New n=2 members of the Li2Srn-1MnO3n+1 family, closely related to the Ruddlesden-Popper phases:: structure and non-stoichiometry

New n = 2 members of the Li2Srn - 1MnO3n + 1 series closely related to the RP phases have been synthesized by solid state reaction for the first time: Li2SrNb2O7, Li2SrTa2O7, Li2EuTa2O7. The detailed structure determination of Li2SrNb2O7 performed by neutron diffraction and electron microscopy shows that it crystallizes in the space group Cmcm [a = 18.0071(5), b = 5.5979(3), c = 5.5920(3) Angstrom], i.e. with a symmetry different from that of LiAM(2)O(7) and Li(2)AM(2)O(7) previously synthesized by chemical or electrochemical intercalation. The double perovskite layers [SrNb2O6] are puckered due to a tilting of their [001] octahedral rows alternately in opposite directions while the tetrahedral [Li2O] layers are significantly distorted. The possibility of cationic non-stoichiometry in both Li and Sr sites is demonstrated by the synthesis of Li(2)Sr(1 - x)Ln(2x/3)Nb(2)O(7) (Ln = La, Nd) and Li2 - xSr1 - xLaxNb2O7 oxides. The stability of these phases is discussed in terms of matching of the octahedral and tetrahedral layers, taking into consideration the tilting of the octahedra, itself influenced by the size of the A site cations and the cationic non-stoichiometry. An extension to n = 3 members is made, with the synthesis of the oxides Li2Sr1.5Nb3O10 and Li2Ca1.5Nb3O10.