
transition is reversible and not volume change is detected between the low- and
high-pressure phases. No additional structural changes or evidence of
decomposition are found up to 21.1 GPa. In contrast nanocrystalline PbMoO4
remains in the scheelite structure at least up to 16.1 GPa. Finally, the
equation of state for bulk and nanocrystalline PbMoO4 are also determined.Comment: 18 pages, 4 figure
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, being the most incompressible orthovanadate studied to
date. The sequence of structural transitions and compressibilities are
discussed in comparison with other zircon-type oxides.Comment: 34 pages, 2 Tables, 11 Figure
New high-pressure phase and equation of state of Ce2Zr2O8
In this paper we report a new high-pressure rhombohedral phase of Ce2Zr2O8
observed from high-pressure angle-dispersive x-ray diffraction and Raman
spectroscopy studies up to nearly 12 GPa. The ambient-pressure cubic phase of
Ce2Zr2O8 transforms to a rhombohedral structure beyond 5 GPa with a feeble
distortion in the lattice. Pressure evolution of unit-cell volume showed a
change in compressibility above 5 GPa. The unit-cell parameters of the
high-pressure rhombohedral phase at 12.1 GPa are ah = 14.6791(3) {\AA}, ch =
17.9421(5) {\AA}, V = 3348.1(1) {\AA}3. The structure relation between the
parent cubic (P2_13) and rhombohedral (P3_2) phases were obtained by
group-subgroup relations. All the Raman modes of the cubic phase showed linear
evolution with pressure with the hardest one at 197 cm-1. Some Raman modes of
the high-pressure phase have a non-linear evolution with pressure and softening
of one low-frequency mode with pressure is found. The compressibility, equation
of state, and pressure coefficients of Raman modes of Ce2Zr2O8 are also
reported.Comment: 33 pages, 8 figures, 6 table
Phonons and Colossal Thermal Expansion Behavior of Ag3Co(CN)6 and Ag3Fe(CN)6
Recently colossal positive volume thermal expansion has been found in the
framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been
measured using the inelastic neutron scattering technique as a function of
temperature and pressure. The data has been analyzed using ab-initio
calculations. We find that the bonding is very similar in both compounds. At
ambient pressure modes in the intermediate frequency part of the vibrational
spectra in the Co compound are shifted to slightly higher energies as compared
to the Fe compound. The temperature dependence of the phonon spectra gives
evidence for large explicit anharmonic contribution to the total anharmonicity
for low-energy modes below 5 meV. We found that modes are mainly affected by
the change in the size of unit cell, which in turn changes the bond lengths and
vibrational frequencies. Thermal expansion has been calculated via the volume
dependence of phonon spectra. Our analysis indicates that Ag phonon modes in
the energy range from 2 to 5 meV are strongly anharmonic and major contributors
to thermal expansion in both compounds. The application of pressure hardens the
low-energy part of the phonon spectra involving Ag vibrations and confirms the
highly anharmonic nature of these modes.Comment: 19 pages, 14 figures and one tabl
High-pressure polymorphs of TbVO4: A Raman and ab initio study
Raman measurements on TbVO4 show the occurrence of three pressure-induced phase transitions. The first one, an irreversible transition from the zircon to the scheelite structure, occurs beyond 6.7 GPa. In addition, two reversible transformations take place at 26.7 and 34.4 GPa. The last transition was never reported before. The experimental findings are supported by structural and lattice-dynamics calculations that helped us to identify the post-scheelite phase as a monoclinic fergusonite structure. According to the calculations, the third transition involves a symmetry increase. An orthorhombic structure is proposed for the phase found above 34.4 GPa. The results have been compared with previous studies in TbVO4 and discussed in comparison with related compounds. The calculated equations of state are reported for the different polymorphs of TbVO4. A compressibility increase is caused by the third transition. It is associated to a bond-strength decrease, which is related to a coordination increase and a delocalization of Tb f-electrons. (C) 2013 Elsevier B.V. All rights reserved.Research supported by the Spanish MINECO (Grants No.: MAT2010-21270-C04-01/03/04), MALTA Consolider (CSD2007-00045), and Vicerrectorado de Investigacion y Desarrollo of UPV (UPV2011-0966/0914). The authors also acknowledge the computing time provided by Red Espanola de Supercomputacion and MALTA-Cluster.Errandonea, D.; Manjón Herrera, FJ.; Muñoz, A.; Rodríguez-Hernández, P.; Panchal, V.; Achary, SN.; Tyagi, AK. (2013). High-pressure polymorphs of TbVO4: A Raman and ab initio study. Journal of Alloys and Compounds. 577:327-335. https://doi.org/10.1016/j.jallcom.2013.06.008S32733557
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