EFFECT OF DEUTERATION ON THE PHASE-TRANSITIONS AND ON THE CRITICAL-DYNAMICS IN AMMONIUM HEXACHLOROMETALLATES

Recent and novel data obtained from chlorine NQR measurements on natur al and deuterated (NH4)2MCl6 compounds are discussed with special rega rd to the influence of the ammonium-ion dynamics on the structural sta bility of these crystals. The temperature dependence (4.2 K to 350 K) of the chlorine NOR frequency nu(Q) and relaxation rates T1(-1), T2(-1 ) obtained from the natural ammonium salts of Sn, Pd, Os, Pb, Te, Se a nd from the deuterated salts of Sn, Te and Se are analysed. Slight dev iations from the normal temperature behaviour of nu(Q) and T1(-1) are found in Sn, Pd and Os compounds which stay cubic in the whole tempera ture range. The ammonium compounds of Pb and Te undergo a structural t ransformation between 80 K and 90 K from the cubic to a trigonal phase which is distinguished by the preservation of the single line spectru m of the chlorine NQR below T(c1). The observed divergence of T1(-1) a t the transition point can be described in terms of a spin-phonon proc ess in the presence of an overdamped soft mode. Deuteration of (NH4)2T eCl6 only slightly affects the transition of T(c1) but leads to new st ructural changes at lower temperatures. Whereas the natural compound s tays trigonal down to 4.2 K the deuterated crystal undergoes two addit ional structural transformations at T(c2) = 48 K and T(c3) = 28 K whic h are correlated with a slowing down of the deuteron motion. Approachi ng T(c2) from the above the spin-lattice relaxation rate and the spin- spin relaxation rate of the chlorine NQR exhibit distinct anomalies wh ich are attributed to limited jumps of the octahedron in a shallow pot ential. The barrier height of this potential deduced from the chlorine NQR spin-lattice relaxation rate is 400 K. The transition at T(c2) is explained by the condensation in one minimum of this potential. At T( c3) a long range correlation is formed which is accompanied by a rotat ion of the octahedron about its fourfold axis. A similar mechanism is adopted for the transitions observed in (NH4)2SeCl6 at T(c) = 24 K and in (ND4)2SeCl6 at T(c) = 48 K.