A family of Ln ₂CrO ₆ (Ln=Pr, Nd, Sm-Tb) compounds has been re-investigated using powder X-ray diffraction and IR spectroscopy. The structure of β-Nd ₂CrO ₆β-[Nd ₂O ₂](CrO ₄) is similar to that of the slag compound [Ba ₂F ₂](S ⁶⁺O ₃S ^2-) in that it exhibits a disordered arrangement of (CrO ₄) ^2- anions between [Nd ₂O ₂] ²⁺ litharge-type blocks. Its structural architecture is also related to other layered α- and γ-[Ln ₂O ₂](AO ₄) species (A=S, Cr, Mo), showing various orientations of the tetrahedral anions within the interlayer space. Size relationships between the incorporated tetrahedral anions and formation of different structure types (denoted as M1-, M2- and T-type) are reviewed. The possible existence of new compounds which are isostructural with, or structurally related to, β-[Nd ₂O ₂](CrO ₄) and bearing other transition metal-centred tetrahedral anions are discussed.