The crystal chemistry of lithium in the alluaudite structure: a study of the (Na_{1− x} Li _x )_1.5Mn_1.5Fe_1.5(PO₄)₃ solid solution (x = 0 to 1)

Summary

Phosphates of compositions (Na_{1− x} Li _x )_1.5Mn_1.5Fe_1.5(PO₄)₃ were synthesized by solid state reactions in air, and pure alluaudite-type compounds were obtained for x = 0.00, 0.25, and 0.50. Rietveld refinements of X-ray powder diffraction data indicate the occurrence of Mn²⁺ in the M(1) site, and of Fe³⁺ and Mn²⁺ in the M(2) site. For x = 0.25 and 0.50, A(1) is occupied by Li⁺ and Na⁺, whereas A(2)’ is occupied by Na⁺ and vacancies. A careful examination of the number of electrons occurring in the A sites of the alluaudite-type compounds (Na_{1− x} Li _x )MnFe₂(PO₄)₃ and (Na_{1− x} Li _x ) CdIn₂(PO₄)₃ confirms that lithium occupies only the A(1) crystallographic site of the alluaudite structure.