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A new family of rare earth – strontium tellurite chlorides, SrLn4(TeO3)4Cl6 (Ln = Ce, Nd, Sm): Synthesis, crystal structures, possible polytypism, and crystal-chemical relationships
An interaction of LnOCl and TeO2 (Ln = Ce, Nd, Sm) in molten SrCl2 results in a new family of layered quinary compounds, SrLn4(TeO3)4Cl6. Their monoclinic structures (P2/c) correspond to a novel structure type, build up by the alternation of [M2Ln3(TeO3)4] layers (M = Ln0.5Sr0.5) and double halide sheets. New compounds are characterized by the modular type of the structures, while the presence of the twinning indicates the possible stacking disorder and polytypic character. An accurate analysis indicates that crystal structures belong to the OD (“order-disorder”) family of category III with one types of nonpolar OD layers and can be described by the same OD family groupoid. Possible maximal degree order polytypes have been predicted and crystal chemical relationships among the oxyhalides of rare earth and lone-pair cations are briefly discussed.