New Ferroelectrics Based on Divalent Metal Ion Alum

A detailed study of two alum-type complexes containing the SeO4 2- anion, 1,4-diazoniabicyclo[2.2.2]octane hexaaquacopper(II) bis(selenate) [(H2dbco)Cu(H2O)6(SeO4)2, 1] and its deuterated analogue (D2dbco)Cu(D2O)6(SeO4)2 (2), has revealed that 1 and 2 are new ferroelectrics that undergo a paraelectric−ferroelectric phase transition at ca. −140 to −138 °C as a result of order−disorder features of the cation ([H2dbco]2+ or [D2dbco]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 ∼1960.