Phase transitions and relaxation dynamics in (NH4I)x(KI)1−x mixed crystals

Transverse optical phonons have been studied in (NH 4 I) 0.3 (KI) 0.7 , (ND 4 I) 0.3 (KI) 0.7 and (NH 4 Br) 0.3 (KBr) 0.7 using Fourier-transform infrared techniques. In all materials three phonon modes have been observed in the far-infrared regime and were followed as a function of temperature in detail. While the first low-frequency mode characterizes a sublattice motion of the alkali ions against the halogenides K + ↔ I − and K + ↔ Br − , the second mode involves vibrations of the ammonium molecules against the halogenide ions NH 4 + ↔ I − , ND 4 + ↔ I − and HH 4 + ↔ Br − . Despite the fact that the average symmetry of these crystals is cubic, the transitional and orientational disorder locally breaks the cubic symmetry, the infra-red selection rules are relaxed and, hence, a third phonon contribution appears. Deviations of the temperature dependence of the eigenfrequencies, as expected for anharmonic modes, are interpreted in terms of a rotation-translation coupling where the phonon modes interact with the orientational degrees of freedom.

Section: Discussionmentioning

confidence: 71%

Section: Introductionmentioning

confidence: 99%

Lattice vibrations in the mixed crystals (NH₄I)_0.3(KI)_0.7, (ND₄I)_0.3(KI)_0.7and (NH₄Br)_0.3(KBr)_0.7

Ohl

Mayr

Reehuis

et al. 2001

“…Below the critical concentration x c ∼ 0.55 (K) 1−x (NH 4 ) x I compounds exhibit in the low temperature region an orientation glass state driven by frustrated dipolar interactions [3,5]. In addition, a new ε phase with a trigonal structure (space group R 3m) was detected in these compounds in a narrow concentration range 0.55 < x < 0.75 [6,7]. This phase shows a long-range order of the NH + 4 -tetrahedra with two inequivalent ammonium sites.…”

Section: Introductionmentioning

confidence: 94%

“…For 0.75 < x < 0.95 (K) 1−x (NH 4 ) x I crystals exhibit the β phase and for x > 0.95 γ phase was observed. The nature of the orientational glass transition and phase diagram of (K) 1−x (NH 4 ) x I compounds have been extensively studied by dielectric [3,8], NMR [9][10][11][12] and Raman [13,14] spectroscopy methods and neutron scattering techniques [6,7,15,16]. In the neutron diffraction studies [15,16] it was shown that the application of rather moderate pressure P ∼ 500 MPa leads to a phase transition from α to β phase in (K) 1−x (NH 4 ) x I systems with 0.35 < x < 0.7.…”

Section: Introductionmentioning

confidence: 99%

A neutron diffraction and NMR study of theP–Tphase diagram of Rb_1 x(NH₄)_xI mixed crystals (x= 0.29,0.77)

Wąsicki¹,

Lewicki²,

Козленко³

et al. 2004

The structure of Rb 1−x (NH 4 ) x I (x = 0.29, 0.77) mixed crystals was studied by means of powder neutron diffraction in the temperature range 20-300 K and at ambient pressure. Measurements of spin-lattice relaxation time T 1 have been performed for polycrystalline samples of Rb 1−x (NH 4 ) x I (x = 0.29, 0.77) in the temperature range 95-300 K and pressure range 0-800 MPa by the proton NMR technique using the saturation method. Activation parameters were obtained for a model of complex ammonium ion reorientations around twofold C 2 and three-fold C 3 axes. The activation volume for different phases of the investigated compounds was determined. The experimental data were used for a construction of P-T phase diagram of Rb 1−x (NH 4 ) x I (x = 0.29, 0.77) mixed crystals. The phase diagram of Rb 0.23 (NH 4 ) 0.77 I has similar features to that of NH 4 I. Four different phases were found to exist in the temperature and pressure range studied: a disordered α phase with an NaCl-type cubic structure, a disordered β phase with a CsCl-type cubic structure, a γ phase with a tetragonal structure and antiparallel ordering of ammonium ions, and a δ phase with a CsCl-type cubic structure and parallel order of ammonium ions. The phase diagram of Rb 0.71 (NH 4 ) 0.29 I contains only α and β phases, and is similar to that of RbI.

“…Furthermore, the disorder could be either static [17], or dynamic [16], or both [19]. On the other hand, a continuum of disordered states would make the ion a free rotator, resulting in the broadening of the characteristic internal vibrations of the tetrahedral ion [20,21]. Existence of discrete orientational disorder has also been found to result in pressure-induced amorphization (PIA) in several compounds such as LiKSO 4 [17], potash alum [18] and scandium molybdate [15].…”

Section: Introductionmentioning

confidence: 99%

Evidence of disorder in cubic zirconium tungstate from the temperature dependence of Raman spectra

Arora¹,

Ravindran²,

Chandra³

2007

The cubic phase of zirconium tungstate exhibits a larger number of vibrational Raman modes in the frequency region of the symmetric stretching mode of tungstate ions than permitted by group-theoretical analysis for a perfectly ordered lattice. This suggests the existence of disorder in the tungstate sublattice. The additional modes are identified and assigned on the basis of their relative intensities and temperature dependences of line-widths. Using a real-space model for misoriented tetrahedral ions, the occurrence of four disorder modes is explained. From the evolution of the intensities of different components of the symmetric stretching mode as a function of temperature, the disorder is found to be largely static (frozen-in) in nature. As each tungstate ion carries a net dipole moment, randomly misoriented tetrahedral units can result in a dipole glass, explaining a recently reported glass-like behaviour of thermal conductivity. The proposed disordered configurations of tungstate ions are indeed found to be energetically stable from first-principles simulation studies.