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The compositions Rb2ZnCl4, Rb2ZnBr4, Rb2CoCl4, Rb2CoBr4 and {N(CH3)4}2CoCl4 have been found to be ferroelectric. The Curie temperatures of these Rb compounds are near −80°C. (N(CH3)4}2CoCl4 is ferroelectric in a narrow temperature range of 4.6–7.1°C.
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Sawada et al. (1978) studied this question.
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