Ferroelectric Behavior of a Hexamethylenetetramine‐Based Molecular Perovskite Structure

Abstract

We report the development of a molecular ferroelectric material inspired by the hexamethylenetetramine (hmta) non-centrosymmetric molecular rotator. The bromide salt of diprotonated hmta (hmtaH₂ ) crystalized as (hmtaH₂ )(NH₄ )Br₃ in a metal-free ABX₃ perovskite-type structure, in which the A and B sites are occupied by hmtaH₂ ²⁺ and ammonium cations, respectively. The compound crystallized in the Pma2 polar space group. A distorted polar perovskite structure formed owing to the distortion of {(NH₄ )Br₆ } octahedrons that are stabilized through the formation of NH⋅⋅⋅Br hydrogen bonds and the orientational ordering of positive charges on the non-centrosymmetric hmtaH₂ molecules. This spontaneous polarization exhibited ferroelectric behavior with a nominally high Curie temperature (>400 K), in which the electrical switching of polarization originates from the rotation of the hmtaH₂ unit.