Mechanism of ferroelectric phase transition in NaNH4SeO4 · 2 H2O

Nuclear magnetic resonance, X-ray, and neutron diffractional laser Raman investigations are carried out. A semi-quantitative model of the phase transition in SASeD-crystals is proposed. The phase transition is of a mixed (trigger) type. Along with the active ionic displacements in the sodium-oxygen octahedra and additional distortions of tetrahedral groups, ordering processes of hydrogen bonds and tetrahedral groups take place. In the process of ordering the nonlinear interaction of SeO4-groups and water vibrations are of great importance. [Russian Text Ignored]