Structural relaxation in superprotonic tetraammonium dihydrogen triselenate single crystals

• Published in: Solid state ionics 1999-10, Vol.125 (1-4), p.163-169
• Main Authors: Hilczer, Bożena, Połomska, Maria, Pawłowski, Antoni
• Format: Article
• Language: English
• Subjects: (NH4)4H2(SeO4)3; Condensed matter: structure, mechanical and thermal properties; Diffusion in solids; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Impedance spectroscopy; Physics; Raman spectroscopy; Self-diffusion and ionic conduction in nonmetals; Solid-solid transitions; Specific phase transitions; Structural relaxation; Superprotonic phase; Transport properties of condensed matter (nonelectronic)
• ISSN: 0167-2738; 1872-7689
• DOI: 10.1016/S0167-2738(99)00171-X

(NH4)4H2(SeO4)3 single crystals undergo a transition to superionic phase at Ts=378 K and the phase is characterized by high structural disorder. When the crystal is cooled from highly disordered superprotonic phase to T≪Ts the order, characteristic of low-temperature phase (trimers formed by three SeO4 tetrahedra linked with two short hydrogen bonds, lying along [02̄1] direction), has to be recovered. The recovery process, termed by us as structural relaxation, was studied by means of impedance and Raman spectroscopy. Results of conductivity recovery measurements allow to distinguish three rates of conductivity decrease. Time variation of SeO4 skeletal modes after cooling the crystal from the superprotonic state to temperatures T≪Ts points to the following processes of the recovery: i) relaxation to the low-temperature molecular structure (formation of trimers is proceeded by formation and disappearance of dimers as a transitional form) and ii) relaxation to the low-temperature crystallographic structure (trimers lying along [02̄1] direction). We observe the process of dimer formation and disappearance within ∼30 min at 303 K, whereas the formation of trimers lasts ∼1 h at the same temperature.