“Improper” ferroelectric properties of NaNO2 and AgNa(NO2)2

The anomalous temperature dependence of the elcetrostrictive coefficients of NaNO2 and AgNa(NO2)2 is interpreted in terms of improper ferroelectricity. A phenomenological formulation of the electrostrictive properties of an improper ferroelectric is derived. The electrostrictive coefficients which are derived from observed piezoelectric coefficients are linear functions of the inverse dielectric susceptibility. A possible origin of the improper nature of the NaNO2-type ferroelectrics is discussed.