Journal of Crystallography

Volume 2014, Article ID 181695, 9 pages

http://dx.doi.org/10.1155/2014/181695

## Synthesis, Crystal Structure, and Electrical Properties of a New Molybdylarsenate LiNa_{5}K_{3}Mo_{11}As_{3}O_{45}

Laboratoire de Matériaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, Manar II, 2092 Tunis, Tunisia

Received 29 December 2013; Accepted 24 April 2014; Published 13 July 2014

Academic Editor: Baoyuan Man

Copyright © 2014 Hamadi Hamza et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} is a new inorganic compound. It was synthesized by a solid state method. The crystal structure has been studied by single crystal X-ray analysis. The *R*-values reached 2.8%. The title compound crystallizes in the triclinic system, space group P-1, with *a* = 10.550 (2) Å, *b* = 11.723 (2) Å, *c* = 17.469 (3) Å, *α* = 102.35 (3)°, *β* = 87.61 (2)°, and *γ* = 111.03 (3)°. The anionic unit [Mo_{11}As_{3}O_{45}]^{9−} is formed by nine MoO_{6} octahedra, two MoO_{5} trigonal bipyramids, and three AsO_{4} tetrahedra. The association of [Mo_{11}As_{3}O_{45}]^{9−} units, running along [010], leads to a one-dimensional framework. Li, K, and Na are located in the space surrounding the anionic ribbons. This material was characterized by SEM microscopy, IR spectroscopy, and powder X-ray diffraction. The electrical conductivity was investigated from 528 K to 673 K by impedance complex followed by DSC spectroscopy.

#### 1. Introduction

The search for new materials, based in arsenic and molybdenum, with an open framework formed by octahedra and tetrahedra showing multiple modes of connections and containing alkali gives a big interest in solid state chemistry [1]. This kind of material shows important energetic property which is the ionic conductivity. We are interested in this field and we have explored the A_{2}O-MoO_{3}-As_{2}O_{5} systems (A = alkali or silver) in which many compounds were characterized: K_{2}MoO_{2}(MoO_{2}As_{2}O_{7})_{2} [2], Na_{2}(MoO_{2})_{3}(As_{2}O_{7})_{2} [3], and NaAg_{2}Mo_{3}O_{9}AsO_{4} [4]. We have succeeded in the synthesis of a new material LiNa_{5}K_{3}Mo_{11}As_{3}O_{45}. It was prepared by a solid state method.

#### 2. Experimental Details

##### 2.1. Synthesis

The LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound was obtained from a mixture of (NH_{4})_{2}Mo_{4}O_{13} (Fluka 69858), NH_{4}H_{2}AsO_{4} (ASTM 01-775), Na_{2}CO_{3} (Prolabo 27778), K_{2}CO_{3} (Pan 121490) and LiOH·H_{2}O (Fluka 62530). The mixture was grinded and preheated in air at 673 K to remove NH_{3}, H_{2}O, and CO_{2}. Then, it was grinded and heated again to 808 K. The mixture was left at this temperature for 2 weeks to promote germination and growth of crystals. The final residue was subjected at a first slow cooling (5 K/24 h) in an interval of 50 K and then at a second faster cooling (50 K/h) to reach room temperature. The yellowish crystals obtained by spontaneous crystallization were separated by flow of hot water to do the preliminary identification.

##### 2.2. Single Crystal X-Ray Data Collection

It was performed with a CAD-4 Enraf-Nonius X-ray diffractometer [5] at 298 K with graphite monochromator using wavelength. All calculations were performed using the Wingx-98 crystallographic software package [6]. An empirical correction of absorption by PSI scan [7] was applied. The structure was solved and refined using, respectively, SHELXS-97 and SHELXL-97 [8] by full-matrix least squares based on . The graphs of the structure were drawn with diamond 2.1 supplied by Crystal Impact [9]. The crystal data and the refinements details are summarized in Table 1. Table 2 contains the main bond distances.

##### 2.3. Powder XRD Analysis

The polycrystalline powder was prepared from a stoichiometric mixture of reagents forming the single crystal. In the beginning, the mixture was heated at 473 K to remove volatile compounds. Then, it was grinded and heated to 673 K and it was rigorously grinded again before heating up to 773 K. The residue was maintained for 48 hours at this temperature. Then, it was cooled down rapidly to room temperature. Obtained powder was characterized using X-ray diffraction. XRD analysis was performed with a PANalytical X’PertPro diffractometer with CuK*α* radiation (*λ* = 1.5406 Å). The comparison of obtained pattern (Figure 1) with reference pattern (Figure 2) shows that powder is pure. The obtained pattern was indexed in Table 3 by the programs X’pert Highscore plus [10] and Diamond 3.2 [11].

The confidence factor calculated by the formula is 1.27%.

##### 2.4. Scanning Electron Microscopy

One single crystal was selected by means of polarizing microscope. Then, it was analyzed by dispersive energy spectroscopy (model FEI type Quanta 200). The SEM analysis was used to observe the crystal morphology (Figure 3). The EDS local microanalysis (Figure 4) confirms the presence of expected chemical elements, particularly: sodium, potassium, molybdenum, arsenic, and oxygen.

##### 2.5. Infrared Spectroscopy FTIR

For this analysis, sample was prepared from a mixture of 2 mg of pure powder of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} and 200 mg of KBr and compressed with hydraulic press under 100 kg/cm^{2} in order to obtain a little pellet. In the transmission spectrum (Figure 5) obtained by spectrometer model Nicolet-IR 200, we have found two main strong and well-resolved bands around 490 and 830 cm^{−1} characterizing, respectively, the stretching of MoO_{6} [12, 13] and AsO_{4} [14] and two absorption bands at 425 and 625 cm^{−1} arising from the vibration of MoO_{6} [15]. A peak around 952 cm^{−1} might be assigned to the stretching and the bending of AsO_{4} [14] and little band, at 995 cm^{−1}, is the result of the vibration of MoO_{5} [12, 16].

##### 2.6. Complex Impedance Analysis

The electrical properties of the LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} material have been investigated using complex impedance spectroscopy. The sample is prepared by pressing the crystal powder at 100 kg/cm^{2} and sintering at 673 K for 24 hours. The thickness and the surface of the obtained pellet are respectively, 0.136 cm and 1.40 cm^{2}. This pellet was placed between two blocking platinum electrodes in order to ensure good electric contacts in a tubular oven to undergo the measurements of complex impedance by using a Hewlett-Packard 4192-A impedance analyzer in the temperature range from 523 K to 673 K and in the frequency range from 5 Hz to 13000 Hz.

##### 2.7. Differential Scanning Calorimetric Analysis

To examine thermal transitions in LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound, a sample of 10.3 mg was analyzed by using a differential scanning calorimeter 822-E made by Mettler Toledo.

#### 3. Results and Discussion

##### 3.1. Structure Description

The asymmetric unit of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound (Figure 6) consists of(i)three Mo_{3}O_{14} motifs; each one is formed by three octahedra sharing edges;(ii)two MoO_{5} bipyramids which are inserted between the three Mo_{3}O_{14} motifs. They are linked by edges and corners;(iii)three AsO_{4} tetrahedra; each one is linked by sharing corners with four octahedra.

The association of two MoO_{5} bipyramids and one Mo_{3}O_{14} motif leads to a Mo_{5}O_{20} semicyclic group.

The charge compensation is ensured by Na^{+}, K^{+}, and Li^{+}. The molybdenum atom Mo (11) is delocalized. It occupies two positions with the distance of 0.949 (8) Å with various occupancies 93.3% and 6.7%.

The combination of asymmetric units by sharing corners between octahedra and tetrahedra leads to (Mo_{11}As_{3}O_{45})^{9−} ribbons (Figure 7). The structure of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} can be described as a one-dimensional framework of ribbons disposed along direction. Na^{+}, K^{+}, and Li^{+} are situated in the space surrounding the ribbons (Figures 8 and 9).

In the structure of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45}, the polyhedra are distorted because of the existence of(i)short atomic bonds of molybdenyl group in MoO_{6} octahedra and MoO_{5} bipyramids;(ii)short atomic bonds in AsO_{4} tetrahedra.

The distortion factors of angles and distances of atomic bonds (resp., A_{d}F and D_{d}F), obtained by the following formula, are summarized in Table 4:
(see [17]). is real distance value, is moyen distance value, is coordination number, is real angle value, is moyen angle value, and is angles number.

Moreover, the calculation of the various valence sums of atomic bonds (BVS), using empirical formula of Altermatt and Brown [18], confirms that they are the expected values of ions charges. All bond valence sums are represented in Table 5.

##### 3.2. Structure Comparison

The studied phase reveals some structure affiliations with the phases found in the literature. In fact, the structures of K_{2}Mo_{3}O_{10} [19] and K_{2}Mo_{4}O_{13} [20] are one dimensional. Their basic units are formed by MoO_{6} octahedra and MoO_{5} bipyramids. The ribbons forms are helicoidal, as the ribbon forms of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound. In the three-dimensional framework of Na_{6}Mo_{5}P_{2}O_{23}·14H_{2}O [21], we have found a cyclic group Mo_{5}O_{21}, but, in the structure of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45}, the Mo_{5}O_{20} group is linear. So, the structure of Na_{6}Mo_{5}P_{2}O_{23}·14H_{2}O differed from the structure of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45}.

In the one-dimensional framework of Na_{2}AgMo_{3}AsO_{13} [4] and the framework of Ag_{12.4}Na_{1.6}Mo_{18}As_{4}O_{71} [1], there are two motifs similar to those found in LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound that are Mo_{3}O_{14}, formed by three MoO_{6} octahedra linked by sharing edges, and Mo_{3}AsO_{17}, composed by Mo_{3}O_{14} motif and AsO_{4} tetrahedron.

##### 3.3. Ionic Conductivity

The geometric data analysis shows that a few interstitial sites are adjacent to those occupied by cations (Figure 10). Furthermore, on the basis of the arrangement of cations in the one-dimensional framework, LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound could be a good ionic conductor. Figure 11 shows the spectrum of complex impedances of LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} in the various temperatures.

The radius of semicircles decreases when temperature increases signifying an ionic conduction with activated mechanism. The intercepts of the semicircles with the real axis give the estimated values of the material’s resistances by using the Zview software [22]. The measured impedance can be modeled as that of an equivalent electrical circuit composed of a resistor* R* connected in parallel with a nonideal capacitor usually known as constant phases elements CPE [23]. After determination of the resistance values at various temperatures and the dimensions of the sample, we have calculated the conductivity values (Table 6).

Figure 12 shows the variation of log( (S·K·cm^{−1}) versus 10000/ (K^{−1})). The values of activation energies (*E*_{a1} and* E*_{a2}) of cations migration deduced from the slopes are equal to(i)*E*_{a1} = 0.559 eV before 340°C;(ii)*E*_{a2} = 0.871 eV after 340°C.

The change of activation energy is assigned to a change of cation migration process or a thermal transition.

Figure 13 shows the variation of the resistance imaginary part versus the relaxation frequency versus . All curvatures in various temperatures have the same wide at midheight which is equal to 60.34 Hz ; this confirms that the variation of activation energy is not due to the change of cation migration process [24, 25]. The DSC diagram in Figure 14 shows the change of baseline from 340°C. Therefore, LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound is the subject of a thermal transition from this temperature and this is the real reason of change of activation energy. This compound shows a medium electric conductivity, compared with the compounds found in literature [1, 15, 26].

#### 4. Conclusion

LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} compound was prepared by solid state reaction. The structure has been resolved by single crystal X-ray diffraction and characterized by dispersive energy spectroscopy, powder X-ray diffraction, FTIR spectroscopy, and DSC. The compound crystallizes in the triclinic system (space group P-1) with the following unit cell parameters:* a* = 10.550 (2) Å,* b* = 11.723 (2) Å,* c* = 17.469 (3) Å, *α* = 102.35 (3)°, *β* = 87.61 (2)°, and *γ* = 111.03 (3)°. This material has one-dimensional structure. The electrical properties are investigated using complex impedance spectroscopy. The conductivity value at 673 K is S·cm^{−1} and the activation energy value is 0.559 eV. LiNa_{5}K_{3}Mo_{11}As_{3}O_{45} presents medium electric properties.

#### Disclosure

The CIF file corresponding to the studied structure has been deposited in the database of FIZ Karlsruhe no. CSD 426635.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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