Abstract
LiNa5K3Mo11As3O45 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 [Mo11As3O45]9− is formed by nine MoO6 octahedra, two MoO5 trigonal bipyramids, and three AsO4 tetrahedra. The association of [Mo11As3O45]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 A2O-MoO3-As2O5 systems (A = alkali or silver) in which many compounds were characterized: K2MoO2(MoO2As2O7)2 [2], Na2(MoO2)3(As2O7)2 [3], and NaAg2Mo3O9AsO4 [4]. We have succeeded in the synthesis of a new material LiNa5K3Mo11As3O45. It was prepared by a solid state method.
2. Experimental Details
2.1. Synthesis
The LiNa5K3Mo11As3O45 compound was obtained from a mixture of (NH4)2Mo4O13 (Fluka 69858), NH4H2AsO4 (ASTM 01-775), Na2CO3 (Prolabo 27778), K2CO3 (Pan 121490) and LiOH·H2O (Fluka 62530). The mixture was grinded and preheated in air at 673 K to remove NH3, H2O, and CO2. 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 CuKα 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 LiNa5K3Mo11As3O45 and 200 mg of KBr and compressed with hydraulic press under 100 kg/cm2 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 MoO6 [12, 13] and AsO4 [14] and two absorption bands at 425 and 625 cm−1 arising from the vibration of MoO6 [15]. A peak around 952 cm−1 might be assigned to the stretching and the bending of AsO4 [14] and little band, at 995 cm−1, is the result of the vibration of MoO5 [12, 16].
2.6. Complex Impedance Analysis
The electrical properties of the LiNa5K3Mo11As3O45 material have been investigated using complex impedance spectroscopy. The sample is prepared by pressing the crystal powder at 100 kg/cm2 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 cm2. 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 LiNa5K3Mo11As3O45 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 LiNa5K3Mo11As3O45 compound (Figure 6) consists of(i)three Mo3O14 motifs; each one is formed by three octahedra sharing edges;(ii)two MoO5 bipyramids which are inserted between the three Mo3O14 motifs. They are linked by edges and corners;(iii)three AsO4 tetrahedra; each one is linked by sharing corners with four octahedra.
The association of two MoO5 bipyramids and one Mo3O14 motif leads to a Mo5O20 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 (Mo11As3O45)9− ribbons (Figure 7). The structure of LiNa5K3Mo11As3O45 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 LiNa5K3Mo11As3O45, the polyhedra are distorted because of the existence of(i)short atomic bonds of molybdenyl group in MoO6 octahedra and MoO5 bipyramids;(ii)short atomic bonds in AsO4 tetrahedra.
The distortion factors of angles and distances of atomic bonds (resp., AdF and DdF), 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 K2Mo3O10 [19] and K2Mo4O13 [20] are one dimensional. Their basic units are formed by MoO6 octahedra and MoO5 bipyramids. The ribbons forms are helicoidal, as the ribbon forms of LiNa5K3Mo11As3O45 compound. In the three-dimensional framework of Na6Mo5P2O23·14H2O [21], we have found a cyclic group Mo5O21, but, in the structure of LiNa5K3Mo11As3O45, the Mo5O20 group is linear. So, the structure of Na6Mo5P2O23·14H2O differed from the structure of LiNa5K3Mo11As3O45.
In the one-dimensional framework of Na2AgMo3AsO13 [4] and the framework of Ag12.4Na1.6Mo18As4O71 [1], there are two motifs similar to those found in LiNa5K3Mo11As3O45 compound that are Mo3O14, formed by three MoO6 octahedra linked by sharing edges, and Mo3AsO17, composed by Mo3O14 motif and AsO4 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, LiNa5K3Mo11As3O45 compound could be a good ionic conductor. Figure 11 shows the spectrum of complex impedances of LiNa5K3Mo11As3O45 in the various temperatures.
(a)
(b)
(c)
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 (Ea1 and Ea2) of cations migration deduced from the slopes are equal to(i)Ea1 = 0.559 eV before 340°C;(ii)Ea2 = 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, LiNa5K3Mo11As3O45 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
LiNa5K3Mo11As3O45 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. LiNa5K3Mo11As3O45 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.