In the heavier homologue Pb 2 SbS 2 I 3 , one could attain a low-temperature phase transition between the disordered orthorhombic room temperature structure and the antipolar ordered structure at lower temperatures. The group-subgroup relationship is given in Figure . In order to detect even small differences at lower temperatures, we conducted low-temperature Guinier diffraction concentrated on a specific range in the diffraction pattern, Figure .