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

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 (FE) 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 hypotheticalCmcmhigh-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 twoB1u(i.e.Γ2-) 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.P_s= 3.06µC·cm^-2andP_s= 42.41µC·cm^-2), emerging from the high symmetry non-polar structure. According to the structural analysis the FE order, of a geometric nature, is driven by theΓ2-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.