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 structure s were refined from powder X-ray diffraction (XRD) patterns by Rietveld ana lysis. These compounds are new members of the Dion-Jacobson series with n = 3 for the general formula M[A(n-1)B(n)O(3n+1)] (M = alkali metal, B = Nb). In addition, a new reduced tantalate, Na2Ca2Ta3O10, was synthesized by int ercalation reaction from the parent sodium compound, NaCa2Ta3O10. This inte rcalation compound is the first example of the Ruddlesden-Popper-type tanta late. The stacking feature of the perovskite layer is discussed on the basi s of the results obtained from the structure refinement. The stacking seque nces are dependent on space volume sizes formed between the perovskite laye rs. (C) 1999 Elsevier Science Ltd.