Manifestation of incommensurate phase in the dielectric properties of NH4HSeO4 crystals

Temperature (220 to 280 K) and frequency (0.02 to 1000 kHz) dependences of the real (epsilon') and imaginary (epsilon") parts of the complex electric permittivity for the ferroelectric b-cut of NH4HSeO4 crystal have been inve stigated. The analysis of the experimental results has shown that the effec tive elastic stiffness coefficient C-eff of the crystal decreases about thr ee to four times in the temperature range of 256 to 262 K when compared to the paraelectric phase. Small oscillations of the temperature dependence of epsilon(T) have been revealed in the temperature range of the incommensura te phase (252 to 265 K), which have been interpreted as break off in the co ntinuous change of the wave vector of the crystal structure caused by numer ous transitions between commensurate and incommensurate subphases.