Structural chemistry of new ion-exchangeable tantalates with layered perovskite structure: new dion–jacobson phase MCa2Ta3O10 (M = alkali metal) and ruddlesden–popper phase Na2Ca2Ta3O10

New layered perovskites, MCa2Ta3O10 (M = alkali metal), were synthesized by a solid-state reaction and an ion-exchange reaction. The crystal structures were refined from powder X-ray diffraction (XRD) patterns by Rietveld analysis. These compounds are new members of the Dion–Jacobson series with n = 3 for the general formula M[An−1BnO3n+1] (M = alkali metal, B = Nb). In addition, a new reduced tantalate, Na2Ca2Ta3O10, was synthesized by intercalation reaction from the parent sodium compound, NaCa2Ta3O10. This intercalation compound is the first example of the Ruddlesden–Popper-type tantalate. The stacking feature of the perovskite layer is discussed on the basis of the results obtained from the structure refinement. The stacking sequences are dependent on space volume sizes formed between the perovskite layers.