Abstract

New molybdenum oxide Na_1.92Mg_2.04Mo₃O₁₂ has been synthesized by the solid state method. The title compound crystallizes in the triclinic system (space group P-1). The unit cell parameters are a = 6.9660(7) Å, b = 8.6352(8) Å, c = 10.2501(8) Å, α = 106.938(1)°, β = 104.825(1)°, γ = 103.206(1)°, V = 538.72(9) ų, and Z = 2. The compound is isotypical to Ag₂M₂(MoO₄)₃ (M = Zn, Mg, Co, Mn). The structure can be described as a three-dimensional anionic mixed framework of MoO₄ tetrahedra and pairs of Mg₂O₁₀ octahedra sharing common edges. The Na⁺ ions are disordered and located in the voids forming infinite channels running along the direction [100]. The electrical conductivity investigated from 693 K to 793 K by AC impedance spectroscopy is low ( S cm⁻¹ at 683 K).

1. Introduction

The interest in the study of alkali molybdenum oxides is primarily in the unique structural and physical properties of some of these compounds which include high anisotropic transport properties [1, 2] and superconductivity and in their important application in the field of energy and electronics as described in several reviews [3, 4]. Some of complex molybdates crystallize in structure types which show significant ionic conductivity. For example, the structure of the Nasicon type molybdate [5] is of great interest because of high ionic conduction. For this reason the characterization of those oxides seems to be an important task. In our investigation, we synthesize new crystal of molybdenum oxide Na_1.92Mg_2.04Mo₃O₁₂.

In our bibliographical search, we found the study of the phase diagram of the system Na₂MoO₄-MgMoO₄ [6]; the system contains a single intermediate compound Na₂Mg₅(MoO₄)₆ [7]. The structural study reveals that the compound is isotypical to Na₂Mg₅(MoO₄)₆ [7], Ag₂M₂(MoO₄)₃ (M = Co, Mn, Mg, Zn) [8–10], and Na_0.5Zn_2.75(MoO₄)₃ [11].

(The cif file corresponding to the studied structure has been deposited in the database of Karlsruhe Number CSD. 426651 (http://www.fiz-karlsruhe.de/icsd.html)).

2. Experimental Details

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 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 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. Results and Discussion

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.

Figure 1 

The asymmetric unit of Na1.92Mg2.04Mo3O12 compound.

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)).

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Figure 2 

A view showing (a) Mg2Mo10O40 and (b) Mg2Mo8O32.

The projection of the structure along and 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 direction (Figure 3) and squared along direction (Figure 4). The monovalent cations are located in tunnels forming infinite channels.

Figure 3 

Projection of Na1.92Mg2.04Mo3O12 structure along direction showing cavities where monovalent cations are located.

Figure 4 

Projection of Na1.92Mg2.04Mo3O12 structure along direction showing layers along a direction.

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

Figure 5 

Projection of chains structure along direction showing layers along a direction.

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 ( (Mg1–O) = 2.056(3)–2.167(3) Å) and ( (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 = 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, , 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.

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Figure 6 

Complex impedance spectra of Na1.92Mg2.04Mo3O12 at various temperatures.

The variation of log ((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 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].

Figure 7 

Arrhenius plot of conductivity of Na1.92Mg2.04Mo3O12 ceramic.

4. Conclusion

New molybdate Na_1.92Mg_2.04Mo₃O₁₂ is synthesized by a solid state method. The structure of our compound has been solved by using X-ray diffraction. The compound is formed by bioctahedra M₂O₁₀ and MoO₄ tetrahedra connected via common vertices. The structure of this material has an open framework having different interconnecting tunnels running along and where the Na⁺ ions are located. The electrical properties of the title compound are investigated using complex impedance spectroscopy. The conductivity value at 683 K is 3.01 × 10⁻⁷ S cm⁻¹ and the activation energy for Na⁺ ions migration is = 1.37 eV. Na_1.92Mg_2.04Mo₃O₁₂ presents a low electric conductivity.

Supplementary Materials