Synthesis, crystal structure, sintering and electrical properties of a new alluaudite-like triple molybdate K0.13Na3.87MgMo3O12

Abstract

A new triple molybdate K_0.13Na_3.87MgMo₃O₁₂ was synthesized by solid state reaction. The crystal structure has been determined by single X-ray diffraction and the electrical conductivity measured by impedance spectroscopy. The title compound crystallizes in the monoclinic space group C2/c with a = 12.9325 (8) Å, b = 13.5537 (9) Å, c = 7.1627 (6) Å, β = 112.212 (9)°, V = 1162.33 (14) ų and Z = 4. The final agreement factors are R = 0.0241, wR (F²) = 0.0584, S(F²) = 1.22. The magnesium–molybdate 3D-framework belongs to the alluaudite type. The structure is formed by infinite chains composed of edge-sharing (Mg/Na)₂O₁₀ dimmers, which are linked together via bridging MoO₄ tetrahedra, yielding to a three-dimensional framework enclosing two distinct types of hexagonal tunnels in which Na⁺ and K⁺ cations reside. The structural model is validated by bond valence sum (BVS) and charge distribution (CD) methods. Ball milling is used as mechanical means to reduce the particles sizes of the synthesized powder. At the optimal sintering temperature of 650 °C, a relative density of 81% was obtained. The microstructures were characterized by scanning electron microscopy. The compound undergoes a phase transformation at 528 °C accompanied by an abrupt increase of the electrical conductivity. Above this phase transition, the electrical conductivity reaches 10⁻² S cm⁻¹. Thus K_0.13Na_3.87Mg(MoO₄)₃ may be considered as a promising compound for developing new materials with high ionic conductivity.