arXiv subjects

F. J. Manjon

Publications and source records attributed to F. J. Manjon.

At least 19 recordsLinked to original sources

arXiv2026

Pressure-induced hypercoordination of iodine and dimerization of I2O6H in strontium di-iodate hydrogen-iodate (Sr(IO3)2HIO3)

In this work, we report evidence of pressure-induced changes in the crystal structure of Sr(IO3)2HIO3 connected to changes the coordination of the iodine atom and the of the configuration of HIO3 and IO3 units. The changes favor iodine hypercoordination and happen in two steps on sample compression. Firstly, at 2.5 GPa, [HIO3]-[IO3] complexes are formed, and secondly, at 4.5 GPa, these complexes form dimers of [HIO3]-[IO3]-[IO3]-[HIO3]. The evidence is obtained from a combined experimental and theoretical study performed up to 20 GPa. Synchrotron powder X-ray diffraction, Raman spectroscopy, and optical-absorption experiments have been complemented with density-functional theory calculations, including the study of the topology of the electron density. The changes observed in the crystal structure are related to the transformation of secondary (halogen) bonds into electron-deficient multicenter bonds. The paper also discusses the effect of pressure on the compressibility of the Sr(IO3)2HIO3 crystal structure, its phonons, the electronic band gap, and the refractive index. Sr(IO3)2HIO3 was found to be highly compressible with an anisotropic compressibility. The softening of the internal I-O vibrations of IO3 units was also observed, together with a decrease of the band-gap energy (from 4.1 eV at 0 GPa to 3.7 eV at 20 GPa), a band-gap crossing, and a change in the topology of the band structure, with Sr(IO3)2HIO3 transforming from a direct gap semiconductor at 0 GPa to an indirect gap semiconductor beyond 6 GPa.

cond-mat.mtrl-sci↗

arXiv2024

Structural and vibrational study of Zn(IO3)2 combining high-pressure experiments and density-functional theory

We report a characterization of the high-pressure behavior of zinc-iodate, Zn(IO3)2. By the combination of x-ray diffraction, Raman spectroscopy, and first-principles calculations we have found evidence of two subtle isosymmetric structural phase transitions. We present arguments relating these transitions to a non-linear behavior of phonons and changes induced by pressure on the coordination sphere of the iodine atoms. This fact is explained as a consequence of the formation of metavalent bonding at high-pressure which is favored by the lone-electron pairs of iodine. In addition, the pressure dependence of unit-cell parameters, volume, and bond is reported. An equation of state to describe the pressure dependence of the volume is presented, indicating that Zn(IO3)2 is the most compressible iodate among those studied up to now. Finally, phonon frequencies are reported together with their symmetry assignment and pressure dependence.

cond-mat.mtrl-sci↗

arXiv2020

High-pressure characterization of multifunctional CrVO4

The structural stability and physical properties of CrVO4 under compression were studied by X-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure X-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is isomorphic to the wolframite structure. Such a phase transition (CrVO4-type - wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, is reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2015

High-pressure structural and elastic properties of Tl2O3

The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic a-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa.

cond-mat.mtrl-sci↗

arXiv2014

Room-temperature vibrational properties of multiferroic MnWO4 under quasi-hydrostatic compression up to 39 GPa

The multiferroic manganese tungstate (MnWO4) has been studied by high-pressure Raman spectroscopy at room temperature under quasi-hydrostatic conditions up to 39.3 GPa. The low-pressure wolframite phase undergoes a phase transition at 25.7 GPa, a pressure around 8 GPa higher than that found in previous works which used less hydrostatic pressure-transmitting media. The pressure dependence of the Raman active modes of both the low- and high-pressure phases are reported and discussed comparing with the results available in the literature for MnWO4 and related wolframites. A gradual pressure-induced phase transition from the low- to the high- pressure phase is suggested on the basis of the linear intensity decrease of the Raman mode with the lowest frequency up to the end of the phase transition.

cond-mat.mtrl-sci↗

arXiv2012

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) Å, ch = 17.9421(5) Å, V = 3348.1(1) Å3. 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.

cond-mat.mtrl-sci↗

arXiv2011

Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4

We have performed an experimental study of the crystal structure, lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band-gap have been accurately determined. X-ray-diffraction, Raman, and optical-absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO4. The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.

cond-mat.mtrl-sci↗

arXiv2011

High-pressure Raman spectroscopy and lattice-dynamics calculations on scintillating MgWO4: A comparison with isomorphic compounds

Raman scattering measurements and lattice-dynamics calculations have been performed on magnesium tungstate under high pressure up to 41 GPa. Experiments have been carried out under a selection of different pressure-media. The influence of non-hydrostaticity on the structural properties of MgWO4 and isomorphic compounds is examined. Under quasi-hydrostatic conditions a phase transition has been found at 26 GPa in MgWO4. The high-pressure phase has been tentatively assigned to a triclinic structure similar to that of CuWO4. 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.

cond-mat.mtrl-sci↗

arXiv2011

Zircon to monazite phase transition in CeVO4

X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to 12 and 16 GPa, respectively. Experiments reveal that 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 Eg and internal B2g bending modes. We attributed 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 non-hydrostatic stresses on the structural sequence is studied and the equations of state of different phases are reported.

cond-mat.mtrl-sci↗

arXiv2010

Post-spinel transformations and equation of state in ZnGa2O4: Determination at high-pressure by in situ x-ray diffraction

Room temperature angle-dispersive x-ray diffraction measurements on spinel ZnGa2O4 up to 56 GPa show evidence of two structural phase transformations. At 31.2 GPa, ZnGa2O4 undergoes a transition from the cubic spinel structure to a tetragonal spinel structure similar to that of ZnMn2O4. At 55 GPa, a second transition to the orthorhombic marokite structure (CaMn2O4-type) takes place. The equation of state of cubic spinel ZnGa2O4 is determined: V0 = 580.1(9) A3, B0 = 233(8) GPa, B0'= 8.3(4), and B0''= -0.1145 GPa-1 (implied value); showing that ZnGa2O4 is one of the less compressible spinels studied to date. For the tetragonal structure an equation of state is also determined: V0 = 257.8(9) A3, B0 = 257(11) GPa, B0'= 7.5(6), and B0''= -0.0764 GPa-1 (implied value). The reported structural sequence coincides with that found in NiMn2O4 and MgMn2O4.

cond-mat.mtrl-sci↗

arXiv2010

High-pressure structural phase transitions in CuWO4

We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase (P-1) to a monoclinic wolframite-type structure (P2/c). Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: V0 = 132.8(2) A3, B0 = 139 (6) GPa and = 4. Furthermore, experiments under different stress conditions show that non-hydrostatic stresses induce a second phase transition at 17 GPa and reduce the compressibility of CuWO4, B0 = 171(6) GPa. The pressure dependence of all Raman modes of the triclinic and high-pressure phases is also reported and discussed.

cond-mat.mtrl-sci↗

arXiv2008

High-pressure x-ray diffraction study on the structure and phase transitions of the defect-stannite ZnGa2Se4 and defect-chalcopyrite CdGa2S4

X-ray diffraction measurements on the sphalerite-derivatives ZnGa2Se4 and CdGa2S4 have been performed upon compression up to 23 GPa in a diamond-anvil cell. ZnGa2Se4 exhibits a defect tetragonal stannite-type structure (I-42m) up to 15.5 GPa and in the range from 15.5 GPa to 18.5 GPa the low-pressure phase coexists with a high-pressure phase, which remains stable up to 23 GPa. In CdGa2S4, we find the defect tetragonal chalcopyrite-type structure (I-4) is stable up to 17 GPa. Beyond this pressure a pressure-induced phase transition takes place. In both materials, the high-pressure phase has been characterized as a defect-cubic NaCl-type structure (Fm-3m). The occurrence of the pressure induced phase transitions is apparently related with an increase of the cation disorder on the semiconductors investigated. In addition, the results allow the evaluation of the axial compressibility and the determination of the equation of state for each compound. The obtained results are compared with those previously reported for isomorphic digallium sellenides. Finally, a systematic study of the pressure-induced phase transition in twenty-three different sphalerite-related ABX2 and AB2X4 compounds indicates that the transition pressure increases as the ratio of the cationic radii and anionic radii of the compounds increases.

cond-mat.mtrl-sci↗

arXiv2008

Combined Raman scattering and ab initio investigation of pressure-induced structural phase transitions in the scintillator ZnWO4

Room-temperature Raman scattering was measured in ZnWO4 up to 45 GPa. We report the pressure dependence of all the Raman-active phonons of the low-pressure wolframite phase. As pressure increases new Raman peaks appear at 30.6 GPa due to the onset of a reversible structural phase transition to a distorted monoclinic b-fergusonite-type phase. The low- and high-pressure phases coexist from 30.6 GPa to 36.5 GPa. In addition to the Raman measurements we also report ab initio total-energy and lattice-dynamics calculations for the two phases. These calculations helped us to determine the crystalline structure of the high-pressure phase and to assign the observed Raman modes in both the wolframite and b-fergusonite phases. Based upon the ab initio calculations we propose the occurrence of a second phase transition at 57.6 GPa from the b-fergusonite phase to an orthorhombic Cmca phase. The pressure evolution of the lattice parameters and the atomic positions of wolframite ZnWO4 are also theoretically calculated and an equation of state reported.

cond-mat.mtrl-sci↗

arXiv2007

High-pressure effects on the optical-absorption edge of CdIn2S4, MgIn2S4, and MnIn2S4 thiospinels

The effect of pressure on the optical-absorption edge of CdIn2S4, MgIn2S4, and MnIn2S4 thiospinels at room temperature is investigated up to 20 GPa. The pressure dependence of their band-gaps has been analyzed using the Urbach rule. We have found that, within the pressure-range of stability of the low-pressure spinel phase, the band-gap of CdIn2S4 and MgIn2S4 exhibits a linear blue-shift with pressure, whereas the band-gap of MnIn2S4 exhibits a pronounced non-linear shift. In addition, an abrupt decrease of the band-gap energies occurs in the three compounds at pressures of 10 GPa, 8.5 GPa, and 7.2 GPa, respectively. Beyond these pressures, the optical-absorption edge red-shifts upon compression for the three studied thiospinels. All these results are discussed in terms of the electronic structure of each compound and their reported structural changes.

cond-mat.mtrl-sci↗

arXiv2006

Lattice dynamics study in PbWO4 under high pressure

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, 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-to-III transition. Consequently, we found the coexistence of the scheelite, PbWO4-III, and fergusonite phases from 7.9 to 9 GPa, and of the last two phases up to 14.6 GPa. Further to the experiments, we have performed ab initio lattice dynamics calculations which have greatly helped us in assigning the Raman modes of the three phases and discussing their pressure dependence.

cond-mat.mtrl-sci↗

arXiv2006

Determination of the high-pressure crystal structure of BaWO4 and PbWO4

We report the results of both angle-dispersive x-ray diffraction and x-ray absorption near-edge structure studies in BaWO4 and PbWO4 at pressures of up to 56 GPa and 24 GPa, respectively. BaWO4 is found to undergo a pressure-driven phase transition at 7.1 GPa from the tetragonal scheelite structure (which is stable under normal conditions) to the monoclinic fergusonite structure whereas the same transition takes place in PbWO4 at 9 GPa. We observe a second transition to another monoclinic structure which we identify as that of the isostructural phases BaWO4-II and PbWO4-III (space group P21/n). We have also performed ab initio total energy calculations which support the stability of this structure at high pressures in both compounds. The theoretical calculations further find that upon increase of pressure the scheelite phases become locally unstable and transform displacively into the fergusonite structure. The fergusonite structure is however metastable and can only occur if the transition to the P21/n phases were kinetically inhibited. Our experiments in BaWO4 indicate that it becomes amorphous beyond 47 GPa.

cond-mat.mtrl-sci↗

arXiv2006

High-pressure lattice dynamics in bulk single-crystal BaWO4

Room-temperature Raman scattering has been measured in barium tungstate (BaWO4) up to 16 GPa. We report the pressure dependence of all the Raman-active first-order phonons of the tetragonal scheelite phase (BaWO4-I, space group I41/a), 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 BaWO4-II phase (space group P21/n). 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, BaWO4-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 behaviour of the observed Raman modes of the three phases.

cond-mat.mtrl-sci↗

arXiv2005

High-pressure structural study of the scheelite tungstates CaWO4 and SrWO4

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.

cond-mat.mtrl-sci↗