Anomalous Low-Temperature Behavior in the Dipolar Glass K1−x(N(H,D)4)xI
Recently, K1−x(N(H,D)4)xI was introduced as a model system for orientationally disordered crystals showing a dipolar glass transition . Despite its tetrahedral shape, the ammonium ion exhibits unexpected dipolar behavior. This property has been ascribed to the C3v-symmetry of the ammonium in the alkali-halide lattice . The (x,T)-phase diagram of the dipolar glass K1−x(NH4)xI was first investigated (see Fig. 1) by combining x-ray diffraction, Raman and neutron scattering measurements . Three compositional ranges are observed at low temperatures. Above the critical ammonium concentration xc, estimated between x = 0.72 and 0.82, concentration dependent phase transitions occur from NaCl to CsC1 cubic structures. It should be noted that, as in pure ammonium iodide, large hysteresis effects were encountered in this range. For 0.3≤ x < xc, an orientational glassy phase is characterized by dipolar short-range order of an antiferroelectric type. This was evidenced using elastic neutron scattering on deuterated compounds at x =0.42 and x = 0.62 for which the onset of ND 4 + freezing corresponds to the appearence of diffuse scattering patterns at the Brillouin zone boundary. Below x ∼ 0.3, the transition is no longer collective, but replaced by a single-ion freezing.