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 Na 1-x Li x MnFe 2 (PO 4 ) 3 series, with the alluaudite structure type, were synthesized by solid-state reaction in air. The crystal structure refinement of the NaMnFe 2 (PO 4 ) 3 end-member (space group C2/ c, Z = 4, a = 12.018(2), b = 12.565(3), c = 6.415(1) A, β = 114.33(3)°), a synthetic compound with a chemical composition corresponding to the idealized composition of the Buranga alluaudite, was carried out to R 1 = 0.026. The following cationic distribution was observed: Na + + □ in A(1) and A(2)′ (□ denotes lattice vacancies), Mn 2+ in M(1), Fe 3+ + Fe 2+ in M(2). The A(2)′ site exhibits a distorted gable disphenoid morphology and is found at the (0, y, ¼) (y ≈ 0) position in channel 2 of the alluaudite structure. The crystal structure of Na 0.5 Li 0.5 MnFe 2 (PO 4 ) 3 (space group C2/ c, Z = 4, a = 11.988(2), b = 12.500(3), c = 6.392(1) A, β = 114.67(3)°), refined to R 1 = 0.034, leads to the cationic distribution: Li + + Na + + □ in A(2)′, Na + + □in A(1), Mn 2+ in M(1), Fe 3+ + Fe 2+ in M(2). Thus, the substitution mechanism involved in the replacement of Na by Li in the Na 1-x Li x MnFe 2 (PO 4 ) 3 alluaudite-like compounds corresponds to □ + Na □ Li + □, with x ranging from 0.00 to 0.90.