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 (P4(2)/mnm, Z = 2) yielded cell constants a = 4.49307 (7) and c = 3.10639 (7) Angstrom, 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) Angstrom, 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)degrees with respect to their orientation in the rutile-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.