Journal of Crystallography

Volume 2014 (2014), Article ID 181695, 9 pages

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

Research Article

Synthesis, Crystal Structure, and Electrical Properties of a New Molybdylarsenate LiNa₅K₃Mo₁₁As₃O₄₅

Hamadi Hamza, Mohamed Faouzi Zid, and Ahmed Driss

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.

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.

Table 1: Crystal data and the refinements details.

Empirical formula
LiNa₅K₃Mo₁₁As₃O₄₅

Crystal system/space group/
Triclinic/P-1/2

(Å)
10.550(2)

(Å)
11.723(2)

(Å)
17.469(3)

α (°)
102.35(3)

β (°)
87.61(2)

γ (°)
111.03(3)

(Å³)
1986.46(2)

μ (mm⁻¹)
6.39

(g·cm⁻³)/formula weight (g/mol)
3.78/2247.34

Crystal size (mm)/Color
0.2 × 0.3 × 0.5/yellow

Tmin/Tmax
0.7026/0.8745

Measured reflections
9789

Independent reflections/rint
8557/0.0157

Observed refl. with > 2σ()
7730

Data/restraints/parameters
8557/2/619

[ > 2σ()]
0.028

()//(000)
0.071/1.22/2072

Δρmax/Δρmin (e·Å⁻³)
1.56/−1.39

Table 2: Main bond distances.

Bond distances (Å)

As1–O30
1.669 (4)
Mo10–O9
2.313(4)

As1–O25
1.677 (4)
Mo10–O14
2.360(4)

As1–O9
1.691 (4)
Mo11–O39
1.707(4)

As1–O10
1.696 (4)
Mo11–O45iii
1.714(5)

As2–O15iii
1.663 (4)
Mo11–O21
1.835(4)

As2–O24
1.679 (4)
Mo11–O35
1.915(4)

As2–O13
1.690 (4)
Mo11–O37
2.285(4)

As2–O37
1.695 (4)
K1–O41
2.751(4)

As3–O23
1.675 (4)
K1–O38vi
2.759(4)

As3–O20
1.677 (4)
K1–O12i
2.802(4)

As3–O14
1.698 (4)
K1–O17vii
2.823(4)

As3–O4
1.699 (4)
K1–O43viii
2.906(4)

Mo1–O28
1.701 (4)
K1–O15
2.912(4)

Mo1–O26
1.731 (4)
K1–O17i
2.958(5)

Mo1–O6
1.826 (4)
K1–O5
3.115(4)

Mo1–O19
1.923 (4)
K2–O7
2.755(4)

Mo1–O10
2.273 (4)
K2–O28v
2.770(4)

Mo2–O22
1.706 (4)
K2–O34v
2.803(4)

Mo2–O41
1.735 (4)
K2–O32x
2.852(4)

Mo2–O5
1.869 (4)
K2–O45
2.867(5)

Mo2–O18i
1.972 (4)
K2–O1
2.932(5)

Mo2–O30
2.217 (4)
K2–O31i
2.979(5)

Mo2–O14
2.285 (4)
K2–O20
3.019(4)

Mo3–O16ii
1.685 (4)
K3–O29
2.615(4)

Mo3–O32
1.724 (4)
K3–O2vii
2.713(4)

Mo3–O29
1.943 (4)
K3–O33
2.726(4)

Mo3–O19
1.972 (4)
K3–O22vii
2.932(5)

Mo3–O24
2.239 (4)
K3–O36ii
2.964(4)

Mo3–O10
2.266 (4)
K3–O30
3.063(4)

Mo4–O11
1.700 (4)
Na1–O4
2.409(4)

Mo4–O40
1.731 (4)
Na1–O32
2.444(5)

Mo4–O8
1.916 (4)
Na1–O11i
2.522(5)

Mo4–O3iii
1.924 (4)
Na1–O26
2.530(5)

Mo4–O4iii
2.263 (4)
Na1–O33
2.608(5)

Mo4–O13
2.317 (4)
Na1–O10
2.719(5)

Mo5–O38ii
1.705 (4)
Na1–O23
2.861(5)

Mo5–O33
1.714 (4)
Na1–O30
2.995(5)

Mo5–O5
1.918 (4)
Na2–O43
2.400(6)

Mo5–O8i
1.943 (4)
Na2–O18i
2.468(5)

Mo5–O15
2.205 (4)
Na2–O41viii
2.521(5)

Mo5–O4
2.308 (4)
Na2–O22
2.642(5)

Mo6–O1
1.702 (5)
Na2–O44viii
2.710(6)

Mo6–O27
1.710 (4)
Na2–O2
2.812(6)

Mo6–O7
1.936 (4)
Na2–O25
2.882(5)

Mo6–O6
1.963 (4)
Na3–O40xi
2.483(4)

Mo6–O9
2.220 (4)
Na3–O1
2.527(6)

Mo6–O23
2.237 (4)
Na3–O39v
2.628(5)

Mo7–O17
1.697 (4)
Na3–O3
2.737(4)

Mo7–O2
1.710 (4)
Na3–O27v
2.749 (5)

Mo7–O29
1.922 (4)
Na3–O31i
2.786(5)

Mo7–O12
1.964 (4)
Na3–O23
2.845(4)

Mo7–O24
2.252 (4)
Na4–O14
2.313(5)

Mo7–O25
2.280 (4)
Na4–O36
2.401(6)

Mo8–O43
1.712 (4)
Na4–O44
2.412(6)

Mo8–O44iii
1.712 (4)
Na4–O45
2.416(6)

Mo8–O12
1.889 (4)
Na4–O37i
2.436(5)

Mo8–O35
1.933 (4)
Na4–O20
2.671(5)

Mo8–O37
2.301 (4)
Na4–O41
2.910(6)

Mo8–O25
2.450 (4)
Na5–O16
2.373(5)

Mo9–O31
1.702 (4)
Na5–O8x
2.401(4)

Mo9–O34
1.721 (4)
Na5–O27
2.439(6)

Mo9–O3iii
1.929 (4)
Na5–O42
2.468(5)

Mo9–O21
1.949 (4)
Na5–O35
2.474(5)

Mo9–O20iii
2.224 (4)
Na5–O39
2.734(5)

Mo9–O13
2.227 (4)
Li–O19
2.037(12)

Mo10–O42
1.710 (4)
Li–O40
2.041(12)

Mo10–O36
1.725 (4)
Li–O34
2.053(13)

Mo10–O18i
1.894 (4)
Li–O26xi
2.075(12)

Mo10–O7
1.927 (4)

Symmetry codes:(i) x, y−1, z; (ii) x−1, y, z;(iii)x, y+1, z;(iv)x−1, y−1, z; (v) −x+1, −y+1, −z; (vi) −x+1, −y, −z+1; (vii) −x, −y+1, −z+1; (viii) −x+1, −y+1, −z+1; (ix) −x, −y, −z+1; (x) x+1, y, z; (xi) −x, −y+1, −z; (xii) x+1, y+1, z.

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].

Table 3: Indexation of powder XRD pattern.

%
2 (°)
2(°)

−1
1
0
6.4
09.786
09.791

−1
0
1
100
10.196
10.199

0
0
2
77.8
10.367
10.369

1
−2
1
49.9
15.567
15.621

−1
2
0
11.8
15.764
15.758

0
2
0
87
16.571
16.574

−1
0
3
11.4
17.703
17.696

1
0
3
10.9
18.309
18.308

0
2
1
10.2
18.409
18.393

−1
2
2
16.2
20.603
20.620

2
0
2
12.8
21.177
21.113

−1
−2
1
14.4
21.243
21.249

2
1
0
13.1
22.420
22.422

1
2
1
11.9
23.096
23.090

3
−1
1
30.1
25.758
25.754

2
−1
4
16.4
26.075
26.077

2
−2
4
15.8
26.574
26.608

−2
1
4
41.3
27.783
27.777

−1
−1
5
45.5
27.890
27.896

−1
2
4
19.6
28.713
28.712

−1
−3
1
19.9
28.976
28.890

−2
−2
2
8.2
29.067
29.069

−3
2
2
7.1
29.246
29.256

−1
−3
2
25.6
29.372
29.466

2
−3
4
22.9
29.573
29.579

−2
−2
3
13.7
30.651
30.650

−3
−1
1
10.9
31.199
31.194

0
3
3
11.5
32.348
32.333

3
−4
1
10.8
34.308
34.305

3
−4
3
11.3
35.677
35.682

−3
1
5
11.5
37.266
37.263

−1
−1
7
10.9
37.577
37.523

−3
−1
5
4.8
39.189
39.184

2
−2
8
5.9
44.250
44.251

4
−3
6
12.6
45.773
45.780

−1
−5
4
2.4
47.146
47.140

2
−5
8
2.7
52.161
52.155

−1
2
9
0.8
54.077
54.076

5
−3
7
2.5
55.893
55.895

−4
−4
1
6.5
58.832
58.836

4
0
9
2.4
63.020
63.018

2
−8
5
2.7
65.199
65.194

Figure 1: Experimental XRD pattern of LiNa₅K₃Mo₁₁As₃O₄₅.

Figure 2: Calculated XRD pattern of LiNa₅K₃Mo₁₁As₃O₄₅.

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.

Figure 3: Crystal morphology.

Figure 4: EDS analysis spectrum.

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].

Figure 5: IR spectrum of LiNa₅K₃Mo₁₁As₃O₄₅.

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.

Figure 6: The asymmetric unit.

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).

Figure 7: View of (Mo₁₁As₃O₄₅)⁹⁻ ribbon.

Figure 8: Projection of the structure of LiNa₅K₃Mo₁₁As₃O₄₅.

Figure 9: Projection of the structure along .

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.

Table 4: Angles and distances distortion factors (%).

Polyhedra
F
F

As(1)–O₄
0.15
2.33

As(2)–O₄
0.15
2.54

As(3)–O₄
0.16
3.70

Mo(1)–O₅
1.75
13.64

Mo(2)–O₆
1.64
7.49

Mo(3)–O₆
1.58
8.45

Mo(4)–O₆
1.77
9.39

Mo(5)–O₆
1.64
8.25

Mo(6)–O₆
1.51
10.61

Mo(7)–O₅
1.66
9.03

Mo(8)–O₆
2.09
9.37

Mo(9)–O₆
1.51
7.49

Mo(10)–O₆
1.94
9.79

Mo(11)–O₆
1.76
9.29

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.

Table 5: Bond valence sums.

Polyhedra
BVS

As(1)–O₄
4.904

As(2)–O₄
4.924

As(3)–O₄
4.859

Mo(1)–O₅
5.963

Mo(2)–O₆
6.101

Mo(3)–O₆
6.042

Mo(4)–O₆
6.087

Mo(5)–O₆
6.117

Mo(6)–O₆
6.117

Mo(7)–O₆
6.083

Mo(8)–O₆
5.991

Mo(9)–O₆
6.142

Mo(10)–O₆
6.007

Mo(11)–O₅
5.988

K(1)–O₈
0.927

K(2)–O₈
0.966

K(3)–O6
0.976

Na(1)-O8
1.001

Na(2)–O8
0.905

Na(3)–O7
0.715

Na(4)–O7
1.052

Na(5)–O6
0.915

Li-O₄
0.727

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.

Figure 10: Arrangements of cations.

(a)

(b)

(c)

Figure 11: Spectrum of complex impedance of LiNa₅K₃Mo₁₁As₃O₄₅.

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).

Table 6: Conductivity value at various temperatures.

T° (C)
(Ω)
σ = ()/ (Ω⁻¹·cm⁻¹)

255
4.500 10⁵
2.14 10⁻⁷

280
2.550 10⁵
3.78 10⁻⁷

310
1.400 10⁵
6.90 10⁻⁷

340
8.650 10⁴
1.11 10⁻⁶

370
4.100 10⁴
2.35 10⁻⁶

400
2.050 10⁴
4.71 10⁻⁶

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.

Figure 12: Variation of log  versus 10⁴/.

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].

Figure 13: Variation of versus .

Figure 14: DSC diagram of LiNa₅K₃Mo₁₁As₃O₄₅ compound.

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.

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