Bonilla, I.R., Holz, A. and Rutt, H.N.
Theory of the order parameter in NaNO2 in the molecular field approximation.
Physica Status Solidi, 63, (1), .
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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 ?°(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 ?°(0) which can be taken from experiment. The long-range order parameter ? is calculated in the molecular field approximation. A first order phase transition is obtained for which the critical values Tc and ?c are calculated as a function of the permanent electric dipole moment of NO2? and ?°(0). Only qualitative agreement with experimental results is obtained. The Made-lung constant of the lattice is obtained as ? = 1.93.
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