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The previously observed pressure-induced amorphization transition at 11 kbar [A.J. Schultz, U. Geiser, H.H. Wang, J.M. Williams, L.W. Finger and B.M. Hazen, Physica C 208 (1993) 277] was avoided and single-crystal X-ray data up to 28 kbar were obtained by replacing mineral oil with a hydrostatic alcohol mixture as the pressure medium in diamond anvil cell. The linear compressibilities of the crystallographic axes are remarkably isotropic given the two-dimensional nature of the structure. A full structure determination was performed at room temperature and 28 kbar with the use of X-ray data. It is noteworthy that, a novel structure feature, short interlayer H…H contacts through a hole in the anion layer, may account for the isotropic compressibilities. Electronic band structures calculated for the crystal structures of κ-(ET) 2 Cu[N(CN) 2 ]Cl at 1 bar, 3 kbar and 28 kbar show that this salt is a two-dimensional metal, and that the density of states at the Fermi level decreases gradually with increasing pressure.

Crystal structures of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z=0 plane: at ( 1 2 00) and close to ( 1 2 , 0.4, 0) - but no oxygen at ( 1 2 1 2 0) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess O at ( 1 2 00) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds.

A reversible symmetry-lowering structural phase transition is observed by X-ray diffraction techniques from single crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl (ET, or BEDT-TTF, is bis(ethylenedithio) tetrathiafulvalene, C 10 H 8 S 8 ) under applied pressure. At room temperature, by use of a diamond anvil cell, a transition above 8 kbar characterized by peak broadening begins to occur. The transition is also observed by cooling crystals coated with Apiezon N vacuum grease, which is a technique that is also used to produce the high pressure superconducting state ( T c =12.8 K, P =0.3 kbar). Given the anisotropic nature of the structure, the ambient pressure linear compressibilities are surprisingly isotropic, with values of β a =2.39(8)×10 -3 , β b =2.47(7)×10 -3 , β c =2.29(9)×10 -3 kbar -1 . At 3 kbar and room temperature, from a complete structure analysis it is shown that the ET molecule conformation is the same as that at ambient pressure although S⋯S contacts are reduced by as much as 0.05 A. A second transition, characterized by reversible disappearance of Bragg intensities consistent with pressure-induced amorphization, was observed between 11.4 and 11.8 kbar.

Undoped La 2 CuO 4 oxide ceramic single crystal and polycrystalline materials were studied up to 25.0 GPa in a diamond-anvil cell (DAC) apparatus. La 2 CuO 4 undergoes a phase transformation from an orthorhombic to a tetragonal structure at about 3.4 GPa, and this result is different from the data previously published by others. Compression of this material is only 9% at 25.0 GPa.