The system TiO 2Cr 2O 3: Electron microscopy of solid solutions and crystallographic shear structures

Abstract

The TiO 2-rich end of the system TiO 2CrO 1.5 has been studied by electron diffraction and electron microscopy, to examine the effects of controlled anion deficiency upon rutile-based structures. Materials equilibrated at 1330 K to 1570 K fall into three ranges. With > 15 mol % CrO 1.5, compounds of the known series Cr 2Ti n−2 O 2 n−1 , with (121) crystallographic shear planes are formed. With < 5 mol % CrO 1.5, rutile structure-defect solid solutions are found, with no shear planes. It appears that the anion deficiency is accommodated by point defects or small defect clusters. The limiting composition of this solid-solution range is strongly temperature dependent, and the rutile solid solutions are readily reduced, with the introduction of shear planes on (132), as for TiO 2 itself. Between these limiting ranges, well-ordered shear phases are formed, in which the slabs of rutile structure (the normal spacing between recurrent shear planes) remain constant but the orientation of the shear planes pivots round progressively from (121) to (132) as the anion : cation ratio is changed.