The lattice parameters of nickel-bromine boracite have been measured between 25 and 200°C by single-crystal X-ray diffraction. A first-order orthorhombic (Pca21) to cubic (F 43c) phase transition was observed at about 120°C. The transition temperature and range of coexistence of the two phases was found to be specimen-dependent. No thermal hysteresis was observed. The behaviour of the spontaneous strains can be predicted using a phenomenological (mean-field) theory for the transition, giving excellent agreement between observed and calculated values. Some remarks regarding the suitability of the material for use in pyroelectric detectors are also made.
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Wood et al. (1981) studied this question.