Correlation between curie temperature and cation radius in the ferroelectric BaFeF5 family

The prediction that BaFeF 5 is a displacive-type ferroelectric was made on the basis of its previously published structure. The present study reports a correlation between T c and cation radius for members of the isomorphous A II B III X 5 family. T c is found to increase with increasing VIII A 2+ ionic radius and decrease with increasing VI B 3+ ionic radius. As example BaFeF 5 exhibits T c = 730 K with r( VIII Ba 2+ ) = 1.42 A whereas Ba 0.5 Sr 0.5 FeF 5 shows T c = 620 K with r( VIII Sr 2+ ) = 1.25 A, for r(A 2+ ) used as the variable. The studied materials are relaxor ferroelectrics.