A new family of ferroelectric materials with composition A₂BMO₃F₃(A, B = K, Rb, Cs, for r_A+ ⩾ r_B+ and M = Mo, W)

Abstract The dielectric, pyroelectric, calorimetric and crystallographic properties of materials with formulas K3M03F3, Rb3M03F3, Cs3M03F3, Rb2KM03F3 and Cs2RbM03F3 (M = Mo, W) are shown to be consistent with ferroelectric behavior. The first seven of these materials exhibit two transitions, the latter three a single transition, above 78 K. A maximum in dielectric constant is associated only with the higher temperature (T2) phase transition. The structure is paraelectric above T2 in point group m3m and ferroelectric below in point group 3. The probable point group below the lower transition temperature is 3 or 1.