CRYSTAL-STRUCTURE OF THE SODIUM-ION CONDUCTOR ALPHA-NA7FE3(P2O7)4 - EVIDENCE FOR A LONG-RANGE ORDERING OF THE NA+ IONS

The new sodium iron(III) diphosphate, Na7Fe3(P2O7)4, exhibits a high-temperature form, "β", monoclinic, isotypic with Na7Fe3(As2O7)4 and a room-temperature form, "α", which is a superstructure of β. The structure of the α-form has been solved in two stages. First, the superstructure reflections were neglected and the average structure (cell constants: a = 9.506(3), b = 8.333(2), c = 27.732(5) Å, β = 93.33(3)°, V = 2193(2) Å3, Z = 4) was solved in the space group C2 c. Final reliability factors are R = 0.030 and Rw = 0.042 for 3180 unique reflections with I ≥ 3 σ(I). The structure consists of FeO6 octahedra sharing their corners with P2O7 groups to form a three-dimensional framework [Fe3(P2O7)4]∝ into which the sodium ions are inserted. With the exception of Na(1), the sodium sites are partially occupied and lie in the vicinity of π-planes parallel to (001), which at high temperature play the role of diffusion planes for the Na+ ions. In a second stage, the superstructure (cell constants: a = 28.519(3), b = 8.333(2), c = 29.834(5) Å, β = 111.88(1)°, V = 6579(4) Å3, Z = 12) was solved in the space group C2 c and refined to R = 0.039 and Rw = 0.041 for 4343 independent reflections with I ≥ σ(I). A distribution of vacancies and totally occupied sodium sites was found in the π-planes. The long-range ordering of the sodium ions is accompanied by slight atomic displacements in the [Fe3(P2O7)4]∝ framework. © 1991.