Theory of the order parameter in NaNO₂ in the molecular field approximation

Bonilla, I.R., Holz, A. and Rutt, H.N. (1974) Theory of the order parameter in NaNO₂ in the molecular field approximation. Physica Status Solidi, 63 (1), 297-305.

Abstract

A static theory of the order-disorder, ferro-paraelectric phase transition in NaNO₂ is developed. The Coulomb energy of the system is calculated in the point charge approximation. The model takes the relative displacement δ⁰(T) of the Na⁺ ions from their symmetrical position as a function of temperature into account; and the strong correlation of the Na⁺ ion position with the dipole orientation of the neighboring NO₂^- ion. The short range force potential acting on the Na⁺ ion is fitted by means of δ⁰(0) which can be taken from experiment. The long-range order parameter Ω is calculated in the molecular field approximation. A first order phase transition is obtained for which the critical values T_c and Ω_c are calculated as a function of the permanent electric dipole moment of NO₂^- and δ⁰(0). Only qualitative agreement with experimental results is obtained. The Made-lung constant of the lattice is obtained as α = 1.93.

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