Abstract
The corundum structure ( alpha -R2O3 where R=Al, Ti, V, Cr, Fe) has been chosen for the first detailed study of inner displacement and internal strain in a moderately complex system. It is shown that the inner displacements of sublattices of ions are governed by two independent internal strain tensors: these relate to O-O and R-R displacements and have eight and six independent components respectively. These results are used to give a proper description of the deformation of octahedral clusters of O2- ions surrounding an R3+ cation when the crystal is subject to uniaxial stress. Such clusters and their deformations are important in the understanding of a variety of recent experiments but have been the subject of controversy and erroneous description. The major components of the internal strain tensors have been estimated using a pseudo-HCP model of the anion structure.

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