PRIMARY ORDER PARAMETER IN ANTI-FERROELECTRIC NANBO3

X-ray scattering experiments on NaNbO3 near the transition to antiferroelectric phase R indicate that the mode that softens - the primary mode or order parameter - is an octahedral rotational mode. It is argued that the antiferroelectric array of static Nb displacements present in phase R constitute the secondary order parameter; in other words non-zero values of the Nb displacements are induced through anharmonic coupling to the soft mode or primary mode. In current terminology, phase R is an improper antiferroelectric. © 1979.