Temperature Dependence of Rutile (TiO2) and Geikielite (MgTiO3) Structures Determined Using Neutron Powder Diffraction

The temperature dependences of the structures of rutile and geikielite are reported up to ~1200C. While the thermal expansion of the unit cell edges for our rutile agree with published data the (Ti-O) bond expansions show significant differences with the longer (T-O) showing a larger thermal expansion coefficient (19.3 x 10 /degree) than the shorter bond (2.6 x 10 /degree). High-temperature crystallographic data for geikielite are reported for the first time and display interesting differences in the thermal behaviour of the (Ti-O) and (Mg-O) octahedra related to edgeand face-sharing. Thus (O-O) edges of faces shared by (Ti-O) and (Mg-O) octahedra contract, (O-O) edges shared by adjacent (Ti-O) (and Mg-O) octahedra show no significant change, and (O-O) edges of unshared faces between (Ti-O) (and Mg-O) octahedra and adjacent vacant sites expand. Mg-Mg and Ti-Ti distances across shared edges diverge at higher temperatures as (TiTi) increases and (Mg-Mg) does not change.