Abstract
The paper deals with the variation of lattice parameters and birefringence with temperature, using 'single crystals'. The change in behaviour of these quantities with annealing (ordering) is investigated. From the variation in lattice parameters and by measuring the intensities of reflections from twin components the structural distortions are followed as temperature and annealing (ordering) are varied. The lattice parameters and structural data derived from these results are used to describe the temperature dependence of the birefringence. The change in transition temperature with annealing time for several annealing temperatures is investigated. Using the X-ray continuous-recording technique the coexistence of the high- and low-temperature phases over the so-called Curie range is demonstrated. Accurate measurements using oscillation photographs taken on a camera of radius 57.3 mm, with the film in the Van Arkel mounting, show an increase in the unit-cell dimensions with annealing (ordering).

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