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High-pressure synchrotron X-ray diffraction studies of cobalt iodate, Co(IO3)2, reveal a counterintuitive pressure-induced expansion along certain crystallographic directions. High-pressure Raman and infrared spectroscopy, combined with density-functional theory calculations, reveal that with increasing pressure, it becomes energetically favorable for certain I–O bonds to increase in length over the full range of pressure studied up to 28 GPa. This phenomenon is driven by the high-pressure behavior of iodate ion lone electron pairs. Two pressure-induced isosymmetric monoclinic–monoclinic phase transitions are observed at around 3.0 and 9.0 GPa, which are characterized by increasing oxygen coordination of the iodine atoms and the probable formation of pressure-induced metavalent bonds. Pressure–volume equations of state are presented, as well as a detailed discussion of the pressure dependences of the observed Raman- and infrared-active modes, which clarifies previous inconsistencies in the literature.

The crystal structure and the vibrational and optical characteristics of flux-grown mixed lanthanide vanadate compounds SmxNd1–xVO4(x= 0, 0.1, 0.25, 0.5, 0.75 and 1) are reported. A linear, monotonic decrease of the unit-cell parameters is observed with increasing Sm content, as expected according to the lanthanide contraction phenomenon and Vegard’s law. The evolution of Raman phonons across the series is explained by the reduction of the unit-cell size and the length of atomic bonds. UV–vis absorption spectra are found to be a weighted superposition of the absorption spectra of SmVO4and NdVO4, while the photoluminescence of the mixed compounds is heavily quenched, which is explained by the energy transfer processes occurring between energy levels of Sm3+and Nd3+. Finally, the influence of pressure on the structure of Sm0.5Nd0.5VO4is reported through an X-ray diffraction study by synchrotron radiation. There is evidence of two structural phase transitions (zircon-scheelite and scheelite-fergusonite) in the pressure range up to 23.1 GPa. The zircon-scheelite-fergusonite transition is partially reversible, as a mixture of zircon and scheelite is found after pressure release. Enthalpy calculations on isostructural orthophosphates explain the appearance of a metastable monoclinic phase in the range 8—14 GPa. Finally, hints of a possible further phase transition to an orthorhombic structure are put in evidence by the trend of the monoclinic angle of the fergusonite polymorph.

We have performed an experimental study of the crystal structure, lattice dynamics, and optical properties of PbCrÛ4 (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 from a monoclinic structure to another monoclinic structure. It maintains the symmetry of the crystal but has important consequences in the physical properties; among others, 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 PbCr04. The results are discussed in comparison with related compounds, and the effects of pressure in the electronic structure are explained. Finally, the room-temperature equation of slate of the low-pressure phases is also obtained [ABSTRACT FROM AUTHOR]