The study of ferroelectric and modulated phase of ammonium hydrogen selenate

The pulse NMR methods, the measurements of the conductivity and dielectric permittivity were used to study the sequence of the phase transitions in NH4HSeO4. In the temperature region between ferroelectric and paraelectric phases (Tcl ÷ Ti) the spontaneous asymmetry of the occupation of the H-bond double well potential is homogeneous in the ab-plan, whereas it is modulated along the c-direction. The deuteration drastically increases the incommensurate region. The long-time processes leading to the decay of the long-range order in the structure above Tcl are discussed both in the terms of the topological phase transition and at the assumption of first-order polymorphic transformation.