Temperature and stress dependence of rhombohedral twinning behavior of
sapphire single crystals was investigated. Specimens were deformed in
compression along the c-axis with an Instron universal testing machin
e below 1100 degrees C in order to avoid basal twinning and dislocatio
n slip. Rhombohedral twinning formation can be explained by nucleation
of a double kink in the twinning dislocation, while twin growth can b
e treated as a twinning dislocation movement which is driven by atomic
diffusion at elevated temperatures and by mechanical slip at relative
ly low temperatures. The activation enthalpy and activation volume of
rhombohedral twinning dislocation and also the activation energy of tw
inning dislocation movement were measured using Smidt's empirical equa
tion and Arrhenius' equation, respectively.