Reorientational motion of the HS- ions in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A quasielastic neutron scattering study

The isostructural hydrogensulfides of the alkali metals sodium, potassium, and rubidium were investigated by quasielastic neutron scattering in order to gain insight into the dynamics of the molecular anion HS- as a function of temperature. A phase transition between the rhombohedral middle temperat ure modification (MTM) and the cubic high temperature modification (HTM) of the title compounds was of special interest. A two site jump model is clea rly supported by the data of the MTM and an eight site jump model is consis tent with the data of the HTM. The jump distances are in reasonable agreeme nt with distances of the hydrogen positions determined by neutron powder di ffraction on the fully deuterated samples. At the MTM --> HTM phase transit ion the correlation times of the reorientational motion decrease by about o ne order of magnitude. Within the investigated temperature range (T=170 K t o T=450 K) they change from 10(-8) s to 10(-12) s. They show Arrhenius beha vior. The activation enthalpies of the MTM decrease with increasing radius of the cation from 26(1) kJ mol(-1), 22(1) kJ mol(-1) to 18(2) kJ mol(-1) f or NaHS, KHS and RbHS respectively. In the HTM they are equal within the es timated error 6(1) kJ mol(-1) and 7(1) kJ mol(-1) for NaHS and KHS. (C) 200 0 American Institute of Physics. [S0021-9606(00)52941-9].