First observation of an inverse Ruddlesden-Popper series: (A(3n+1)ON(n-1))Bin+1 with A = Sr, Ba and n=1, 3

Single crystals ((Ba0.78(1)Sr0.22)(4)O)Bi-2 and ((Ba0.61(1)Sr0.38)(10)N2O)Bi-4 were successfully prepared from melt beads of Ba, Sr, and Bi in nitrogen atmosphere with oxygen impurities. The phases can be prepared in single phase from the appropriate mixtures of alkaline-earth metal, bismuth, and bismuth oxide upon heating in pure nitrogen atmosphere. ((Ba0.78(1)Sr0.22)(4)O)Bi-2 crystallizes in the K2NiF4 structure type (space group I4/mmm, No. 139, a 522.34(5) pm, c = 1844.0(2) pm, Z = 2, R-gt(F) = 0.039) with layers of vertex-sharing octahedra ((Ba,Sr)(4/2)Ba2O). ((Ba0.62(1)Sr0.38)(10)N2O)Bi-4 crystallizes as an isotype of Sr4Ti3O10 (space group I4/mmm, No. 139, a = 531.3(1) pm, c = 3983.2(4) pm, Z = 2, R-gt(F) = 0.050) containing slabs of three layers of vertex-sharing octahedra further connected via corners. These compounds are interpreted in terms of members of an inverse Ruddlesden-Popper series with the general formula n (A(3)ON(n-1))Bi . ABi or (A(3n+1)ON(n-1))Bin+1, respectively, with n = 1, 3. Partial order of the alkaline-earth metal ions is analyzed.