Abstract
Abstract A family of Ln 2CrO6 (Ln=Pr, Nd, Sm–Tb) compounds has been re-investigated using powder X-ray diffraction and IR spectroscopy. The structure of β-Nd2CrO6≡β-[Nd2O2](CrO4) is similar to that of the slag compound [Ba2F2](S6+O3S2−) in that it exhibits a disordered arrangement of (CrO4)2− anions between [Nd2O2]2+ litharge-type blocks. Its structural architecture is also related to other layered α- and γ-[Ln 2O2](AO4) 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-[Nd2O2](CrO4) and bearing other transition metal-centred tetrahedral anions are discussed.
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