Reductive lithium insertion into B-cation deficient niobium perovskite oxides.

Reaction between LiH and the A(n)B(n-1)O(3n) cation deficient perovskite phases Ba(5)Nb(4)O(15), Ba(6)TiNb(4)O(18) and Ba(3)LaNb(3)O(12) proceeds by reductive lithium insertion, leading to the formation of Ba(5)LiNb(4)O(15), Ba(6)LiTiNb(4)O(18) and Ba(3)LaLiNb(3)O(12) respectively. During lithium insertion into Ba(5)Nb(4)O(15) and Ba(6)TiNb(4)O(18) the respective ccchh and cccchh stacking sequences are converted into entirely cubic stacking sequences, while the B-cation vacancy order of the two phases is faithfully converted into Li-Nb or Li-Nb/Ti cation order in the lithiated products. In contrast lithium insertion into Ba(3)LaNb(3)O(12) leads to no gross change in structure, with the inserted lithium cations displacing some of the niobium cations leading to a cation disordered material. Transport measurements indicate semiconducting behaviour consistent with variable range hopping for Ba(5)LiNb(4)O(15) and insulating behaviour for Ba(6)LiTiNb(4)O(18) and Ba(3)LaLiNb(3)O(12). Detailed analysis of the crystal structure of Ba(6)LiTiNb(4)O(18) suggests crystallographic charge ordering in this phase.