Electric Field Gradient and Long Range Order Parameter of Ferroelectric Sodium Nitrite

Abstract

In order to obtain any relation between an electric field gradient (E.F.G.) at the site of 23 Na nucleus in Sodium Nitrite crystals and spontaneous polarization, the observed E.F.G. along the b -axis in the ferroelectric phase is formally expanded with the long range order parameter S . The E.F.G. in a fictitious perfectly ordered state and in a disordered one are calculated as a function of temperature. The E.F.G. of both fictitious states are calculated in a point charge approximation on the basis of a simplified model. The calculated S from the first order expansion series in terms of the observed E.F.G. and the calculated E.F.G. shows a good agreement with the S obtained from the observed spontaneous polarization except in the vicinity of the transition temperature. An effect of the order-disorder rearrangement of NO 2 - ion is concluded to reflect the temperature dependence of the E.F.G. observed in the ferroelectric phase.