The temperature dependence of the optical nonlinear susceptibility of sodium nitrite

Information about the order‐disorder transition of NaNO2 is obtained from the temperature dependence of the optical nonlinear susceptibility χ(2). The decrease of χ(2)(T) in the ferroelectric phase can be related to increasing rotational motion of the NO ‐ions. While χ(2) indicates the degree of order within the NO ‐sublattice, the spontaneous polarization Ps may be regarded as an order parameter of the Na+‐sublattice. The difference between Ps(T) and χ(2)(T) is explained by a different order‐disorder behaviour of the Na+‐and the NO ‐ions.