We report a high-pressure study of monoclinic monazite-type SrCrO 4 up to 26 GPa. Therein we combined x-ray diffraction, Raman and optical-absorption measurements with ab initio calculations, to find a pressure-induced structural phase transition of SrCrO 4 near 8-9 GPa. Evidence of a second phase transition was observed at 10-13 GPa. The crystal structures of the high-pressure phases were assigned to the tetragonal scheelite-type and monoclinic AgMnO 4 -type structures. Both transitions produce drastic changes in the electronic band gap and phonon spectrum of SrCrO 4 . We determined the pressure evolution of the band gap for the low-and high-pressure phases as well as the frequencies and pressure dependences of the Raman-active modes. In all three phases most Raman modes harden under compression; however the presence of low-frequency modes which gradually soften is also detected. In monazite-type SrCrO 4 , the band gap blue-shifts under compression, but the transition to the scheelite phase causes an abrupt decrease of the band gap in SrCrO 4 . Calculations showed good agreement with experiments and were used to better understand the experimental results. From x-ray diffraction studies and calculations we determined the pressure dependence of the unit-cell parameters of the different phases and their ambient-temperature equations of state. The results are compared with the high-pressure behavior of other monazites, in particular PbCrO 4 . A comparison of the high-pressure behavior of the electronic properties of SrCrO 4 (SrWO 4 ) andPbCrO 4 (PbWO 4 ) will also be made. Finally, the possible occurrence of a third structural phase transition is discussed. † ERASMUS student from Imperial College London, London SW7 2AZ, United Kingdom * Corresponding author, email: daniel.errandonea@uv.es 2 I. IntroductionPhotocatalytic materials which respond to ultra-violet (UV) and visible (VIS) light can be used in a wide variety of environmental applications [1]. As a consequence, they have received much attention in recent years. In particular, progress has been made thanks to the development of chromium-based compounds [1]. Among them, lead chromate (PbCrO 4 ) and strontium chromate (SrCrO 4 ) are the most studied materials due to their unique properties [2 -5]. The crystal structures of these ternary oxides have been determined accurately [6], both being assigned to a monazite-type structure (space group P2 1 /n, Z = 4). A schematic view of the monazite structure is given in Fig. 1.The structural arrangement is based on the nine-fold coordination of the Pb (Sr) cation and the fourfold coordination of the Cr cation. The ambient-pressure lattice vibrations and electronic band structures of PbCrO 4 and SrCrO 4 have already been studied too [7]. During the last decade, high pressure (HP) has been shown to be an efficient tool for improving the understanding of the physical properties of ternary oxides [8 -15]. In particular, numerous monazite-type oxides have already been the subject of HP studies [16 -19], which have concentrat...

We have conducted Raman spectroscopy experiments on liquid ethane (C 2 H 6 ) at 300 K, obtaining a large amount of data at very high resolution. This has enabled the observation of Raman peaks expected but not previously observed in liquid ethane and a detailed experimental study of the liquid that was not previously possible. We have observed a transition between rigid and nonrigid liquid states in liquid ethane at ca. 250 MPa corresponding to the recently proposed Frenkel line, a dynamic transition between rigid liquid (liquidlike) and nonrigid liquid (gaslike) states beginning in the subcritical region and extending to arbitrarily high pressure and temperature. The observation of this transition in liquid (subcritical) ethane allows a clear differentiation to be made between the Frenkel line (beginning in the subcritical region at higher density than the boiling line) and the Widom lines (emanating from the critical point and not existing in the subcritical region). Furthermore, we observe a narrow transition at ca. 1000 MPa to a second rigid liquid state. We propose that this corresponds to a state in which orientational order must exist to achieve the expected density and can view the transition in analogy to the transition in the solid state away from the orientationally disordered phase I to the orientationally ordered phases II and III.

We report experimental evidence for a crossover between a liquidlike state and a gaslike state in fluid methane (CH_{4}). This crossover is observed in all of our experiments, up to a temperature of 397 K, 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and x-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line-a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquidlike and gaslike properties. On the liquidlike side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gaslike side this competes with the attractive van der Waals potential leading the vibration frequency to decrease as pressure is increased.

We have conducted a Raman study of methane (CH4), a major constituent of the outer planets, at pressures up to 165 GPa. We observe splitting of the principal Raman‐active vibrational mode above 45 GPa and a nonlinear dependence of Raman peak position on pressure. A discontinuous change in the pressure derivative of the ν3 peak position is observed at approximately 75 GPa, corresponding to the phase change previously observed using X‐ray diffraction. The Grüneisen parameters for the principal Raman‐active modes of methane in the simple cubic and high‐pressure cubic phases are calculated. The predicted dissociation of methane at ultrahigh pressure to form C2H6 and H2 is not observed, but an additional discontinuous change in the pressure‐induced shift of the Raman peaks is observed at 110 GPa. We suggest that this may be due to some reorientation or reordering of the methane molecules within the framework of the known cubic lattice.

We studied the electronic and vibrational properties of monazite-type SrCrO under compression. The study extended the pressure range of previous studies from 26 to 58 GPa. The existence of two previously reported phase transitions was confirmed at 9 and 14 GPa, and two new phase transitions were found at 35 and 48 GPa. These transitions involve several changes in the vibrational and transport properties with the new high-pressure phases having a conductivity lower than that of the previously known phases. No evidence of chemical decomposition or metallization of SrCrO was detected. A tentative explanation for the reported observations is discussed.