The title compound, lithium/sodium iron(III) bis­[ortho­molyb­date(VI)], was obtained by a solid-state reaction. The main structure units are an FeO6 octa­hedron, a distorted MoO6 octa­hedron and an MoO4 tetra­hedron sharing corners. The crystal structure is composed of infinite double MoFeO11 chains along the b-axis direction linked by corner-sharing to MoO4 tetra­hedra so as to form Fe2Mo3O19 ribbons. The cohesion between ribbons via mixed Mo—O—Fe bridges leads to layers arranged parallel to the bc plane. Adjacent layers are linked by corners shared between MoO4 tetra­hedra of one layer and FeO6 octa­hedra of the other layer. The Na+ and Li+ ions partially occupy the same general position, with a site-occupancy ratio of 0.631 (9):0.369 (1). A comparison is made with AFe(MoO4)2 (A = Li, Na, K and Cs) structures.

This alluaudite-type structure is constituted of infinite layers formed by links between M 2O10 (M = Cr1/Na1) dimers and MoO4 tetra­hedra. The Na+ and Cr3+ cations are located in the same genel site with, respectively, 0.25 and 0.75 occupancies. The layers are related by sharing corners with MoO4 tetra­hedra, resulting an in open three-dimensional framework with hexa­gonal-form cavities occupied by Na+ cations.

Key indicators: single-crystal X-ray study; T = 298 K; mean (As-O) = 0.003 Å; R factor = 0.027; wR factor = 0.075; data-to-parameter ratio = 12.0. Condensation of the hydrogen arsenate groups with FeO 6 coordination octahedra via common corners results in an overall three-dimensional framework containing interconnected channels parallel to the a-, b-and c-axis directions. The NH 4 + cations are located in three intersecting tunnels, which is promising as an ion exchange. Hydrogen bonding of the types O-HÁ Á ÁO and N-HÁ Á ÁO consolidates the packing of the structure. The distortion of the coordination polyhedra is analyzed by means of the effective coordination number and distortion indices. Structural relationships with other compounds of general formula M I M III [HXO 4 )] 2 (X = P, As) are discussed. Related literature

Key indicators: single-crystal X-ray study; T = 298 K; mean (Fe-O) = 0.005 Å; disorder in main residue; R factor = 0.029; wR factor = 0.076; data-to-parameter ratio = 13.0. Related literature

The anionic framework of (Na0.4,Li0.6)(Fe,Li2)(MoO4)3 is built up from two distinct MO6 octa­hedra, each containing disordered Li+ and Fe3+ ions, and two MoO4 tetra­hedra, which link by vertex-sharing of their O atoms. These tetra­meric units are further linked by sharing edges between octa­hedra and by formation of M—O—Mo (M = Fe/Li) bridges, forming ribbons propagating in the [100]. The ribbons are cross-linked in both the b- and c-axis directions, giving rise to a three-dimensional framework having [100] tunnels in which the monovalent Na+/Li+ cations lie.