Electron Microscopic Observations for the High-index Crystallographic Shear Phases in the System Cr₂O₃-TiO₂

Abstract

Electron diffraction and lattice image observations have been applied to study the high-index crystallographic shear (CS) phases derived from rutile-type structure in the system Cr₂O₃-TiO₂. At 1390°C, there existed many CS phases where the orientation of the CS planes continuously changed from (121)_r to (5, 12, 7)_r and the spacing of CS planes increased from 12.9Å to 18.2Å according to the decrease of Cr₂O₃ contents in the range of 13.7-24.1wt% Cr₂O₃. The high-index (5, 12, 7)_r CS phase coexisted with rutile solid solution in the range of 7.5-13.7wt% Cr₂O₃. The highest end member of (121)_r CS phase appeared to be Cr₂Ti₆O₁₅ instead of Cr₂Ti₇O₁₇ which was previously regarded as a member of (121)_r CS phase by X-ray diffraction analysis. The crystals had ordered features with equal spaced CS planes within each fragment, whereas the structures of high-index CS phases were slightly different from one fragment to another obtained from a bulk specimen with constant composition.
The structural model for (5, 12, 7)_r CS plane was presented on the basis of the structural units for (011)_r and (121)_r boundaries and it seemed that (5, 12, 7)_r CS plane had zig-zag feature rather than straight line along CS plane.