Dynamics and Thermodynamics of Layered Isostructural Lattices

Systems with a layered structure exhibit a host of interesting properties including phase transition behavior, that can be attributed in part to their pseudo, two-dimensional character. In addition, if the materials are isostructural, a study of the systematic variations in the series, makes it possible to focus on the underlying microscopic characteristics driving the phase transitions, irrespective of the complexity of the fundamental crystal structure. The ammonium lanthanide sulfate tetrahydrates and rubidium lanthanide sulfate tetrahydrates (ML(SO_4)_2.4H_2 O, M = NH_4^+ and Rb^+, L = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) form two such isostructural series of layered crystals. It has been demonstrated for the first time that most of these crystals undergo two successive phase transitions at 100 K < T < 300 K. Specific heat measurements, the single -crystal X-ray diffraction technique, and temperature dependent vibrational studies were utilized to conduct a comprehensive investigation of the static and dynamic processes occurring in the above systems. It is concluded that the monovalent cations do not participate actively in the phase transition mechanisms. Nevertheless, the ammonium ions show interesting reorientational behavior which has been extracted by a novel analytic technique from the vibrational dephasing channels. Unlike other isostructural lanthanide series, the phase transition temperatures of ML(SO_4) _2.4H_2O show no straight forward correlation with the trivalent lanthanide ionic radius. On the other hand, a detailed structural analysis suggests that the relaxation of the transverse rigidity of the layers in the lattice is critically coupled to the low temperature phase transition behavior. From vibrational analysis of water oscillators, evidence is presented that the water molecules participating in intra-layer hydrogen bonding are responsible for triggering the low temperature transitions in these crystals. The high temperature transitions appear to be driven by proton fluctuations which results in the inter-layer hydrogen bonds assuming a more critical role in establishing structural stability. Thus, the interplay between the inter- and intra-layer variables appears to dominate the structural stability and phase transition behavior in these layered crystals.