Abstract

Members of the Na1-xLixMnFe2(PO4)3 series, with the alluaudite structure type, were synthesized by solid-state reaction in air. The crystal structure 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) Å, β = 114.33(3)°), a synthetic compound with a chemical composition corresponding to the idealized composition of the Buranga alluaudite, was carried out to R1 = 0.026. The following cationic distribution was observed: Na+ + □ in A(1) and A(2)′ (□ 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, ¼) (y ≈ 0) position in channel 2 of the alluaudite structure. The crystal structure of Na0.5Li0.5MnFe2(PO4)3 (space group C2/c,Z = 4, a = 11.988(2), b = 12.500(3), c = 6.392(1) Å, β = 114.67(3)°), refined to R1 = 0.034, leads to the cationic distribution: Li+ + Na+ + □ in A(2)′, Na+ + □in A(1), Mn2+ in M(1), Fe3+ + Fe2+ in M(2). Thus, the substitution mechanism involved in the replacement of Na by Li in the Na1-xLixMnFe2(PO4)3 alluaudite-like compounds corresponds to □ + Na □ Li + □, with x ranging from 0.00 to 0.90.

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