Ferro- and antiferroelectric phases of the alkali hydroxides

Abstract

The hydroxides MOX (where M = Na, K, Rb, Cs; X = H, D) possess electrically ordered (FE and AFE) phases with transition temperatures in the range 150 K - 310 K^1-7. These have been established by a combination of nuclear resonance, dielectric constant, DTA and specific heat measurements. High resolution powder neutron diffraction shows that the principal feature of the ordering is the displacement of the H(D) along the b-axis of the orthorhombic (or monoclinic) structure. At low temperature NaOD, KOH/D and CsOH/D are AFE whereas RbOH/D is FE. These phases are an entirely new group of FE/AFE materials. It not clear that hydrogen bonding plays any significant part in the structural ordering, viz. large O-H…O distance, and no shift in the O-H IR stretching frequency. However a persistent feature in the low temperature phases is the formation of zig-zag O-H…O chains along the b-direction. Alos full deuteration increases T_c by approximately 30 K in the K, Rb and Cs compounds. The Na compound exhibits a more spectacular isotope effect, in that NaOH shows no electrical ordering down to 6 k, although NaOD transforms at 150 K.