The size of nanocrystals provides a limitation on dislocation activity and associated stress-induced deformation. Dislocation-mediated plastic deformation is expected to become inactive below a critical particle size, which has been proposed to be between 10 and 30 nanometers according to computer simulations and transmission electron microscopy analysis. However, deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3-nanometer crystals. Substantial texturing is observed at pressures above 3.0 gigapascals for 500-nanometer nickel and at greater than 11.0 gigapascals for 20-nanometer nickel. Surprisingly, texturing is also seen in 3-nanometer nickel when compressed above 18.5 gigapascals. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometers-length scale.

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...

To extend the range of high-temperature, high-pressure studies within the diamond anvil cell, a Liermann-type diamond anvil cell with radial diffraction geometry (rDAC) was redesigned and developed for synchrotron X-ray diffraction experiments at beamline 12.2.2 of the Advanced Light Source. The rDAC, equipped with graphite heating arrays, allows simultaneous resistive and laser heating while the material is subjected to high pressure. The goals are both to extend the temperature range of external (resistive) heating and to produce environments with lower temperature gradients in a simultaneously resistive- and laser-heated rDAC. Three different geomaterials were used as pilot samples to calibrate and optimize conditions for combined resistive and laser heating. For example, in Run#1, FeO was loaded in a boron-mica gasket and compressed to 11 GPa then gradually resistively heated to 1007 K (1073 K at the diamond side). The laser heating was further applied to FeO to raise temperature to 2273 K. In Run#2, Fe-Ni alloy was compressed to 18 GPa and resistively heated to 1785 K (1973 K at the diamond side). The combined resistive and laser heating was successfully performed again on (Mg0.9Fe0.1)O in Run#3. In this instance, the sample was loaded in a boron-kapton gasket, compressed to 29 GPa, resistive-heated up to 1007 K (1073 K at the diamond side), and further simultaneously laser-heated to achieve a temperature in excess of 2273 K at the sample position. Diffraction patterns obtained from the experiments were deconvoluted using the Rietveld method and quantified for lattice preferred orientation of each material under extreme conditions and during phase transformation.