Ferroelectric polymorphic phenomena in the layered antiferromagnet Cu(OH)₂

Abstract Ferroic orders and their associated structural phase transitions are paramount in the understanding of a multitude of unconventional condensed matter phenomena. On this note, our investigation focuses on the polymorphic ferroelectric phase transitions of Copper(II) hydroxide, Cu(OH)2, considering an antiferromagnetic ground state. By employing the first-principles studies and group theory analysis, we have provided a systematic theoretical investigation of vibrational properties in the hypothetical Cmcm high-symmetry phase to unveil the symmetry-allowed ferroic phases. We identified a non-polar to polar (Cmc21) phase transition, in which the displacive transformation is primarily responsible for the phase change induced by two B1u (i.e. Γ- 2 ) phonon modes within the centrosymmetric phase. We also observed the existence of two polar structures with the same space group and different degrees of polarization (i.e Ps = 3.06 µC.cm-2 and Ps = 42.41 µC.cm-2), emerging from the high symmetry non-polar structure. According to the structural analysis the ferroelectric order, of a geometric nature, is driven by the Γ- 2 mode in which the O- and H sites displacements lead the polar distortion with a minor contribution from the Cu-sites. Interestingly, the 3d9:Cu2+ Jahn-Teller distortion coupled with the orientational shifts of O-H atoms enhances the polarization.