Infrared spectra of the ammonium ion in crystals. Part 10—Low-temperature transitions in the perovskites NH4MF3(M = Mn, Co, Zn)

Abstract

Investigation of the perovskites NH₄MF₃(M = Mn, Co, Zn) using the NH₃D⁺ ion as an i.r. probe has confirmed the existence of low-temperature structural transitions from a cubic (Pm3m, Z= 1) phase to a phase of lower symmetry in all three compounds. Although in the cubic phase the site symmetry at the N site is O_h, the effective symmetry of the ammonium ion is only D_2d. This is due to the tendency of the ion to establish as strong normal N—H⋯F bonds as is possible within the coordination cuboctahedron of F atoms. The actual behaviour of the ion in the cubic phase is the result of a compromise between satisfying this tendency by “static”(distortion of the ion) and dynamic (large-amplitude librational and bending motions) means. The tetragonal distortion of the ion decreases with increasing temperature. To maintain an overall cubic symmetry the directions of the S₄ axes of the elongated NH₄ tetrahedra in the crystal must be distributed at random and, in view of the rapid reorientations of the ammonium ion known to occur in these fluorides, undergo constant change. The low-temperature phase is not derived from the cubic phase by a simple tetragonal distortion. The site symmetry at the N atom is C_2h or C_s and the overall hydrogen bonding in this phase is strengthened. A model is proposed which accounts for the observed entropy of transition.