The space group of NaNbO₃and (Na0.995K0.005)NbO₃

A method is described for the determination of possible space groups for pseudosymmetric structures, i.e. structures derived from an idealised structure of higher symmetry by small displacements of the atoms from special positions. Conventionally, systematic absences are used to infer the presence of symmetry operators; this method is difficult to apply in the case of a pseudosymmetrie structure where the space group absences occur among a set of systematically weak reflections. The new method uses systematic variations of intensity among these weak reflections to infer the sense and magnitude of the displacements of atoms from their idealised positions; symmetry operators are then introduced in a manner consistent with the displacements. The detailed application of the method to NaNbO a is described and the possible space groups are shown to be t:'bma or one of its subgroups Pb21a or P21212.