The "lone pairs" present in the Bismuth ion, i.e., the 6s2 lone pair present in Bi3+ ion of BiFeO3 repulses each other by going far apart, and therefore expanding and distorting the structure. Now because of this they form local dipoles. These local dipoles are the ones which respond with electric field by polarisation and hence ferroelectricity. Please refer to the following paper. It has far better explanation than what I said.
https://arxiv.org/ftp/arxiv/papers/1005/1005.3102.pdf
Article Visualizing the Role of Bi 6s “Lone Pairs” in the Off-Center...
The second paper is by N.A.Spaldin herself. But I found the 1st paper more easily described. For farthur assistance you can go for the second paper.
It has been two years since you have posted this question. Probably you already know the answer. I just answered here for those who hovers on internet searching for the answer of this question. Your question was the first suggestion by google but it was embarrassing for me to find it out to be unanswered when I was also looking for the answer of this question. Wish that not to happen with others.
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