OPTICAL AND ELECTRIC MANIFESTATIONS OF INCOMMENSURATE PHASE IN NH_{4}HSeO_{4} CRYSTALS

Abstract

Temperature dependences of the optical path difference by the variable part of refractive index D = (n − 1)l and the components ε ′ and ε ′′ 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.

The roles of “ammonium” and “hydrogen-bond” protons in single crystals of the superionic conductor NH4HSeO4 by 1H NMR relaxation

“…The phase II is paraelectric monoclinic with space B2, and the phase III is an incommensurate. The NH 4 HSeO 4 crystal exhibits the ferroelectric phase IV in the temperature range between 98 and 250 K. The phase V below T C2 ¼ 98 K is nonferroelectric [12][13][14].…”

Study on nuclear magnetic resonance of superionic conductor NH4HSeO4in rotating frame

“…The crystal then transits to the incommensurate phase (III) at 261 K (=T i ) and to the ferroelectric phase (IV) at 250 K (=T C1 ). When the temperature drops below 98 K (=T C2 ), the crystal commences its low temperature phase and becomes non-ferroelectric [12][13][14]. Previous studies of NH 4 HSeO 4 with 1 H nuclear magnetic resonance (NMR) spectra, nuclear spin-spin relaxation times T 2 , in the laboratory frame and conductivity measurements in the vicinity of T SI concluded that the diffusions of protons and NH 4 + cations in the crystals were isotropic [15,16].…”