New ferroelectrics based on divalent metal ion alum.

A detailed study of two alum-type complexes containing the SeO(4)(2-) anion, 1,4-diazoniabicyclo[2.2.2]octane hexaaquacopper(II) bis(selenate) [(H(2)dbco)Cu(H(2)O)(6)(SeO(4))(2), 1] and its deuterated analogue (D(2)dbco)Cu(D(2)O)(6)(SeO(4))(2) (2), has revealed that 1 and 2 are new ferroelectrics that undergo a paraelectric-ferroelectric phase transition at ca. -140 to -138 degrees C as a result of order-disorder features of the cation ([H(2)dbco](2+) or [D(2)dbco](2+)) and anion (selenate). These are the first examples of ferroelectrics based on divalent metal anion alum analogues since the discovery of ferroelectrics based on trivalent metal anion alum in approximately 1960.