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-xLi_xMnFe₂(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 Na_0.5Li_0.5MnFe₂(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 Na_1-xLi_xMnFe₂(PO₄)₃ alluaudite-like compounds corresponds to □ + Na □ Li + □, with x ranging from 0.00 to 0.90.