Paraelasticity, a mechanical analog of paramagnetism

Abstract

O − 2 molecule ions can be substituted for halide ions in alkali halides. The internuclear axis has six equilibrium orientations namely, along the [110] axes of the crystal. In the unstressed crystal, all six orientations are equally populated. A mechanical stress applied to the host crystal lifts the sixfold orientational degeneracy and thus induces population differences that correspond to an alignment of the molecule-ions. This effect has been studied by means of paramagnetic resonance for uniaxial stress along [111], [110] and [100] and KC1, KBr and KI, in the temperature range between 1·5°K and 33°K. The experiments show that the distribution of the molecule-ions over the orientations that become nonequivalent through the stress is given by Boltzmann statistics, and that the energy differences occurring in the Boltzmann factors are proportional to the stress. The results are analyzed in terms of the continuum theory of lattice defects. The kinetics of the alignment and reorientation process was studied at and below 4·2°K. The O − 2 molecule-ions do not freeze in. The reorientation time τ at these low temperatures is inversely proportional to the absolute temperature and depends upon the host crystal and upon the stress axis. The dependence of τ on the stress axis can be understood in terms of two transition probabilities, one involving a 90° rotation of the molecule-ion in a (100)-type plane, the other a 60° rotation out of this plane.