The phase diagram of (NH4I)x(KI)1−x

Ammonium resonance modes and orientational glass state in the K1-X(NH4)XI mixed crystals

“…The temperature dependencies of the neutron diffuse scattering intensity around the rcciprocal lattice point Q=(2,-1,0) and of the Raman response intensity of the low-energy mode at 27 cm-' were used for the determination of the freezing temperature and the values obtained from both dependencies were close -58 and 55 K, respectively. The authors of [7] ascribed the low-energy modes at 27 and 44 cm-' observed by Raman response spectra, in accordance with [ 2 0 ] , to the zone-boundary transverse-acoustic (TA) and longitudinal-acoustic (LA) phonon branches at the X point from which the diffuse neutron scattering was also [8,9] showcd that a sample with the concentration x=0.62 could belong to the &-phase and the observed pcculiarities in (71 could be determined by the a-E phase transition. The authors of [7] did not investigate the concentration dependence of thc low energy modes in the Kl-x(NH4)xI mixed crystals in the full concentration region 0<x<0.75.…”

“…The &-phase lies in the concentration region 0.55<x<0.75 181. It has a trigonal structure (R3m) of almost indeformable lattice of NaC1-type with ordered orientations of four ammonium ions per unit cell which occupy two non-equivalent NH4 sites [9]. The &-phase was discovered first in the Kl-,(NH4),1 mixed crystals and the crystal structure of this new phase was investigated for the concentration x=0.72 191.…”

“…the dynamic orientationally disordered a-phase exists down to low temperature [ l l ] . It should be noted that the phase boundaries of the orientational glass regions on the concentration-temperature phase plane in different publications [7][8][9][10] have different positions.…”

Hysteresis effects and mesoscopic domains: Precursors for the formation of orientational glass states

“…In order to clarify this issue and to see how domain sizes evolve upon dilution of an OG parent compound, we have studied the phase transitions in several (NH 4 I) x (KI) 1−x mixed crystals with ammonium concentrations x > x c . Here x c is about 0.55 [10]. For x c < x < 0.8 the trigonal -phase is stable at low temperatures.…”

“…Its center-of-mass lattice exhibits only slight deviations from cubic symmetry [11]. For 0.8 < x < 1 the crystals undergo a first-order transition from the high-temperature α-phase, exhibiting the rock salt structure, to a bodycentered low temperature (β) phase [10]. For NH 4 I it is known that this transformation is characterized by a broad α, β coexistence range, showing up for 210 K < T < 256 K [12].…”