Phase formation and crystal structure of ternary compound Na2Li2Ti6O14

A systematic study into phase formation in the system Na2O-Li2O-TiO2 has been carried out at subsolidus temperatures, ca. 900-1000 deg C. An experimental study of compound formation and phase equilibrium in this ternary system for compositions containing > 50% TiO2 was made. Five ternary phases were encountered; three of them have variable composition. Solid solution called phase A, is centered on Na5.4Li1.6Ti6.5O16.5 (isostructural to Na0.68Ni0.34Ti0.66O2), phase B is centered on Na2Li2Ti6O14 (isostructural to SrLi2Ti6O14), and phase C is based on Na2LiTi8.5O18.5 (isostructural to Na2Fe2Ti6O16). Two ternary compounds were also found with composition: Na5.6Li2.8Ti5.8O15.8 and Na7Li3Ti5O15 (isostructural to Na0.75Fe0.75Ti0.25O2 and Na2.76Li1.24Ti2O6 respectively). The sodium lithium titanate with composition Na2Li2Ti6O14 has been synthesized by solid state reaction at 970 deg C. The characterization was made by X-ray powder diffraction technique. The structure of Na2Li2Ti6O14 was solved using X-ray data (CuK(alpha1,2) radiation), which were indexed using space group Cmca (64), with cell parameters a = 16.4749(1), b = 11.2161(1) and c = 11.4654(1). Crystal structure refinement was performed by means of Rietveld method, with the conventional factors R(wp) = 19.2; R(exp) = 14.40. Na2Li2Ti6O14 phase structure is described as a network of intercalated edge and corner-sharing distorted TiO6 octahedra. Na ions are surrounded by 11 oxygen atoms, while Ti and Li atoms maintain a coordination of 6 and 4, respectively. This arrangement provides tunnels along the z-axis, in which the lithium atoms are located, forming distorted tetrahedra.