Structural relaxation in superprotonic tetraammonium dihydrogen triselenate single crystals

(NH4)(4)H-2(SeO4)(3) single crystals undergo a transition to superionic pha se at T-s = 378 K and the phase is characterized by high structural disorde r. When the crystal is cooled from highly disordered superprotonic phase to T much less than T-s the order, characteristic of low-temperature phase (t rimers formed by three SeO4 tetrahedra linked with two short hydrogen bonds , lying along [0 (2) over bar 1] direction), has to be recovered. The recov ery process, termed by us as structural relaxation, was studied by means of impedance and Raman spectroscopy. Results of conductivity recovery measure ments allow to distinguish three rates of conductivity decrease. Time varia tion of SeO4 skeletal modes after cooling the crystal from the superprotoni c state to temperatures T much less than T-s points to the following proces ses of the recovery: i) relaxation to the low-temperature molecular structu re (formation of trimers is proceeded by formation and disappearance of dim ers as a transitional form) and ii) relaxation to the low-temperature cryst allographic structure (trimers lying along [0 (2) over bar 1] direction). W e observe the process of dimer formation and disappearance within similar t o 30 min at 303 K, whereas the formation of trimers lasts similar to 1 h at the same temperature. (C) 1999 Elsevier Science B.V. All rights reserved.