The high-pressure (HP) behavior of Fe­(IO3)3 was studied up to 35 GPa using powder X-ray diffraction, infrared micro-spectroscopy, and ab initio density-functional theory calculations. Fe­(IO3)3 shows a pressure-induced structural phase transition at 15–22 GPa. Powder X-ray diffraction was employed to obtain the structure of the HP phase. This phase can be described by the same space group (P63) as the low-pressure phase but with a substantial different c/a ratio. This conclusion is supported by our computational simulations. The discovered phase transition involves a large volume collapse and a change in the coordination polyhedron of iodine, being a first-order transition. It also produces substantial changes in the infrared and Raman vibrational spectra. The pressure dependences of infrared and Raman phonon frequencies and unit-cell parameters have been obtained. A mode assignment is proposed for phonons based upon ab initio calculations. The bulk modulus of the two phases was obtained by fitting a Birch–Murnaghan equation of state to synchrotron X-ray powder diffraction data resulting in B 0 = 55(2) GPa for the low-pressure phase and B 0 = 73(9) GPa for the HP phase. Calculations gave B 0 = 36(1) GPa and B 0 = 48(3) GPa for the same phases, respectively. The results are compared with other iodates, in particular LiIO3, for which we have also performed density-functional theory calculations. A possible mechanism driving the observed phase transition will be discussed.

No abstract

We report high-pressure Raman spectroscopy studies of Fe(IO3)3 up to nearly 21 GPa that have been interpreted with the help of density-functional theory calculations, that include the calculation of phonon dispersion curves and elastic constants at different pressures.Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a three-fold to almost six-fold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair, so that likely real six-fold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong nonlinear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 GPa and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.

Lacunar spinels, GaM4X8 (M = V, Nb, Mo, Ta, W; X = S, Se, Te), constitute a rare class of compounds with multiferroic properties. Recently, one member of this family, GaV4S8, received global attention due to the Néel-type skyrmions discovered in this material. Previous investigations strongly indicate the important role of the structure behind the multiferroicity, for example, the strong impact of ferroelectric transition on the exchange interactions at ∼40 K. Inspired by the delicate entanglement of lattice, spin, and charge degrees of freedom, in the present work, we aimed to use pressure to alter the structure and thus change the material’s properties to establish the inter-relation between the structural, optical, and electrical properties for a better understanding of this skyrmion host material. Upon this objective, in situ high-pressure measurements of single crystal/powder X-ray diffraction, electrical conductivity, and Raman spectroscopy were carried out by using a diamond anvil cell. These studies revealed the pressure-induced structural transformation from cubic to orthorhombic, along with a transition from semiconductor to metallic state in GaV4S8. The phase changes coincide with the variation in the optical property in this material explored by Raman spectra. We also determined the bulk modulus of the two phases of GaV4S8 by fitting the data set of unit cell volumes against pressure with the second-order birth-Murnaghan equation of state, and explained the mechanisms of phase transitions by means of the Jahn–Teller effect and the anisotropic changes in bonding lengths during compression.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3̅m1) to another trigonal structure (S-G-P3̅c1). Additionally, at 16 GPa, the P3̅c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3̅m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature (T c) of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides.