Theory of the Order Parameter of NaNO 2 in Molecular Field Approximation

A static theory of the order‐disorder, ferro‐paraelectric phase transition in NaNO 2 is developed. The Coulomb energy of the system is calculated in the point charge approximation. The model takes the relative displacement δ°( 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 NO 2 − ion. The short range force potential acting on the Na + ion is fitted by means of δ°(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 NO 2 − and δ°(0). Only qualitative agreement with experimental results is obtained. The Made‐lung constant of the lattice is obtained as α = 1.93.