Title: OPTICAL AND ELECTRIC MANIFESTATIONS OF INCOMMENSURATE PHASE IN NH_4HSeO_4 CRYSTALS
Author(s): B.V.Andriyevsky, Z.Czapla, S.Dacko, O.Ya.Myshchyshyn

Temperature dependences of the optical path difference by the variable part of refractive index $D =(n-1) l$ and the components $\varepsilon'$ and $\varepsilon''$ of complex dielectric permittivity for NH$_4$HSeO$_4$ single crystal are studied in the temperature range of 220-300 K. Anomalous behaviour of these dependences in the range of incommensurate phase 250-262 K was observed. Analysis of the results of dielectric investigation predicts a considerable decrease of the coefficient of elastic stiffness $C$ in the narrow temperature range of 256-260 K at the transition from the paraelectric to incommensurate phase.

Comments: Figs. 3, Refs. 13, Tabs. 0.