Complete high-pressure single-crystal structure determinations have been performed up to 90 kbar on ruby (Al2O3 with less than 0.05 wt% Cr2O3) using an automatic diffractometer. The rhombohedral lattice constant aR is found to decrease linearly within the given error limits from aR=5.136±0.004 to aR=5.075±0.005 Å at 90 kbar. The rhombohedral angle αR=55.30°±0.06° as well as the atom-position parameters u (Al, Cr) =0.3520±0.0004 for Al (or Cr) and u (0) =0.556±0.003 for oxygen remain constant up to at least 86 kbar. These results are in very good agreement with earlier measurements up to 25 kbar by Schiferl and Jamieson as well as with the Birch and Murnaghan equations of state with the literature values of the isothermal compresssibility K0 and its pressure derivative K′.

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Table 1. Au-C1 distances in the coordination octahedra at different pressuresUsing a gasketed diamond-anvil high-pressure cell, which has been adapted to the automatic four-circle diffractometer, we have studied the structural changes e (Pax 10 -8) of single crystals of the mixed-valence compound 10_3 Cs2AuIAuIIIC16 under high hydrostatic pressure.

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