The observed disorder on the Sb position may be implied through the symmetry of the assumed space group and it is, therefore, necessary to regard the possible subgroups that allow for ordering of the Sb position. Two possible subgroups allow for this distortion in different ways: 1) the polar space group Cmc2 1 no longer contains a mirror plane in the (004) plane, Figure and) the mirror plane is replaced by a glide plane in the centrosymmetric space group P 2 1 /c, Figure .