Synthesis, crystal structure, phonon, magnetic and electrical properties of new molybdate Na2Mn2(MoO4)3

Abstract

The single crystals of Na2Mn2(MoO4)3 were successfully grown using the solid-state synthesis method. This compound crystallizes in a triclinic system with space group P1‾ (Ci). Its structure can be described as a three-dimensional anionic framework of Mn2O10 double octahedra units sharing edges with the MoO4 tetrahedra. The Na+ ions are disordered and located in the voids forming infinite channels running along the [100], [010] and [001] directions. The structural model was validated by the bond valence sum (BVS) and charge distribution (CHARDI) methods. The polycrystalline powder of Na2Mn2(MoO4)3 was prepared as well using citrate process. The obtained sample was characterized by powder X-ray diffraction, infrared spectroscopy and Raman scattering at room temperature. The vibrational study confirms the existence of the MoO42− functional groups. Thermal stability of the studied material was analyzed using differential scanning calorimetry. The Na2Mn2(MoO4)3 compound is thermally stable up to 500 °C. Magnetic susceptibility measurements reveal that the title compound is antiferromagnetic with Néel temperature TN = 6.5 (5) K. At higher temperatures, its susceptibility follows a Curie–Weiss law. Electrical measurements of this material were studied by means of the impedance spectroscopy technique, in 0.1 Hz–4 MHz frequency and in 443–691 K temperature ranges. The conductivity of Na2Mn2(MoO4)3 is 2.30 μScm−1 at 691 K and it is thermally activated with energy of 0.47 eV. The sodium migration pathways in the crystal structure were investigated using the bond valence sum pathways (BVSP) analysis and the bond valence site energy (BVSE) model.