Neutron Diffraction Study of the Ambient-Pressure, Rutile-Type and the High-Pressure, CaCl₂-Type Phases of Ruthenium Dioxide

The structures of the ambient-pressure, rutile-type and the high-pressure, CaCl₂-type phases of RuO₂ were refined by time-of-flight neutron powder diffraction. Refinement of the data obtained from the ambient-pressure phase (P4₂/mnm, Z = 2) yielded cell constants a = 4.49307 (7) and c = 3.10639 (7) Å and an oxygen positional parameter x = 0.3056 (1). The principal axes of the displacement ellipsoids of the oxygen ions lie along the directions which correspond to the normal coordinate of the B_1g soft mode that drives the transition to the CaCl₂-type structure. The CaCl₂-type phase (Pnnm, Z = 2) was refined in situ at 5.3 (3) GPa and the following cell constants and oxygen positional parameters were obtained: a = 4.4865 (5), b = 4.4347 (5), c = 3.0934 (3) Å, x = 0.3101 (5) and y = 0.3005 (5). In the CaCl₂-type phase at 5.3 (3) GPa the RuO₆ octahedra are rotated by 1.1 (2)° with respect to their orientation in the ruffle-type phase. There is little change in the internal angles of the octahedron between 0.1 MPa and 5.3 GPa. Compression of the octahedron is anisotropic with a much greater reduction of the Ru-O contacts in the ab plane.

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