X-ray diffraction and structural ordering of $B$-cations in $A_2B'B''\mathrm{O}_6$ double perovskites

A method for quantitatively assessing the degree of structural ordering $s$ from the intensities of superstructural X-ray diffraction reflections for double perovskites of the $A_2B'B''\mathrm{O}_6$ type is analyzed. It is shown that the technique proposed in 1980 and still widely used cannot always give the correct result for the most practically important polar phases of perovskites described by non-centrosymmetric groups. Using the examples of $\mathrm{Pb}_2\mathrm{Sc}\mathrm{TaO}_6$ and $\mathrm{Ca}_2\mathrm{CrSbO}_6$, it is shown that the dependence of $s$ on the probability of the lattice sites filling can be nonlinear, different for different XRD reflections, and at strong disorder, its values can differ by up to $10$–$50$% from the value corresponding to the definition of its physical meaning.