Compounds with the fresnoite structure type possess remarkable chemical diversity and a range of unique physical properties, including nonlinear optical (NLO) activity. K₂Sb(P₂O₇)F, which was reported recently, belongs to this group and its structure is not novel in terms of either the topology or local coordination geometry around the Sb³⁺ cations. The crystallographic disorder observed in the fresnoite-type structures can be explained by twinning, which is frequent for non-centrosymmetric crystals.