Structural characterisation of the Pa(3)over-bar-type, high pressure phase of ruthenium dioxide

Cubic Pa(3) over bar-type RuO2 was prepared from the ambient pressure, rutile-structured phase al 20 GPa and 1100 degrees C in a multianvil device. The structure of this Pa(3) over bar phase was refined by time-of-flight, neutron powder diffraction on the quenched sample yielding a cell constant a = 4.85892(3) Angstrom and an oxygen positional parameter u = 0.35115(8). The ruthenium cation is rhombohedrally coordinated with six anions at 1.9893(4) Angstrom and two more distant anions at 2.9552(4) Angstrom. The minimum interpolyhedral O-O distance of 2.5045(5) Angstrom in this structure is the shortest known in any solid and is shorter than the intrapolyhedral O-O distances, which are of 2.6208(5) Angstrom. These short distances are the origin of the very low compressibility of this oxide phase, which approaches that of diamond. The Raman spectrum of Pa(3) over bar-type RuO2 is consistent with group-theoretical calculations. (C) 1997 Elsevier Science Ltd. All rights reserved.