First experimental evidence of alluaudite-like phosphates with high Li-content: the (Na1-xLix)MnFe2(PO4)(3) series (x=0 to 1)

Members of the Na1-xLixMnFe2(PO4)(3) series, with the alluaudite structure type, were synthesized by solid-state reaction in air. The crystal structur e refinement of the NaMnFe2(PO4)(3) end-member (space group C2/c, Z = 4, a =12.018(2), b = 12.565(3), c = 6.415(1) Angstrom, beta = 114.33(3)degrees), a synthetic compound with a chemical composition corresponding to the idea lized composition of the Buranga alluaudite, was carried out to R-1 = 0.026 . The following cationic distribution was observed: Na+ + square in A(1) an d A(2)' (square denotes lattice vacancies), Mn2+ in M(1), Fe3+ + Fe2+ in M( 2). The A(2)' site exhibits a distorted gable disphenoid morphology and is found at the (0, y, 1/4) (y approximate to 0) position in channel 2 of the alluaudite structure. The crystal structure of Na0.5Li0.5MnFe2(PO4)(3) (spa ce group C2/c, Z = 4, a = 11.988(2), b = 12.500(3), c = 6.392(1) Angstrom, beta = 114.67(3)degrees), refined to R-1 = 0.034, leads to the cationic dis tribution: Li+ + Na+ + square in A(2)', Na+ + square in A(1), Mn2+ in M(1), Fe3+ + Fe2+ in M(2). Thus, the substitution mechanism involved in the repl acement of Na by Li in the Na1-xLixMnFe2(PO4)(3) alluaudite-like compounds corresponds to square + Na --> Li + square, with x ranging from 0.00 to 0.9 0.