AMMONIUMDYNAMICS AND STRUCTURAL PHASE TRANSITION IN K1-x(NH4)xI MIXED SALTS AT 10 K. I. Natkaniec, L.S. Smirnov Frank Laboratory of Neutron Physics, JINR, 141980 Dubna,Russia
The K1-x(NH4)xI mixed saltshave attracted increasing interest since they exhibit a low-temperature orientational glass phase. At high concentrations (x> xc @ 0.75), ammonium ions are ordered asin pure NH4I, in a slightly distorted CsCl structure of tetragonal symmetry.For 0.3<x<xc, an orientational glassy phase withan antiferroelectric short range order manifest themselves in cubicstructure of NaCl type [1].
We have studied the K1-x(NH4)xI system in the entire concentration range, simultaneously by neutrondiffraction and inelastic scattering (INS), on the high resolution NERA spectrometer at the IBR-2 pulsed reactor. The concentration dependence ofthe lattice parameters and the ammonium excitation energies at 10 K, will bepresented and discussed. The energy of the torsional excitations ofNH4 vary from 36.5 meV in the ordered phase to ca. 30 meV in the disorderedphase. The average energy of the translational NH4 vibrations at ca. 20 meV, does not change significantly at phase transitionand slightly increases with decreasing ammonium concentration in thedisordered phase. For NH4 concentrations belowxc, the corresponding INS spectra display two additional low energy bands atca. 2.5 and 9 meV, while only one band at ca. 10 meV was observed in ref.[2]. Both low frequency bands reflect the localised dynamics of ammoniumions in the crystal. However, their concentration dependence is even clearer than for higher frequency modesand reflects ammonium-ammonium interactions. This implies that theC3v rotational potential for NH4 ions directly depends on such interactions. Direct ammonium-ammoniuminteractions contradict the presentation of this mixed salt as a puredipolar glass [1,2].
1. J.F.Berret, C.Bostoen, B.Hennion, Phys.Rev.,B46 (1992) 13747-13750.
2. J. Tomkinson, B.A. Dasannacharya, P.S. Goyal and R. Chakravarthy,
J. Chem. Soc. Faraday Trans.,87 (1991) 3431-3433.