Synthesis, Crystal Structure and Electrical Properties of the Molybdenum Oxide Na_1.92Mg_2.04Mo₃O₁₂

2.1. Synthesis

The compound is synthesized by the solid state method. A stoichiometric mixture of NaCO₃ (Fluka, 71350), (NH₄)₂Mo₄O₁₃ (Fluka, 69858), and Mg(NO₃)₂·6H₂O (Fluka, 63079), placed in a porcelain crucible, is slowly annealed in air at 350°C for 12 hours, in order to eliminate volatile products. The resulting mixtures were heated at 540°C for 7 days in air. Then, they were slowly cooled at 5°C/day to 490°C and finally, they were cooled at 50°C/day to room temperature. Single colorless crystals of double molybdates were grown by spontaneous crystallizations from stoichiometric melts. The compound is isotypical to Na₂Mg₅(MoO₄)₆ [4] and Ag₂M₂(MoO₄)₃(M = Co, Mn, Mg, Zn) [7–9] and contains mixed frameworks of MoO₄ tetrahedra and pairs of MgO₆ octahedra sharing common edges.

2.2. X-Ray Data Collection

Data collection was performed with a CAD-4 Enraf-Nonius X-ray diffractometer [12] at 298 K with graphite monochromator using MoK_α wavelength. An empirical psi-scan [13] absorption correction was applied. The structure was solved and refined by full-matrix least squares based on F² using SHELXS-97 and SHELXL-97 [14, 15], respectively. In the closest solution proposed by the program, only some molybdenum atoms and magnesium atoms were located. Using SHELXL-97 program, refinements followed by Fourier differences were necessary to find the positions of the other atoms remaining in the lattice yielding a good R factor of 2.18% for all reflections and allowing the revolution of the positions of 12 peaks of oxygen atoms on the difference Fourier map. The structure graphics were drawn with diamond 2.1 supplied by Crystal Impact [16]. Crystal data and structure refinements details are summarized in Table 1. The atomic coordinates and isotropic thermal factors are presented in Table 2. Table 3 contains the main interatomic distances in the coordination polyhedra of the studied structure.

3.1. Structure Description

The title compound is a new member of isostructural phases family including Na₂Mg₅(MoO₄)₆ [7], Ag₂M₂(MoO₄)₃ (M = Co, Mn, Mg, Zn) [8–10], and to Na_0.5Zn_2.75(MoO₄)₃ [11]. The cell parameters of those compounds are given in Table 4.

The asymmetric unit in Na_1.92Mg_2.04Mo₃O₁₂ compound is shown in Figure 1. The structure is composed of two octahedra Mg₂O₁₀ and three tetrahedra MoO₄ sharing corners and forming a cyclic group Mg₂Mo₃O₁₉. The charge compensation in the asymmetric unit is ensured by Na⁺ cations.

The structure may be represented as a three-dimensional mixed framework of pairs of MgO₆ octahedra sharing common edges forming Mg₂O₁₀ group, linked to MoO₄ tetrahedra by corners. Each two Mg₂O₁₀ octahedra sharing edges are surrounded either by ten tetrahedra forming Mg(1)₂Mo₁₀O₄₀ group (Figure 2(a)) or by eight tetrahedra forming Mg(2)₂Mo₈O₃₂ group (Figure 2(b)).

The projection of the structure along [100] and [001] directions shows the presence of layers in the ac plane which are connected by Mo(2)O₄ polyhedra forming two types of tunnels hexagonal along [100] direction (Figure 3) and squared along [001] direction (Figure 4). The monovalent cations are located in tunnels forming infinite channels.

The projection of a layer along the direction [010] shows the presence of two types of Mg(1)₂Mo₁₀O₄₀ and Mg(2)₂Mo₈O₃₂ chains connected by MoO₄ tetrahedra (Figure 5).

In the crystal structure of Na_1.92Mg_2.04Mo₃O₁₂, the Mo atom has a tetrahedral oxygen coordination, with Mo–O distances varying within 1.718(3)–1.800(4) Å with the average of 1.760 Å close to the common values [14]. Magnesium cations occupy two crystallographically independent sites Mg1 and Mg2, which are located in an octahedral environment with (d (Mg1–O) = 2.056(3)–2.167(3) Å) and (d (Mg2–O = 1.989(4)–2.158(3) Å) with the average of 2.100 Å.

Sodium atoms occupy three different positions Na(1), Na(2), and the third site which is occupied by (Na⁺, Mg²⁺). The (Na1/Mg11)–O bond lengths vary from 2.15 to 2.29 Å (Table 3). In fact, we remark that the short distance Na–O is shorter than normal which indicates that the occupation is partial with the M²⁺ atom.

The (Na1/Mg11)O₅ is surrounded by five oxygen atoms (CN = 5). The voids between layers are occupied by sodium atoms: Na2 and Na3 are in the middle of concavities; (Na1/Mg11) lie on the extremity of the concavity attracted to polyhedra of two adjacent layers (Figure 4).

3.2. Electrical Measurements

The electrical properties of Na_1.92Mg_2.04Mo₃O₁₂ ceramic have been investigated using complex impedance spectroscopy. Impedance spectroscopy measurements were carried out in a Hewlett-Packard 4192-A automatic bridge monitored by an HP microcomputer. The electrical measurements are realized in the thermal range 693–743 K and frequency range of 5 Hz–13 MHz. Pellet was prepared by isostatic pressing at 4 kbar and sintering at 480°C for 2 h in air with 5 K·min⁻¹ heating and cooling rates. The thickness and surface of pellet were about 0.159 cm and 0.807 cm² having a geometric factor of e/S = 0.197 cm⁻¹. Platinum electrodes were painted in the two faces of the pellet with a platinum paste to ensure good electric contacts. The Nyquist plots at different temperatures are shown in Figure 6. When temperature increases, the radius of semicircles decreases, which indicates an activated conduction mechanism. The intercepts of the semicircular arcs with the real axis give an estimation of the resistance of material. We have used the Zview software [17] to fit these curves. The measured impedance can be modeled as an equivalent electrical circuit composed of a resistor, R, connected in parallel with a constant phase element, CPE [18]. By knowing the value of resistance and the dimensions of the sample, the conductivity has been calculated at each temperature.

The variation of log (σT(S·K·cm⁻¹)) versus 1000/T (K⁻¹) is represented in Figure 7. The conductivity value at 683 K is 3.01 × 10⁻⁷ S cm⁻¹ and the activation energy for Na⁺ ions migration deduced from the slope is  Ea = 1.37 eV; Na_1.92Mg_2.04Mo₃O₁₂ shows a low electric conductivity, when compared to those found for other molybdenum oxide compounds (Table 5) [19, 20].