RSC Advances PAPER
Cite this: RSC Adv., 2015, 5, 38918
Synthesis, crystal structure, sintering and electrical properties of a new alluaudite-like triple molybdate K0.13Na3.87MgMo3O12† Ines Ennajeh,*a Samuel Georges,b Youssef Ben Smida,a Abderrahmen Guesmi,ac Mohamed Faouzi Zida and Habib Boughazalaa A new triple molybdate K0.13Na3.87MgMo3O12 was synthesized by solid state reaction. The crystal structure has been determined by single X-ray diffraction and the electrical conductivity measured by impedance spectroscopy. The title compound crystallizes in the monoclinic space group C2/c with a ¼ 12.9325 (8) A ˚, b ¼ 13.5537 (9) A ˚ 3 and Z ¼ 4. The final agreement ˚ , c ¼ 7.1627 (6) A ˚ , b ¼ 112.212 (9) , V ¼ 1162.33 (14) A factors are R ¼ 0.0241, wR (F2) ¼ 0.0584, S(F2) ¼ 1.22. The magnesium–molybdate 3D-framework belongs to the alluaudite type. The structure is formed by infinite chains composed of edge-sharing (Mg/Na)2O10 dimmers, which are linked together via bridging MoO4 tetrahedra, yielding to a threedimensional framework enclosing two distinct types of hexagonal tunnels in which Na+ and K+ cations reside. The structural model is validated by bond valence sum (BVS) and charge distribution (CD) methods. Ball milling is used as mechanical means to reduce the particles sizes of the synthesized powder. At the optimal sintering temperature of 650 C, a relative density of 81% was obtained. The microstructures were characterized by scanning electron microscopy. The compound undergoes a
Received 5th February 2015 Accepted 22nd April 2015
phase transformation at 528 C accompanied by an abrupt increase of the electrical conductivity. Above
this
phase
transition,
the
electrical
conductivity
reaches
102
S
cm1.
Thus
DOI: 10.1039/c5ra02276b
K0.13Na3.87Mg(MoO4)3 may be considered as a promising compound for developing new materials with
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high ionic conductivity.
Introduction Complex molybdates containing alkaline elements are interesting because of their physical properties and potential applications as ferroelectric, piezoelectric and especially ionic conductors.1–4 During the last years, an increasing number of structural studies of double and triple molybdates with a rich variety of structure types were reported in the literature. Among complex molybdates of interest, lyonsite-structure such as NaCo2.31(MoO4)3,5 Li3Fe(MoO4)3 and Li2Fe2(MoO4)3,6 Nasicon structure such as Na2xM2Sc2(1x)(MoO4)3 where M ¼ Zn, Cd and Mg7 and alluaudite-type like Na5Sc(MoO4)4 (ref. 8) were described. Double and triple molybdates, due to these structural features are believed to have a high ionic conductivity.9–13 Indeed, the ionic conductivity reaches 103–101 S cm1 for the a Universit´e de Tunis El Manar, Laboratoire de Mat´eriaux et Cristallochimie, Facult´e des Sciences, El Manar II, 2092, Tunis, Tunisia. E-mail:
[email protected] b Laboratoire d'Electrochimie et de Physicochimie des Mat´eriaux et des Interfaces LEPMI, UMR 5279, CNRS: Grenoble INP, Universit´e de Savoie, Universit´e Joseph Fourier, BP75, 38402 Saint Martin d'H`eres, France c Universit´e de Tunis El Manar, Institut Pr´eparatoire aux Etudes d'Ing´enieurs d'El Manar, El Manar II, 2092 Tunis, Tunisia
† CCDC 1045308. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra02276b
38918 | RSC Adv., 2015, 5, 38918–38925
NASICON-type molybdates Na3Sc(MoO4)3 and Cs2Zr(MoO4)3.14 The conductivity appears to be restricted to the signicant cation mobility. It was also noted that complex molybdenum oxides undergo various phase transitions that lead to crystalline structures with high ionic conductivity. The open framework, the mobility of cations and the phase transformation were features interesting enough to motivate us to study double and triple molybdate. In this work, we report the synthesis, structure and ionic conductivity of a new triple molybdate K0.13Na3.87Mg(MoO4)3 with an alluaudite-type structure. The structural study showed that the title compound presents a three-dimensional framework with large hexagonal tunnels containing the alkaline cations. The proposed structural model based on a careful investigation of the single crystal X-ray diffraction data is supported by charge-distribution (CHARDI) analysis and bond-valence-sum (BVS) calculations. The correlation between the X-ray renement and the validation results is discussed. The thermal stability of the molybdate was investigated. The electrical properties of the ceramics shaped from the powders have been studied using complex impedance spectroscopy. Correlation attempts between the electrical behaviour and the microstructure of the samples were made.
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Paper
Experimental Synthesis The crystals used for the structural determination were prepared by solid-state reaction. For the synthesis of the single crystals, Na2CO3 (Fluka, 71350), K2CO3 (Fluka, 60109), (NH4)2Mo4O13 (Fluka, 69858) and Mg(NO3)2$6H2O (Fluka, 63079) were used as starting materials with the following proportions (Na: 3.8, K: 0.2, Mo: 3, Mg: 1). In this study, we are not currently in the process of investigating the complete Na–K range. We put potassium in small amounts in order to create a random cationic distribution, to rise the cationic disorder and increase the cationic conductivity. A mixture of the starting materials placed in a porcelain crucible was slowly annealed in air at 400 C for 12 hours, in order to eliminate volatile compounds. The resulting mixtures were heated at 700 C for 7 days in air. The sample was slowly cooled down at 5 /24 h to 600 C and nally quenched to room temperature. Single colorless crystals of triple molybdates were grown by spontaneous crystallization.
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Impedance spectra were recorded in the 13 MHz to 5 Hz frequency range between 250 and 620 C. Platinum lms (0.1 mm) were deposited as electrodes on both faces of each sample by RF magnetron sputtering. The pellet was connected to F.R.A using platinum grids and wires, mounted in a stainless steel sample holder placed inside an alumina tube, and positioned in a Pyrox furnace. Differential thermal analysis. Thermal analysis have been carried out to determine the sample transition temperature. Differential thermal analyses (DTA) were performed on a Setaram Type 92 microbalance. The experiments were carried out in air, up to 600 C, with a heating rate of 5 min1. SEM analysis. For SEM observations, the sintered sample was optically polished and thermally etched at 600 C, 50 C underneath the sintering temperature for 30 min with heating and cooling rates of 5 C min1. The SEM observations were performed using a Philips XL 30 microscope.
Results and discussion Crystal structure determination
Characterisation Single XRD analysis. Single crystal X-ray diffraction data for structure determination were collected on a CAD-4 EnrafNonius X-ray diffractometer15,16 using the MoKa (l ¼ 0.71069 ˚ radiation at room temperature. A) Powder XRD analysis. The quantitative synthesis of the polycrystalline powder was carried out in the same experimental conditions as described above, from a stoichiometric mixture of the reagents. Phase purities were carefully monitored by the method of X-ray powder diffraction using a PANalytical X'Pert ˚ Pro MPD diffractometer with Cu Ka radiation (l ¼ 1.5418 A) over the range of 0 # 2q # 70 . Ball milling. In order to reduce the mean particle size of the synthesized powders, ball milling was realized using a FRITSCH planetary micromill pulverisette apparatus equipped with agate jars and agate balls. A small powder quantity (5 g) and six agate balls (around 10 mm diameter) are placed into agate jars. The milling was realized at 700 rpm in ethanol to reduce pollution and excessive heating during the experiment. Alternation of 15 min milling sequences with 10 min pause sequences have been applied whatever the total duration of the experiment. The powders were shaped into pellets of 10 mm in diameter and 2–3 mm in thickness using a cylindrical steel mold. The uniaxial pressure was initially applied to give the shape of a cylinder by applying a force in the range of 1–2 T cm2 followed by isostatic pressing at 2.5 kbar. A melting point of 700 C was determined by series of annealing at increasing temperatures. Accordingly, the sintering temperature was set to 650 C. Relative densities were calculated from accurate measurements of the pellet size and weight, with comparison to the theoretical density deduced from the crystal structure. Impedance spectroscopy. Impedance spectroscopy measurements were carried out using a Hewlet-Packar 4192-A automatic bridge monitored by a HP microcomputer.
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Data collection was performed with a CAD-4 Enraf-Nonius X˚ radiaray diffractometer15,16 using the MoKa (l ¼ 0.71069 A) 17 tion at room temperature. An empirical psi-scan absorption correction was applied. The structure was solved by direct methods using SHELXS-97 program.18 Many following renements by Fourier differences were necessary to nd the positions of all atoms using SHELXL-97 program.18 All calculations were performed using the winGX-98 crystallographic soware package.19 The Mg1 and Na1 atoms occupying the same site were constrained using the EXYZ and EADP instructions of SHELXL97 (ref. 18) to have the same positional and displacement parameters. The Na (4) and K (1) atoms were sharing the same site M1, the free variable restraints (SUMP) was required to restrain the sum of their occupation factors and the EXYZ and EADP. A qualitative EDX (energy-dispersive X-ray spectroscopy) analysis detected the presence of Na, Mo, Mg, K and oxygen (Fig. 1). Crystal and X-ray analysis data of the title compound are summarized in Table 1. The atomic coordinates, equivalent isotropic displacement parameters and selected interatomic
Fig. 1
SEM spectrum of K0.13Na3.87Mg(MoO4)3.
RSC Adv., 2015, 5, 38918–38925 | 38919
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Paper
Table 1 Crystal data K0.13Na3.87Mg(MoO4)3
Empirical formula Formula weight (g mol1) Crystal system, space group Unit cell dimensions ˚) (A ˚ 3)/Z (A Calculated density (g cm3) Crystal size (mm) m(MoKa) (mm1) q range (deg) for data collection Miller index ranges Measured reections Independent reections No. of variables R[F2 > 2s(F2)] wR(F2) GooF ¼ S ˚ 3) Drmax/Drmin (e A
and
structure
refinement
details
for
K0.13Na3.87Mg(MoO4)3 598.18 Monoclinic, C2/c a ¼ 12.9325 (8), b ¼ 13.5537 (9), c ¼ 7.1627 (6), b ¼ 112.212 (9) 1162.33 (14)/4 3.418 0.16 0.14 0.12 3.490 2.3 # q # 27 16 # h # 16, 1 # k # 17, 9 # l # 9 2712 1261 (Rint ¼ 0.0272) 97 0.0241 0.0584 1.23 0.81/0.66
and the structure renement details were obtained from Crystal X-ray diffraction, the purpose of the Rietveld renement is only to show the purity of powder and to show that there are no other additional peaks due to the presence of impurity. The Rietveld renement was carried out using Topas Academic soware,26 allowed a Quantitative Phase Analysis (QPA) and led to a weight proportion of 100% of K0.13Na3.87Mg(MoO4)3. In addition, the background is linear: this is an indicator of the good crystallinity of the powder. The nal agreement factors are RB: 4.689%; Rexp: 0.9; Rexp: 8%; Rp: 4.64%; Rwp: 7.44%.
Table 3 Selected interatomic distances (˚ A) for K0.13Na3.87Mg(MoO4)3. Symmetry codes: (i) x 1/2, y + 1/2, z + 1/2; (ii) x + 1/2, y + 1/2, z + 1; (iii) x + 1/2, y 1/2, z + 3/2; (iv) x 1/2, y 1/2, z; (v) x 1/2, y + 1/2, z 1/2; (vi) x + 1/2, y + 3/2, z; (vii) x + 1/2, y + 3/2, z + 1; (viii) x + 1/2, y + 1/2, z + 1/2; (ix) x, y + 1, z 1/2; (x) x + 1/2, y 1/ 2, z + 1/2; (xi)x + 1, y + 1, z + 1; (xii) x, y + 1, z + 1/2; (xiii) x + 1, y, z + 1/2; (xiv) x + 1/2, y + 1/2, z + 3/2; (xv) x 1/2, y + 1/2, z; (xvi) x 1/2, y + 3/2, z + 1/2
Mo1O4
distances are listed in Tables 2 and 3. The structure graphics were drawn with diamond 2.1 supplied by Crystal Impact.20 The structural model is validated by the two structural tools, Bond Valence Sum BVS21,22 and Charge Distribution analysis CD.23–25 Both BVS and CD show adequate valences (V) and charges (Q) of all the cation sites. The structural model is thus validated, as shown by the dispersion factor of 3.1% which measures the deviation of the computed charges (Q) with respect to the formal oxidation numbers. The Bond Valence computation and Charge Distribution analysis are summarized in Table 4. Rietveld analysis was performed in the 10–70 range using the single crystal structure data (Fig. 2). In fact, the crystal data
Mo1–O2i Mo1–O2ii Mo1–O4iii Mo1–O4iv
Mo2O4 1.758 (3) 1.758 (3) 1.771 (3) 1.771 (3)
Mo2–O3 Mo2–O1v Mo2–O5vi Mo2–O6vii
1.745 (3) 1.759 (3) 1.765 (3) 1.781 (3)
Na2O6
Na3O6 Na3–O3 Na3–O3xi Na3–O2xi Na3–O2 Na3–O3xii Na3–O3xiii
Mg1O6
2.523 (3) 2.523 (3) 2.543 (3) 2.543 (4) 2.686 (4) 2.686 (4)
Na3–O3 Na3–O3xi Na3–O2xi Na3–O2 Na3–O3xii Na3–O3xiii
2.523 (3) 2.523 (3) 2.543 (3) 2.543 (4) 2.686 (4) 2.686 (4)
K1O8
Na2–O4iv Na2–O4xv Na2–O5xv Na2–O5iv Na2–O3iv Na2–O3x
2.393 (3) 2.393 (3) 2.447 (3) 2.447 (3) 2.468 (4) 2.468 (4)
K1–O1i K1–O1ii K1–O1xiv K1–O1xv K1–O4iv K1–O4iii K1–O5iviii K1–O5xvi
2.684 (4) 2.684 (4) 2.749 (4) 2.749 (4) 3.093 (4) 3.093 (4) 3.192 (4) 3.192 (4)
Table 2 Atomic coordinates and isotropic thermal factors of K0.13Na3.87Mg(MoO4)3. *Ueq is defined as one third of the trace of the orthogonalized Uij
Atom
x
y
z
˚ U*eq (A)
Mo1 Mo2 Mg1 Na1 Na2 Na3 Na4 K1 O1 O2 O3 O4 O5 O6
0.0000 0.23694 (3) 0.21624 (11) 0.21624 (11) 0 0.5 0 0 0.6702 (3) 0.3951 (3) 0.3789 (3) 0.4566 (3) 0.3261 (3) 0.2769 (3)
0.21604 (3) 0.60958 (2) 0.34005 (10) 0.34005 (10) 0.23648 (18) 0.5 0.4915 (2) 0.4915 (2) 0.0051 (3) 0.3654 (2) 0.5894 (2) 0.7802 (2) 0.8305 (2) 0.8203 (3)
0.7500 0.12685 (5) 0.12690 (19) 0.12690 (19) 0.25 0.5 0.75 0.75 0.6099 (5) 0.2540 (5) 0.1839 (5) 0.5302 (4) 0.1080 (4) 0.6735 (4)
0.01849 (13) 0.02006 (11) 0.0161 (3) 0.0161 (3) 0.0250 (5) 0.0412 (7) 0.0440 (11) 0.0440 (11) 0.0434 (9) 0.0387 (8) 0.0306 (7) 0.0265 (6) 0.0314 (7) 0.0326 (7)
38920 | RSC Adv., 2015, 5, 38918–38925
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