Stacking faults and dislocations in titanium dioxide, with special reference to non-stoichiometry

Abstract

A detailed electron microscopical investigation has been made of the stacking faults and dislocations observed in thin films of titanium dioxide grown on the (100) faces of titanium carbide crystals. The large stacking faults formed during the growth process lie on a {101} plane, but they often change from one plane to another of the same family, sometimes on too fine a scale to be clearly resolved. The fault is terminated by a partial dislocation having a vector of the $\frac{1}{2}\langle 101 \rangle$-type; if the specimen is heated in the microscope, when it becomes non-stoichiometric, the fault anneals out by one of two mechanisms. The first mechanism involves the glide of the partial dislocation terminating the fault, and the second the growth of small dislocation loops formed by the condensation of vacancies introduced as a result of deviations from the stoichiometric composition. Contrast experiments show that the observed dislocations are of two types. The first are dissociated dislocations having a partial $\frac{1}{2}\langle 101 \rangle$ vector, glissile on {101} planes and associated with a stacking fault. The second type of dislocation are undissociated and have a $\langle 001 rangle$ Burgers vector. Interaction between the leading partials of the [101]-type dislocations moving on intersecting {101} slip planes gives rise to a stair-rod dislocation with either a $\langle 001 \rangle$ or $\frac{1}{2}\langle 110 \rangle$ Burgers vector. A sessile configuration is also formed by an interaction between dislocations with $\frac{1}{2}\langle 101 \rangle$ and $\langle 001 \rangle$ Burgers vector. An interaction between glissile partial dislocations and vacancy clusters also occurs, and it is suggested that this is a possible mechanism for the increased yield stress produced when TiO$_2$ becomes substoichiometric.