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[A_n-1B_nO_3n+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.