Phase diagram of the system Na2MoO4–Li2MoO4, crystal structures of Na7Li(MoO4)4, Na3-xLi1+x(MoO4)2, and a metastable spinel-type form of Li2MoO4

Abstract

The T-x phase diagram of the Na₂MoO₄–Li₂MoO₄ system was revised and two compounds, Na₇Li(MоO₄)₄ (exists above 435 °C) and Na_3-xLi_1+x(MoO₄)₂ (0 ≤ x ≤ 0.2) were found, which melt incongruently at 544 and 515 °C, respectively. Orthorhombic Na₇Li(MоO₄)₄ of a new structure type contains the [Li(MoO₄)₄]^7– clusters of the central LiO₄ tetrahedron sharing vertices with four MoO₄ tetrahedra; the clusters are connected through NaO₆ and NaO₅ polyhedra in two types of layers, which alternate along the c axis. In monoclinic Na_2.82Li_1.18(MoO₄)₂, isostructural with Na₃Li(MoO₄)₂, the trigonal bipyramids of NaO₅ and (Na, Li)O₅ share edges to form six-membered ribbons arranged along (101) in parquet-like layers resembling cuspidine-like layers with four-membered ribbons in Na₇Li(MоO₄)₄. The layers in Na_2.82Li_1.18(MoO₄)₂ are connected by the MoO₄ and LiO₄ tetrahedra into a 3D framework. Under metastable crystallization conditions, the spinel-type Li₂MoO₄(sp) crystals were also obtained and structurally studied. Calculations of bond-valence-based energy barriers for potential diffusion of the lithium and sodium ions show probable 1D sodium-ion conductivity for Na₇Li(MоO₄)₄. Possible 3D lithium-ion transport pathway in Li₂MoO₄(sp) passes through common triangular faces surrounding Li in octahedral 16d position and empty tetrahedral 8b site.