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The vibrational and structural behaviors of NiVO and CoVO orthovanadates have been studied up to around 20 GPa by means of X-ray diffraction, Raman spectra, and theoretical simulations. Both materials crystallize in an orthorhombic Kagomé staircase structure (space group: Cmca) at ambient conditions, and no phase transition was found in the whole pressure range. In order to identify the symmetry of the detected Raman-active modes under high pressure, single crystal samples of those materials were used in a polarized Raman and infrared setup. Moreover, high-pressure powder X-ray diffraction measurements were performed for CoVO, and the results confirmed the structure stability also obtained by other diagnostic techniques. From this XRD analysis, the anisotropic compressibilities of all axes were calculated and the unit-cell volume vs pressure was fitted by a Birch-Murnaghan equation of state, obtaining a bulk modulus of 122 GPa.

Ni V O and Co V O orthovanadates have been studied for decades due to their rich magnetic polymorphism [1] and multiferroic properties [2]. They also present several industrial applications in nanostructured systems, such as the water splitting process, electrochemical energy storage [3], nitrogen fixation and glucose detection. Ni V O have already been studied under high-pressure by means of powder XRD diffraction up to 23 GPa [4]. No phase transition has been found. In this work we would like to complement previous studies performed in Ni V O with new Raman experiments, and extend the research to a new compound, Co V O. We will analyse the results with the help of ab-initio calculations. Both compounds present a Kagomé staircase structure (Cmca) at ambient conditions and remain structurally stable up to 20 GPa. As there is a lack of information concerning the basic properties of these compounds, in a first step we have characterise the vibrations in ambient conditions. To this end, we have oriented single crystals to perform polarised Raman and Infrared measurements in ambient conditions, which we have interpreted thanks to DFT calculations. With the ambient pressure modes properly characterised, we have been able to follow the evolution of 24 (17) out of the 36 Raman active modes that powder Ni V O (Co V O) presents up to 19.5(1) (20.3(1)) GPa. Raman experiments confirm that Ni V O remains in the low-pressure phase in the studied pressure range. Co3V2O8 is also observed to be stable both by Raman and by XRD experiments. The analysis of the equation of state yields a bulk modulus of 122(1) GPa, which is slighty smaller than that of Ni V O [139(3) GPa]. Comparing the results of both compounds, we observe that all modes increase in wavenumber with the pressure, and the pressure coefficients obtained experimentally are in a good agreement with the simulated ones. In addition, all modes in Ni orthovanadate are higher in wavenumber than the Co orthovanadate ones. This

[EN] The structural, vibrational and electronic properties of the compressed ß-Sb2O3 polymorph, a.k.a. mineral valentinite, have been investigated in a joint experimental and theoretical study up to 23 GPa. The compressibility of the lattice parameters, unit-cell volume and polyhedral unit volume as well as the behaviour of its Raman- and IR-active modes under compression have been interpreted on the basis of ab initio theoretical simulations. Valentinite shows an unusual compressibility up to 15 GPa with four different pressure ranges, whose critical pressures are 2, 4, and 10 GPa. The pressure dependence of the main structural units, the lack of soft phonons, and the electronic density charge topology address the changes at those critical pressures to isostructural phase transitions of degree higher than 2. In particular, the transitions at 2 and 4 GPa can be ascribed to the changes in the interaction between the stereochemicallyactive lone electron pairs of Sb atoms under compression. The changes observed above 10 GPa, characterized by a general softening of several Raman- and IR-active modes, point to a structural instability prior to the 1st-order transition occurring above 15 GPa. Above this pressure, a tentative new high-pressure phase (s.g. Pcc2) has been assigned by single-crystal and powder X-ray diffraction measurements.

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/page/policy/articlesonrequest/index.html", [EN] The high-pressure behavior of technologically important visible-light photocatalytic semiconductor In.NbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calculations. The experimental results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compound beyond 10.8 GPa. The large volume collapse associated with the phase transition and the coexistence of two phases observed over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination number around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The experimental pressure volume data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low and high-pressure phases, respectively. The pressure dependence of the Raman mode frequencies and Gruneisen parameters was determined for both phases by experimental and theoretical methods. The same information is obtained for the infrared modes from first-principles calculations. Results from theoretical calculations corroborate the experimental findings. They also provide information on the compressibility of interatomic bonds, which is correlated with the macroscopic properties of InNbO4.

Compression of arsenolite has been studied from a joint experimental and theoretical point of view. Experiments on this molecular solid at high pressures with different pressure-transmitting media have been interpreted thanks to state-of-the-art ab initio calculations. Our results confirm arsenolite as one of the most compressible minerals and provide evidence for ordered helium trapping above 3 GPa between adamantane-type As4O6 cages. Our calculations indicate that, at relatively small pressures, helium establishes rather localized structural bonds with arsenic forming a compound with stoichiometry As4O6 center dot 2He. All properties of As4O6 center dot 2He are different from those of parent As4O6. In particular, pressure-induced amorphization, which occurs in arsenolite above 15 GPa, is impeded in As4O6 center dot 2He, thus resulting in a mechanical stability of As4O6 center dot 2He beyond 30 GPa. Our work paves the way to explore the formation of alternative compounds by pressure-induced trapping and reaction of gases, small atomic or molecular species, in the voids of molecular solids containing active lone electron pairs.

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Inorganic Chemestry, copyright © American Chemical Society after peer review. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/ic402043x, A new wolframite-type polymorph of InVO4 is identified under compression near 7 GPa by in situ high-pressure (HP) X-ray diffraction (XRD) and Raman spectroscopic investigations on the stable orthorhombic InVO4. The structural transition is accompanied by a large volume collapse (Delta V/V = -14%) and a drastic increase in bulk modulus (from 69 to 168 GPa). Both techniques also show the existence of a third phase coexisting with the low- and high-pressure phases in a limited pressure range close to the transition pressure. XRD studies revealed a highly anisotropic compression in orthorhombic InVO4. In addition, the compressibility becomes nonlinear in the HP polymorph. The volume collapse in the lattice is related to an increase of the polyhedral coordination around the vanadium atoms. The transformation is not fully reversible. The drastic change in the polyhedral arrangement observed at the transition is indicative of a reconstructive phase transformation. The HP phase here found is the only modification of InVO4 reported to date with 6-fold coordinated vanadium atoms. Finally, Raman frequencies and pressure coefficients in the low- and high-pressure phases of InVO4 are reported.