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Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4
Accepted Manuscript Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4 Najoua Ouerfelli, Youssef Ben Smida, Mohamed Faouzi Zid PII:
S0925-8388(15)30825-2
DOI:
10.1016/j.jallcom.2015.08.129
Reference:
JALCOM 35124
To appear in:
Journal of Alloys and Compounds
Received Date: 29 June 2015 Revised Date:
9 August 2015
Accepted Date: 17 August 2015
Please cite this article as: N. Ouerfelli, Y. Ben Smida, M.F. Zid, Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.08.129. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4 Najoua Ouerfelli, Youssef Ben Smida* and Mohamed Faouzi Zid
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Laboratoire de Matériaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia.
* Corresponding author: phone +216 96 01 94 45
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E-mail address: [email protected] (Y. Ben Smida)
Abstract
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A new iron arsenate Na2.77K1.52Fe2.57(AsO4)4 has been synthesized by a solid state reaction and characterized by single crystal X-ray diffraction. The title compound crystallize in orthorhombic system, space group Cmce, with a = 10.854(4) Å, b = 20.985(8) Å, c = 6.536(2) Å, V = 1488.7(9) Å3 and Z = 4. The crystal structure consists of FeO6 octahedra and AsO4 tetrahedra sharing corners and edges to form a two-dimensional framework. This arrangement leads to infinite anionic layers perpendicular to the b-axis. The K+ cations reside in the interlayer space, whereas the
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Na+ cations are located in the cavities of the anionic framework. The structural model was validated by the bond valence sum (BVS) and charge distribution (CHARDI) methods. The material was characterized by scanning electron microscopy and infrared spectroscopy. The electrical properties
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were measured using impedance spectroscopy. The conductivity value is σ = 7.22 10−6 S·cm−1 at 298 °C and the activation energy is 0.50 eV. The BVS model is extended to simulate the ionic
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migration pathways of alkali cations in the anionic framework.
Keywords: X-ray diffraction; Crystal structure; Ionic conductivity; Bond valence analysis; pathways transport simulation.
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ACCEPTED MANUSCRIPT 1. Introduction Since the discovery of potentially interesting ionic conductivity for NaSICON family [1] and highly interesting electrochemical properties as positive electrode materials for secondary lithium batteries for phosphates of the general formula LiMPO4 (M=Mn, Fe, Co, Ni) [2-5], the search for novel polyanion-based insertion hosts is intense. The advances in technology and the
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need for new materials in device applications have triggered the rapid growth of solid state chemistry. As a result, exploring new materials with novel properties or modifying known structures must start from understanding the basics, such as structure and bonding, which determine the properties. From the understanding of these properties, applications can be enhanced. The
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variety and complexity of these structures and the evaluation of the factors that control and influence the crystal structures and physical properties are the elements that made solid state
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chemistry a dynamic area of research.
The aim of this work is to elucidate the crystal structure of the new arsenate Na2.77K1.52Fe2.57(AsO4)4. The structural model was confirmed by the means of bond valence sum (BVS) [6-7] and charge distribution (CHARDI) [8-9] methods. To revise the correlation between the structure and the alkali migration in the anionic framework, the electrical properties were
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determined using complex impedance spectroscopy and the alkali migration pathways were simulated using the BVS model [10-11]. The new compound was characterized by the FTIR and Semiqualitative energy-dispersive X-ray spectroscopy (EDX) spectroscopy.
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2. Experimental section
Single crystals of Na2.77K1.52Fe2.57(AsO4)4 were obtained by solid state reaction from a
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mixture of Na2CO3, K2CO3, Fe(NO3)3.9H2O, and NH4H2AsO4 with a Na:K:Fe:As molar ratio of 1:1:1:5. After a fine grinding, the resulting mixture has been poured into platinum crucible and heated to 400 °C for 24h. The resulting powder was ground and heated again at 850 °C for 72h and finally slowly cooled at 20 K h-1 to room temperature. The green single crystals obtained were separated from the excess flux by washing the product in boiling water. EDX analysis of the several like crystals was performed on a JEOL-JSM 5400 scanning electron microscope. It revealed the presence of only Na, K, Fe, As and O elements (Fig. 1). Micrographs of these crystals are shown in the insert of this figure. The formula of the title compound, Na2.77K1.52Fe2.57(AsO4)4, has been established as a result of crystal structure.
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ACCEPTED MANUSCRIPT The Fourier Transform Infrared (FTIR) measurements were performed at room temperature, on a Perkin-Elmer FT-IR Pragon 1000 PC spectrometer over the 1400-400 cm-1 region, in a KBr pellet. Suitable single crystal was chosen under polarizing microscope for the structure determination and refinement. It was analyzed at room temperature using an Enraf Nonius CAD-4
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diffractometer equipped with graphite monochromated Mo Kα radiation (λ=0.71069 Å) [12-13]. The unit cell parameters were determined upon 25 reflections in the 2°≤θ≤10° range and they were refined using a least-squares method in the 10°≤θ≤15° range. The reflections intensities were collected upon θ=27° with two standard reflections for intensities and orientation control. Intensity
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data were corrected for Lorentz and polarization effects and secondary extinction [14]. The absorption correction was obtained via psi-scan [15]. The details of the data collection are given
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in Table 1.
The polycrystalline powder is obtained by grinding the selected single crystals after separating from the excess flux. XRD pattern was obtained using a D8 Bruker diffractometer equipped with a Cu anticathode (CuKα radiation λ= 1.54056 Å) at room temperature. The measurement was performed under Bragg-Brentano geometry at 2θ with step 0.02° in the 5–70° range. The Rietveld refinement was performed using the single crystal structure. The final
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agreement factors are Rp=0.039, Rwp=0.045 and R(F2)=0.99 (Fig. 2). Electrical measurements were carried out in air by complex impedance spectroscopy. The crystal samples of Na2.77K1.52Fe2.57(AsO4)4 were crushed and pressed into disks at 10 t/cm2 with 13
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mm in diameter and 1.16 mm in thickness. Then the pellet was sintered at 600 °C for 24h. A layer of silver paint was deposited on the edges to ensure good electrical contact between the sample and
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the platinum electrical junctions. The AC impedance measurements were made using HP 4192A impedance analyzer in the 5Hz-13 MHz range and between 298°C and 509°C. 3. Results and discussion
3.1. Structure refinement and validation The crystal structure was solved by direct methods using SHELXS-97 program [14]. It was found that new compound crystallized in the orthorhombic system, space group Cmce. In the closest solution proposed by the program, only iron and arsenic atoms were located. The use of SHELXL-97 [14] program refinements based on F2, included in the WinGX software package [16], followed by Fourier differences were necessary to find the positions of the others atoms. The 3
ACCEPTED MANUSCRIPT presence of both alkalis cations (Na+ and K+) in crystal structure is confirmed by the semiquantitative energy-dispersive spectroscopy (EDS) analysis (Fig. 1). In the first step, the constraint SUMP instruction in SHELX-97 was applied to the Na+/K+ cation occupation rates to achieve electro-neutrality. When psi-scan intensities correction [15] was applied and all the atomic positions were refined anisotropically, the agreement reliability factors R(F) and wR(F2) converged to 0.051
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and 0.151, respectively. The alert checkCIF indicate that SHELXL Second Parameter in WGHT is large (47.533) and a high value of the difference density picks (2.622/-2.404 eÅ-3). In the second step, we refined without constraint and the Fe3+ cations are distributed over two partially occupied special crystallographic sites. The parameter WGHT decreases to 8.017, All ADPs are positive with normal standard deviations and the residual peaks are 2.30/-1.43 eÅ3 and the agreement R(F) and
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wR(F2) converged to 0.043 and 0.113, respectively. The crystallographic data and structural determination is listed in Table 1. The atomic coordinates, fractional occupancies and isotropic
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thermal factors atomic are reported in Tables 2. The main interatomic distances in coordination polyhedra are given in Table 3. The structure graphics were drawn with diamond 2.1 supplied by Crystal Impact [17]. Further details of the crystal structure investigation can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2
1EZ,
UK;
fax:
+44(0)1223-762911;
email:
[email protected]
(or
at
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www.ccdc.cam.ac.uk/conts/retrieving.html) on quoting the deposit number CCDC 1041791. The structural model is validated by both Charge Distribution analysis (CHARDI) [8-9] and Bond Valence Sum (BVS) methods [6-7] (Table 4). These computations have been obtained with the CHARDI-IT [18] and SoftBV [19] programs, respectively. The CHARDI method confirms the
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structure with a dispersion factor on the cations charges of σ=0.10 and the BVS analysis shows that the calculated valences V(i) are in agreement with the oxidation numbers with global instability
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index [20] GII = 0.16 v.u ( Both GII and σ measure the deviation of the computed valences and charges respectively from the formal oxidation number). 3.2. Structure description and discussion The new compound Na2.77K1.52Fe2.57(AsO4)4 is isostructural to K3Fe3(AsO4)4 phase [21]. It is member of family AI3MIII3(AsO4)4 (A= alkali: M= trivalent metal). The asymmetric unit is formed by cycle unit containing two octahedra FeO6 and one tetrahedron As(1)O4 sharing corners (Fig. 3). A second tetrahedron As(2)O4 is grafted on octahedron Fe(2)O6 by pooling of one edge.
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of the anionic framework faces of the hexagonal sections windows (Fig. 5).
The two crystallographically distinct iron atoms exhibit a slightly distorted octahedral coordination with effective coordination numbers ECoN(Fe1)=5.790 and ECoN(Fe2)=5.563, weighted average distances=1.974Å and =1.979 Å (Table 4) and the distortion
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indices (DI) for the coordination polyhedra around Fe varying from 3 to 5% (Table 5). The As1O4 tetrahedron is more regular than As2O4 tetrahedron. Thus it is explained by distortion parameters, ECoN(As1)=3.981, ECoN(As2)=3.935, DI(As1)=1~2% and DI(As2)=2~5%. The DIo(=5%) values octahedron and As2O4 tetrahedron.
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for both Fe2 and As2 are slightly higher. They are due to shared edge between the Fe2O6
The K1 coordination sphere is formed by seven oxygen atoms and the K2 by six oxygen atoms. The K2 polyhedron (ECoN =5.882) is slightly distorted than K1 (ECoN =7.080). But the
distorted (Table 4). 3.3. Infrared spectroscopy
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effective coordination value numbers of sodium coordination polyhedra indicate that they are highly
Figure 6 shows IR spectra of Na2.77K1.52Fe2.57(AsO4)4. According to a vibrational study done
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by Nakamoto [22], and other similar compounds [21][23], the asymmetrical stretching
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mode, νas(As-O), appears at frequencies 870 cm−1, the symmetrical stretch, νs(As-O), is detected at a frequency of 796 cm−1 . The symmetrical stretch, νs(Fe-O-Fe), is shown at a frequency of 718 cm−1 . The bands centered at 570, 447 and 362 cm−1 are due to the (δas+δs) [(AsO4) + Fe-O-Fe)]. 3.4. Ionic conductivity
The electrical conductivity was investigated using complex impedance spectroscopy. The electrical parameters were obtained in air in the 298–509 °C temperature range after stabilization at each temperature. Figure 7 illustrates the variation recorded for the imaginary part of impedance (Z’’) with the real part (Z’) at several temperatures. A conventional electrical circuit R//CPE was used, where CPE is a constant phase element: 5
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1 (1) A( jω ) p
The parameters obtained for Na2.77K1.52Fe2.57(AsO4)4 sample at different temperatures are
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given in Table 6.
The electrical conductivity increase from 0.722 10-5 S.cm-1 at 298°C to 8.323 10-5 S.cm-1 at 509 °C. The activation energy was obtained by linear fitting of the ionic conductivity values at
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different temperatures by applying the Arrhenius equation: σT = σ0exp(−Ea/kbT) (2)
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Where σ is the temperature dependent ionic conductivity, σ0 is the ionic conductivity at absolute zero temperature, Ea is the activation energy of ion migration, and kb and T have their usual meanings. The activation energy deduced from the slope is 0.50 eV (Fig. 8).
3.5. Alkali pathways transport simulation
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In order to study the relationship between the structure and alkali motion, the transport pathways in the crystal bulk of the title compound have been simulated by the mean of the bond valence sum model [6]. This model was used with success to determine the pathways migration of Li+ in Li4GeS4 and Li0.16La0.62TiO3 [24], Na+ in NaCo2As3O10 [10], K+ in KCoP3O9 [11], etc.
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Accessible sites for mobile ions A in a local structure model are identified using empirical
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relationships between the bond length R and bond valence
[24-26]:
The mismatch of the bond valence sum |∆V(A)| is obtained as: = exp[(R0−R)/b]
(3)
The mismatch of the bond valence sum |∆V(A)| is obtained as: |∆V(A)|= |∑ Where
|
∑
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, named penalty function, discriminates against sites that achieve a matching V(A) by
strongly asymmetric coordinations:
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0
" (5)
!
Bond valence sum map (BVSM) model visualizes the transport pathways of the mobile ions in solid electrolytes, provided that the ion migration from one equilibrium site to the next one
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follows pathways along which the valence mismatch |∆V| remains ideally as small as possible. The bond valence sum calculation was carried out using 3DBVSMAPPER computer program [27]. The results of simulation are shown in figures 9 and 10.
The crystal structure study shows two cations: K+ in inlayer space (Fig. 11) and Na+ in
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interconnected cavities along b-axis (Fig. 12). Figure 9 shows the isosurface with bond valence mismatch |∆V(Na)|=0.2 v.u which link Na+ atoms. Sodium atoms form a two-dimensional
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conduction pathway in the ab plane. During its migration along the b-axis, sodium atoms pass through windows with minimum size equal to 4.6 Å (Fig. 11) which is close to the sum of Na+ and O2- diameters (4.82 Å) and thus the interconnected cavities may be expected to allow Na+ transport. This observation may explain the low value of bond valence mismatch (0.2 v.u): this is the minimum valence mismatch value necessary to form a continuous isosurface which links all Na+ atoms in the structure. Figure 10 shows the isosurface with bond valence mismatch |∆V(K)|=0.1
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v.u. This figure shows one-dimensional migration pathways for potassium atoms along c-axis with zigzag form (Fig. 10). The bond valence value is very low (0.1 v.u) and this may be explained by the minimum dimension of the section which is equal to 6.4 Å (Fig. 12) .This value is slightly
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higher than the sum of K+ and O2- diameters (5.6 Å). In similar cases, where there are two types of alkali in the structure, it is possible that one of
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the two cation blocks the migration of the other [28], in other cases, an alkali maintains the structure and the other migrates [28]. In the case of Na2.77K1.52Fe2.57(AsO4)4 structure, the two cations migrate through two different spaces and the simulation shows no interference between the two cations. The low values of the bond valence mismatch of Na+ or K+ are in good agreement with the value of the activation energy. 4. Conclusion This work deals with the synthesis of a new arsenate iron Na2.77K1.52Fe2.57(AsO4)4 by solid state reaction. This material was characterized by X-ray diffraction. The structure of the title compound has an open two-dimensional framework. The K+ cations reside in the interlayer space, 7
ACCEPTED MANUSCRIPT whereas the smaller Na+ cations are located in the cavities of the anionic framework. The electrical properties of Na2.77K1.52Fe2.57(AsO4)4 sample were studied using complex impedance spectroscopy. The electrical conductivity is 0.722 10-5 S·cm-1 at 298°C and 8.323 10-5 S·cm-1 at 509 °C. The activation energy deduced from the slope is 0.5 eV. The BVS analysis is extended to simulate the ionic conduction properties of alkali cations in the anionic framework. Pathways migration of
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potassium in Na2.77K1.52Fe2.57(AsO4)4 are one-dimensional along c-axis. However, pathways migration of sodium is two-dimensional in the ab plane. References
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[1] H. Hong, Mater. Res. Bull. 11 (1976) 173–182.
[2] O. Shigeto, S. Shoichiro, E. Minato, Y. Jun-ichi, J. Power Sources 97–98 (2001) 430–432.
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[3] K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid-State Lett. 3 (2000) 178–179. [4] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188– 1194.
[5] G. Li, H. Azuma, M. Tohda, Electrochem. Solid-State Lett. 5 (2002) 135–137. [6] I.D. Brown, The Chemical Bond in Inorganic Chemistry – The Bond Valence Model. IUCr Monographs on Crystallography, Vol. 12, Oxford University Press, 2002.
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[7] S. Adams, Acta Crystallogr. B57 (2001) 278-287.
[8] M. Nespolo, G. Ferraris, G. Ivaldi, R. Hoppe, Acta Crystallogr. B57 (2001) 652–664. [9] A. Guesmi, M. Nespolo, A. Driss, J. Solid State Chem.179 (2006) 2466–2471.
(2015) 132–139.
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[10] Y. Ben Smida, R. Marzouki, A. Guesmi, S. Georges, M.F. Zid, J. Solid State Chem. 221
[11] Y. Ben Smida, A. Guesmi, S. Georges, M.F. Zid, J. Solid State Chem. 221 (2015) 278–284.
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[12] Enraf-Nonius, CAD-4 EXPRESS, Enraf–Nonius, Delft, The Netherlands, 1995. [13] A.J.M. Duisenberg, J. Appl. Crystallogr. 25 (1992) 92–96. [14] G.M. Sheldrick, Acta Crystallogr. A 64 (2008) 112–122. [15] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr. A 24 (1968) 351–359. [16] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837–838. [17] K. Brandenburg, M. Berndt, Diamond Version 2.1. Crystal Impact Bonn 2001. [18] M. Nespolo, CHARDT-IT A Program to Compute Charge Distributions and Bond Valences in Non-molecular Crystalline Structures, LCM3B, University Henri Poincaré Nancy I, France, 2001. [19] Softbv web page by Pr.Stefan Adams:/
ACCEPTED MANUSCRIPT [21] N. Ouerfelli, M.F. Zid, T. Jouini, Acta Crystallogr. E 61 (2005) i67. [22] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1977. [23] B. Bouzemi Friaa, H. Boughzala, T. Jouini, J. Solid State Chem. 173 (2003) 273–279. [24] S. Adams,J. Power Sources.159 (2006) 200–204.
[26] S. Adams, Solid State Ion.136–137 (2000) 1351–1361. [27] M. Sale,M. Avdeev, J. Appl. Crystallogr. 45 (2012) 1054–1056.
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[25] S. Adams,Solid State Ion. 177 (2006) 1625–1630.
[28] C. Déportes, M. Duclot, P. Fabray, J. Fouletier, A. Hammou, M. Kleitz, E. Siebert, J.L.
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Souquet. Electrochimie des Solides (1994). Collection Grenoble Sciences, France.
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conditions
and
structure
refinement
results
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Na2.77K1.52Fe2.57(AsO4)4 Orthorhombic ; Cmce a = 10.854(4) Å ; b = 20.985 (8) Å ; c = 6.536 (2) Å ; 1488.7 (9) Å3 ; 4 822.41 g·mol-1; 3.669 g·cm−3 11.90 mm−1 Prism, green 0.08 × 0.02 × 0.02 mm3 Enraf-Nonius CAD-4 λMo Kα=0.71069 Å ; 298 K
3.7° ≤ θ ≤ 27° −13≤ h ≤ 13; 0≤ k ≤ 26; 0≤ l ≤ 8 ω/2θ Psi-scan ; 0.752 ; 0.807 2 ; 120 ; 1 1619 857 [Rint.= 0.037] 779
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Crystal data Empirical formula Crystal system ; space group Unit cell dimensions Volume; Z Formula weight; ρcalc. Absorption coefficient (µ) Crystal sharp ; Color Crystal size Data collection Diffractometer Wave length ; Temperature Theta range for data collection Limiting indices Scan mode Absorption correction ; Tmin ; Tmax Standards; frequency (min); decay (%) Reflections collected Independent reflections Observed reflections [I > 2σ(I)] Refinement Refinement method Final R indices [I > 2σ(I)] Reflections ; parameters ∆ρmax ; ∆ρmin (e.Å−3) Goodness of fit (S)
recording
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data,
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Summary of crystallographic Na2.77K1.52Fe2.57(AsO4)4.
Full-matrix least-squares on F2 R= 0.043; wR= 0.113 857 ; 89 2.30 ; −1.43 1.24
for
ACCEPTED MANUSCRIPT Table 2 Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) in Na2.77K1.52Fe2.57(AsO4)4.
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Atom Wyck. x y z Ueqa (Å2) Occ. (<1) 8f 0 0.15537 (4) 0.06542 (12) 0.0131 (3) As1 8e 1/4 −0.04501 (4) 1/4 0.0105 (3) As2 4a 0 0 0 0.0080 (4) 0.79 Fe1 8e 1/4 0.09220 (6) 1/4 0.0097 (3) 0.89 Fe2 8f 0 0.2176 (3) −0.0895 (11) 0.0294 (15) O1 8f 0 0.0878 (3) −0.0755 (8) 0.0158 (11) O2 16g 0.1237 (4) 0.15448 (19) 0.2246 (6) 0.0186 (9) O3 16g 0.2119 (4) −0.09030 (17) 0.4482 (5) 0.0152 (8) O4 16g 0.1378 (3) 0.01189 (17) 0.2058 (5) 0.0120 (7) O5 8f 0 0.1633 (4) −0.4357 (10) 0.0419 (15) 0.733 (7) Na1 16g −0.0774 (18) 0.1801 (9) −0.423 (2) 0.0419 (15) 0.206 (8) Na2 8f 0 0.059 (2) 0.508 (6) 0.0419 (15) 0.115 (11) Na3 8f 0 0.129 (2) −0.443 (6) 0.0419 (15) 0.127 (14) Na4 16g 0.2266 (10) 0.2719 (4) 0.132 (3) 0.077 (5) 0.311 (13) K1 8e 1/4 0.2731 (19) 0.2500 0.077 (5) 0.14 (2) K2 a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
ACCEPTED MANUSCRIPT Table3 Main interatomic distances (Å) in the coordination polyhedra for Na2.77K1.52Fe2.57(AsO4)4. As2 tetrahedron As2—O4iv As2—O4 As2—O5iv As2—O5 Fe2 octahedron Fe2—O3iv Fe2—O3 Fe2—O4xiii Fe2—O4vi Fe2—O5 Fe2—O5iv
1.901 (4) 1.901 (4) 2.016 (4) 2.016 (4) 2.100 (4) 2.100 (4)
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1.907 (6) 1.907 (6) 2.027 (4) 2.027 (4) 2.027 (4) 2.027 (4) 2.53 (1) 2.601 (7) 2.601 (7) 2.695 (12) 2.764 (6) 2.764 (6) 2.837 (9) 2.411 (13) 2.411 (13) 2.66 (3) 2.66 (3) 2.79 (4) 2.82 (4) 2.82 (4)
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1.659 (4) 1.659 (4) 1.730 (4) 1.730 (4)
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1.653 (7) 1.691 (6) 1.699 (4) 1.699 (4)
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As1 tetrahedron As1—O1 As1—O2 As1—O3i As1—O3 Fe1 octahedron Fe1—O2 Fe1—O2vii Fe1—O5vii Fe1—O5i Fe1—O5 Fe1—O5xi Na1 polyhedron Na1—O1 Na1—O3xxi Na1—O3xii Na1—O1xviii Na1—O4vii Na1—O4xi Na1—O2 Na3 polyhedron Na3—O4v Na3—O4xiii Na3—O5i Na3—O5 Na3—O2ii Na3—O5v Na3—O5xiii K1 polyhedron K1—O3 K1—O4xxiv K1—O1xix K1—O1xvi K1—O1 K1—O3iv K1—O3xix
2.77 (1) 2.94 (1) 2.989 (10) 3.068 (11) 3.07 (2) 3.098 (14)
Na2 polyhedron Na2—O4vii Na2—O3xxi Na2—O1 Na2—O1xviii
2.39 (2) 2.42 (2) 2.47 (2) 2.55 (2)
Na4 polyhedron Na4—O4vii Na4—O4xi Na4—O2 Na4—O3xxi Na4—O3xii Na4—O1
2.439 (18) 2.439 (18) 2.55 (4) 2.61 (3) 2.61 (3) 2.97 (4)
K2 polyhedron K2—O3 K2—O3iv K2—O1xvi K2—O1xix K2—O4xxvi K2—O4xxiv
2.85 (4) 2.85 (4) 2.916 (4) 2.916 (4) 3.17 (4) 3.17 (4)
3.23 (1)
Symmetry codes: (i) −x, y, z; (ii) x, y, z+1; (iv) −x+1/2, y, −z+1/2; (v) −x, −y, −z+1; (vi) −x+1/2, −y, z−1/2; (vii) x, y−1/2, −z+1/2; (xi) x, −y, −z; (xii) x, y, z−1; (xiii) x, −y, −z+1; (xvi) x, −y+1/2, z+1/2; (xviii) −x+1/2, −y+1/2, −z; (xix) x−1/2, −y+1/2, −z; (xxi) −x, y, z−1; (xxiv) x, y+1/2, −z+1/2; (xxvi) −x+1/2, y+1/2, z.
ACCEPTED MANUSCRIPT Table 4 CHARDI and BVS analysis of cation polyhedra in Na2.77K1.52Fe2.57(AsO4)4. Q(i) 0.701 0.195 0.119 0.123 0.292 0.132 2.639 2.622 4.872 5.139
V(i).sof(i) 0.746 0.161 0.107 0.107 0.225 0.098 2.533 2.743 4.982 4.987
CN(i) 7 4 7 6 7 6 6 6 4 4
ECoN(i) 6.542 4.028 6.017 5.168 7.080 5.882 5.790 5.563 3.981 3.935
dar(i) 2.685 2.458 2.653 2.603 2.967 2.979 1.987 2.006 1.686 1.695
dmed(i) 2.653 2.451 2.562 2.525 2.985 2.946 1.974 1.979 1.684 1.690
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q(i).sof(i) 0.733 0.206 0.115 0.127 0.311 0.140 2.370 2.670 5.000 5.000
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Cation Na1 Na2 Na3 Na4 K1 K2 Fe1 Fe2 As1 As2
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Notes: q(i) = formal oxidation number; sof(i) = site occupation factor; dar(i) = arithmetic average distance; dmed(i) = weighted average distance; CNs= coordination number; ECoN(i)= effective coordination number; σ = dispersion factor on cationic charges measuring the deviation of the computed charges (Q) with respect to the formal oxidation numbers; σ = [Σi(qi-Qi)2/N-1]1/2=0.10.
ACCEPTED MANUSCRIPT Table 5 Distortion indices (DI) for the coordination polyhedra around Fe and As in Na2.77K1.52Fe2.57(AsO4)4. Fe1
Fe2
As1
As2
DId
0.027
0.035
0.010
0.021
DIa
0.033
0.038
0.011
0.035
0.041 0.048 0.019 0.050 ∑ |d d | ⁄n d ; ∑ |a a | ⁄n a and ∑ |o o | ⁄n o . d, a and o signify Fe/As-O bond distance, O-Fe/As-O angle and O-O edge within the relevant polyhedron; index i indicates individual values, index m the mean value for the polyhedron. n1and n2 are 4 and 6 for the arsenate tetrahedral; 6 and 12 for the iron octahedral.
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R (Ω)
σ (10-5 S·cm-1)
1000/T (103 K-1)
Ln(σT)
298 326 346 366 401 442 465 509
571 599 619 639 674 715 738 782
12100 7940 6242 5200 3363 2126 1650 1050
0.722 1.101 1.400 1.681 2.598 4.110 5.297 8.323
1.751 1.669 1.616 1.565 1.484 1.399 1.355 1.279
-5.491 -5.022 -4.748 -4.534 -4.045 -3.527 -3.242 -2.732
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T (°C)
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Fig. 1. EDS spectrum and SEM micrograph (inset of the figure) of Na2.77K1.52Fe2.57(AsO4)4. Fig.2. Rietveld reffinement patterns of Na2.77K1.52Fe2.57(AsO4)4 sample. Fig. 3. The asymmetric unit with atom-labelling scheme. Displacement ellipsoids are drawn at the
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50% probability level. [Symmetry codes: (i) -x, y, z; (iv) ½-x, 1+y, ½-z; (vii) -x, -y, -z; (xi) x, -y, -z; (xiii) x, -y, 1-z]
Fig. 4. Projection of the structure of Na2.77K1.52Fe2.57(AsO4)4 along the c axis.
windows.
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Fig. 6. The infrared spectrum of the Na2.77K1.52Fe2.57(AsO4)4.
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Fig. 5. Layer of the Na2.77K1.52Fe2.57(AsO4)4 structure viewed along [010] showing the hexagonal
Fig.7. Impedance spectrums of Na2.77K1.52Fe2.57(AsO4)4 temperatures rang.
recorded in air in the 298°C–509°C
Fig.8. Arrhenius plot of conductivity of Na2.77K1.52Fe2.57(AsO4)4 sample.
|∆V(Na)|=0.2 v.u.
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Fig.9. 2D pathways link Na+ atoms perpendicular to c-axis with bond valence mismatch
Fig.10. 1D pathways link K+ along c-axis with bond valence mismatch |∆V(K)|=0.1 v.u.
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Fig.11. Dimension of the hexagonal windows sections.
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Fig.12. Dimension of interlayer space along c-axis.
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A new single crystal Na2.77K1.52Fe2.57(AsO4)4 was grown by solid state reaction.
•
The structure was determined by single-crystal X-ray diffraction.
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The electrical properties were determined complex impedance spectroscopy.
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The conduction pathways for the Na+ and K+ cations are simulated using the BVSM model.
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S0925-8388(15)30825-2
DOI:
10.1016/j.jallcom.2015.08.129
Reference:
JALCOM 35124
To appear in:
Journal of Alloys and Compounds
Received Date: 29 June 2015 Revised Date:
9 August 2015
Accepted Date: 17 August 2015
Please cite this article as: N. Ouerfelli, Y. Ben Smida, M.F. Zid, Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.08.129. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, crystal structure and electrical properties of a new iron arsenate Na2.77K1.52Fe2.57(AsO4)4 Najoua Ouerfelli, Youssef Ben Smida* and Mohamed Faouzi Zid
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Laboratoire de Matériaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Tunis, Tunisia.
* Corresponding author: phone +216 96 01 94 45
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E-mail address: [email protected] (Y. Ben Smida)
Abstract
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A new iron arsenate Na2.77K1.52Fe2.57(AsO4)4 has been synthesized by a solid state reaction and characterized by single crystal X-ray diffraction. The title compound crystallize in orthorhombic system, space group Cmce, with a = 10.854(4) Å, b = 20.985(8) Å, c = 6.536(2) Å, V = 1488.7(9) Å3 and Z = 4. The crystal structure consists of FeO6 octahedra and AsO4 tetrahedra sharing corners and edges to form a two-dimensional framework. This arrangement leads to infinite anionic layers perpendicular to the b-axis. The K+ cations reside in the interlayer space, whereas the
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Na+ cations are located in the cavities of the anionic framework. The structural model was validated by the bond valence sum (BVS) and charge distribution (CHARDI) methods. The material was characterized by scanning electron microscopy and infrared spectroscopy. The electrical properties
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were measured using impedance spectroscopy. The conductivity value is σ = 7.22 10−6 S·cm−1 at 298 °C and the activation energy is 0.50 eV. The BVS model is extended to simulate the ionic
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migration pathways of alkali cations in the anionic framework.
Keywords: X-ray diffraction; Crystal structure; Ionic conductivity; Bond valence analysis; pathways transport simulation.
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ACCEPTED MANUSCRIPT 1. Introduction Since the discovery of potentially interesting ionic conductivity for NaSICON family [1] and highly interesting electrochemical properties as positive electrode materials for secondary lithium batteries for phosphates of the general formula LiMPO4 (M=Mn, Fe, Co, Ni) [2-5], the search for novel polyanion-based insertion hosts is intense. The advances in technology and the
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need for new materials in device applications have triggered the rapid growth of solid state chemistry. As a result, exploring new materials with novel properties or modifying known structures must start from understanding the basics, such as structure and bonding, which determine the properties. From the understanding of these properties, applications can be enhanced. The
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variety and complexity of these structures and the evaluation of the factors that control and influence the crystal structures and physical properties are the elements that made solid state
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chemistry a dynamic area of research.
The aim of this work is to elucidate the crystal structure of the new arsenate Na2.77K1.52Fe2.57(AsO4)4. The structural model was confirmed by the means of bond valence sum (BVS) [6-7] and charge distribution (CHARDI) [8-9] methods. To revise the correlation between the structure and the alkali migration in the anionic framework, the electrical properties were
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determined using complex impedance spectroscopy and the alkali migration pathways were simulated using the BVS model [10-11]. The new compound was characterized by the FTIR and Semiqualitative energy-dispersive X-ray spectroscopy (EDX) spectroscopy.
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2. Experimental section
Single crystals of Na2.77K1.52Fe2.57(AsO4)4 were obtained by solid state reaction from a
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mixture of Na2CO3, K2CO3, Fe(NO3)3.9H2O, and NH4H2AsO4 with a Na:K:Fe:As molar ratio of 1:1:1:5. After a fine grinding, the resulting mixture has been poured into platinum crucible and heated to 400 °C for 24h. The resulting powder was ground and heated again at 850 °C for 72h and finally slowly cooled at 20 K h-1 to room temperature. The green single crystals obtained were separated from the excess flux by washing the product in boiling water. EDX analysis of the several like crystals was performed on a JEOL-JSM 5400 scanning electron microscope. It revealed the presence of only Na, K, Fe, As and O elements (Fig. 1). Micrographs of these crystals are shown in the insert of this figure. The formula of the title compound, Na2.77K1.52Fe2.57(AsO4)4, has been established as a result of crystal structure.
2
ACCEPTED MANUSCRIPT The Fourier Transform Infrared (FTIR) measurements were performed at room temperature, on a Perkin-Elmer FT-IR Pragon 1000 PC spectrometer over the 1400-400 cm-1 region, in a KBr pellet. Suitable single crystal was chosen under polarizing microscope for the structure determination and refinement. It was analyzed at room temperature using an Enraf Nonius CAD-4
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diffractometer equipped with graphite monochromated Mo Kα radiation (λ=0.71069 Å) [12-13]. The unit cell parameters were determined upon 25 reflections in the 2°≤θ≤10° range and they were refined using a least-squares method in the 10°≤θ≤15° range. The reflections intensities were collected upon θ=27° with two standard reflections for intensities and orientation control. Intensity
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data were corrected for Lorentz and polarization effects and secondary extinction [14]. The absorption correction was obtained via psi-scan [15]. The details of the data collection are given
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in Table 1.
The polycrystalline powder is obtained by grinding the selected single crystals after separating from the excess flux. XRD pattern was obtained using a D8 Bruker diffractometer equipped with a Cu anticathode (CuKα radiation λ= 1.54056 Å) at room temperature. The measurement was performed under Bragg-Brentano geometry at 2θ with step 0.02° in the 5–70° range. The Rietveld refinement was performed using the single crystal structure. The final
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agreement factors are Rp=0.039, Rwp=0.045 and R(F2)=0.99 (Fig. 2). Electrical measurements were carried out in air by complex impedance spectroscopy. The crystal samples of Na2.77K1.52Fe2.57(AsO4)4 were crushed and pressed into disks at 10 t/cm2 with 13
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mm in diameter and 1.16 mm in thickness. Then the pellet was sintered at 600 °C for 24h. A layer of silver paint was deposited on the edges to ensure good electrical contact between the sample and
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the platinum electrical junctions. The AC impedance measurements were made using HP 4192A impedance analyzer in the 5Hz-13 MHz range and between 298°C and 509°C. 3. Results and discussion
3.1. Structure refinement and validation The crystal structure was solved by direct methods using SHELXS-97 program [14]. It was found that new compound crystallized in the orthorhombic system, space group Cmce. In the closest solution proposed by the program, only iron and arsenic atoms were located. The use of SHELXL-97 [14] program refinements based on F2, included in the WinGX software package [16], followed by Fourier differences were necessary to find the positions of the others atoms. The 3
ACCEPTED MANUSCRIPT presence of both alkalis cations (Na+ and K+) in crystal structure is confirmed by the semiquantitative energy-dispersive spectroscopy (EDS) analysis (Fig. 1). In the first step, the constraint SUMP instruction in SHELX-97 was applied to the Na+/K+ cation occupation rates to achieve electro-neutrality. When psi-scan intensities correction [15] was applied and all the atomic positions were refined anisotropically, the agreement reliability factors R(F) and wR(F2) converged to 0.051
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and 0.151, respectively. The alert checkCIF indicate that SHELXL Second Parameter in WGHT is large (47.533) and a high value of the difference density picks (2.622/-2.404 eÅ-3). In the second step, we refined without constraint and the Fe3+ cations are distributed over two partially occupied special crystallographic sites. The parameter WGHT decreases to 8.017, All ADPs are positive with normal standard deviations and the residual peaks are 2.30/-1.43 eÅ3 and the agreement R(F) and
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wR(F2) converged to 0.043 and 0.113, respectively. The crystallographic data and structural determination is listed in Table 1. The atomic coordinates, fractional occupancies and isotropic
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thermal factors atomic are reported in Tables 2. The main interatomic distances in coordination polyhedra are given in Table 3. The structure graphics were drawn with diamond 2.1 supplied by Crystal Impact [17]. Further details of the crystal structure investigation can be obtained free of charge from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2
1EZ,
UK;
fax:
+44(0)1223-762911;
email:
[email protected]
(or
at
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www.ccdc.cam.ac.uk/conts/retrieving.html) on quoting the deposit number CCDC 1041791. The structural model is validated by both Charge Distribution analysis (CHARDI) [8-9] and Bond Valence Sum (BVS) methods [6-7] (Table 4). These computations have been obtained with the CHARDI-IT [18] and SoftBV [19] programs, respectively. The CHARDI method confirms the
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structure with a dispersion factor on the cations charges of σ=0.10 and the BVS analysis shows that the calculated valences V(i) are in agreement with the oxidation numbers with global instability
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index [20] GII = 0.16 v.u ( Both GII and σ measure the deviation of the computed valences and charges respectively from the formal oxidation number). 3.2. Structure description and discussion The new compound Na2.77K1.52Fe2.57(AsO4)4 is isostructural to K3Fe3(AsO4)4 phase [21]. It is member of family AI3MIII3(AsO4)4 (A= alkali: M= trivalent metal). The asymmetric unit is formed by cycle unit containing two octahedra FeO6 and one tetrahedron As(1)O4 sharing corners (Fig. 3). A second tetrahedron As(2)O4 is grafted on octahedron Fe(2)O6 by pooling of one edge.
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of the anionic framework faces of the hexagonal sections windows (Fig. 5).
The two crystallographically distinct iron atoms exhibit a slightly distorted octahedral coordination with effective coordination numbers ECoN(Fe1)=5.790 and ECoN(Fe2)=5.563, weighted average distances
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indices (DI) for the coordination polyhedra around Fe varying from 3 to 5% (Table 5). The As1O4 tetrahedron is more regular than As2O4 tetrahedron. Thus it is explained by distortion parameters, ECoN(As1)=3.981, ECoN(As2)=3.935, DI(As1)=1~2% and DI(As2)=2~5%. The DIo(=5%) values octahedron and As2O4 tetrahedron.
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for both Fe2 and As2 are slightly higher. They are due to shared edge between the Fe2O6
The K1 coordination sphere is formed by seven oxygen atoms and the K2 by six oxygen atoms. The K2 polyhedron (ECoN =5.882) is slightly distorted than K1 (ECoN =7.080). But the
distorted (Table 4). 3.3. Infrared spectroscopy
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effective coordination value numbers of sodium coordination polyhedra indicate that they are highly
Figure 6 shows IR spectra of Na2.77K1.52Fe2.57(AsO4)4. According to a vibrational study done
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by Nakamoto [22], and other similar compounds [21][23], the asymmetrical stretching
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mode, νas(As-O), appears at frequencies 870 cm−1, the symmetrical stretch, νs(As-O), is detected at a frequency of 796 cm−1 . The symmetrical stretch, νs(Fe-O-Fe), is shown at a frequency of 718 cm−1 . The bands centered at 570, 447 and 362 cm−1 are due to the (δas+δs) [(AsO4) + Fe-O-Fe)]. 3.4. Ionic conductivity
The electrical conductivity was investigated using complex impedance spectroscopy. The electrical parameters were obtained in air in the 298–509 °C temperature range after stabilization at each temperature. Figure 7 illustrates the variation recorded for the imaginary part of impedance (Z’’) with the real part (Z’) at several temperatures. A conventional electrical circuit R//CPE was used, where CPE is a constant phase element: 5
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The parameters obtained for Na2.77K1.52Fe2.57(AsO4)4 sample at different temperatures are
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given in Table 6.
The electrical conductivity increase from 0.722 10-5 S.cm-1 at 298°C to 8.323 10-5 S.cm-1 at 509 °C. The activation energy was obtained by linear fitting of the ionic conductivity values at
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different temperatures by applying the Arrhenius equation: σT = σ0exp(−Ea/kbT) (2)
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Where σ is the temperature dependent ionic conductivity, σ0 is the ionic conductivity at absolute zero temperature, Ea is the activation energy of ion migration, and kb and T have their usual meanings. The activation energy deduced from the slope is 0.50 eV (Fig. 8).
3.5. Alkali pathways transport simulation
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In order to study the relationship between the structure and alkali motion, the transport pathways in the crystal bulk of the title compound have been simulated by the mean of the bond valence sum model [6]. This model was used with success to determine the pathways migration of Li+ in Li4GeS4 and Li0.16La0.62TiO3 [24], Na+ in NaCo2As3O10 [10], K+ in KCoP3O9 [11], etc.
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Accessible sites for mobile ions A in a local structure model are identified using empirical
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relationships between the bond length R and bond valence
[24-26]:
The mismatch of the bond valence sum |∆V(A)| is obtained as: = exp[(R0−R)/b]
(3)
The mismatch of the bond valence sum |∆V(A)| is obtained as: |∆V(A)|= |∑ Where
|
∑
4
, named penalty function, discriminates against sites that achieve a matching V(A) by
strongly asymmetric coordinations:
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0
" (5)
!
Bond valence sum map (BVSM) model visualizes the transport pathways of the mobile ions in solid electrolytes, provided that the ion migration from one equilibrium site to the next one
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follows pathways along which the valence mismatch |∆V| remains ideally as small as possible. The bond valence sum calculation was carried out using 3DBVSMAPPER computer program [27]. The results of simulation are shown in figures 9 and 10.
The crystal structure study shows two cations: K+ in inlayer space (Fig. 11) and Na+ in
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interconnected cavities along b-axis (Fig. 12). Figure 9 shows the isosurface with bond valence mismatch |∆V(Na)|=0.2 v.u which link Na+ atoms. Sodium atoms form a two-dimensional
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conduction pathway in the ab plane. During its migration along the b-axis, sodium atoms pass through windows with minimum size equal to 4.6 Å (Fig. 11) which is close to the sum of Na+ and O2- diameters (4.82 Å) and thus the interconnected cavities may be expected to allow Na+ transport. This observation may explain the low value of bond valence mismatch (0.2 v.u): this is the minimum valence mismatch value necessary to form a continuous isosurface which links all Na+ atoms in the structure. Figure 10 shows the isosurface with bond valence mismatch |∆V(K)|=0.1
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v.u. This figure shows one-dimensional migration pathways for potassium atoms along c-axis with zigzag form (Fig. 10). The bond valence value is very low (0.1 v.u) and this may be explained by the minimum dimension of the section which is equal to 6.4 Å (Fig. 12) .This value is slightly
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higher than the sum of K+ and O2- diameters (5.6 Å). In similar cases, where there are two types of alkali in the structure, it is possible that one of
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the two cation blocks the migration of the other [28], in other cases, an alkali maintains the structure and the other migrates [28]. In the case of Na2.77K1.52Fe2.57(AsO4)4 structure, the two cations migrate through two different spaces and the simulation shows no interference between the two cations. The low values of the bond valence mismatch of Na+ or K+ are in good agreement with the value of the activation energy. 4. Conclusion This work deals with the synthesis of a new arsenate iron Na2.77K1.52Fe2.57(AsO4)4 by solid state reaction. This material was characterized by X-ray diffraction. The structure of the title compound has an open two-dimensional framework. The K+ cations reside in the interlayer space, 7
ACCEPTED MANUSCRIPT whereas the smaller Na+ cations are located in the cavities of the anionic framework. The electrical properties of Na2.77K1.52Fe2.57(AsO4)4 sample were studied using complex impedance spectroscopy. The electrical conductivity is 0.722 10-5 S·cm-1 at 298°C and 8.323 10-5 S·cm-1 at 509 °C. The activation energy deduced from the slope is 0.5 eV. The BVS analysis is extended to simulate the ionic conduction properties of alkali cations in the anionic framework. Pathways migration of
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potassium in Na2.77K1.52Fe2.57(AsO4)4 are one-dimensional along c-axis. However, pathways migration of sodium is two-dimensional in the ab plane. References
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[1] H. Hong, Mater. Res. Bull. 11 (1976) 173–182.
[2] O. Shigeto, S. Shoichiro, E. Minato, Y. Jun-ichi, J. Power Sources 97–98 (2001) 430–432.
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[3] K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid-State Lett. 3 (2000) 178–179. [4] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188– 1194.
[5] G. Li, H. Azuma, M. Tohda, Electrochem. Solid-State Lett. 5 (2002) 135–137. [6] I.D. Brown, The Chemical Bond in Inorganic Chemistry – The Bond Valence Model. IUCr Monographs on Crystallography, Vol. 12, Oxford University Press, 2002.
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[7] S. Adams, Acta Crystallogr. B57 (2001) 278-287.
[8] M. Nespolo, G. Ferraris, G. Ivaldi, R. Hoppe, Acta Crystallogr. B57 (2001) 652–664. [9] A. Guesmi, M. Nespolo, A. Driss, J. Solid State Chem.179 (2006) 2466–2471.
(2015) 132–139.
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[10] Y. Ben Smida, R. Marzouki, A. Guesmi, S. Georges, M.F. Zid, J. Solid State Chem. 221
[11] Y. Ben Smida, A. Guesmi, S. Georges, M.F. Zid, J. Solid State Chem. 221 (2015) 278–284.
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[12] Enraf-Nonius, CAD-4 EXPRESS, Enraf–Nonius, Delft, The Netherlands, 1995. [13] A.J.M. Duisenberg, J. Appl. Crystallogr. 25 (1992) 92–96. [14] G.M. Sheldrick, Acta Crystallogr. A 64 (2008) 112–122. [15] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Crystallogr. A 24 (1968) 351–359. [16] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837–838. [17] K. Brandenburg, M. Berndt, Diamond Version 2.1. Crystal Impact Bonn 2001. [18] M. Nespolo, CHARDT-IT A Program to Compute Charge Distributions and Bond Valences in Non-molecular Crystalline Structures, LCM3B, University Henri Poincaré Nancy I, France, 2001. [19] Softbv web page by Pr.Stefan Adams:/
ACCEPTED MANUSCRIPT [21] N. Ouerfelli, M.F. Zid, T. Jouini, Acta Crystallogr. E 61 (2005) i67. [22] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1977. [23] B. Bouzemi Friaa, H. Boughzala, T. Jouini, J. Solid State Chem. 173 (2003) 273–279. [24] S. Adams,J. Power Sources.159 (2006) 200–204.
[26] S. Adams, Solid State Ion.136–137 (2000) 1351–1361. [27] M. Sale,M. Avdeev, J. Appl. Crystallogr. 45 (2012) 1054–1056.
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[25] S. Adams,Solid State Ion. 177 (2006) 1625–1630.
[28] C. Déportes, M. Duclot, P. Fabray, J. Fouletier, A. Hammou, M. Kleitz, E. Siebert, J.L.
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Souquet. Electrochimie des Solides (1994). Collection Grenoble Sciences, France.
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conditions
and
structure
refinement
results
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Na2.77K1.52Fe2.57(AsO4)4 Orthorhombic ; Cmce a = 10.854(4) Å ; b = 20.985 (8) Å ; c = 6.536 (2) Å ; 1488.7 (9) Å3 ; 4 822.41 g·mol-1; 3.669 g·cm−3 11.90 mm−1 Prism, green 0.08 × 0.02 × 0.02 mm3 Enraf-Nonius CAD-4 λMo Kα=0.71069 Å ; 298 K
3.7° ≤ θ ≤ 27° −13≤ h ≤ 13; 0≤ k ≤ 26; 0≤ l ≤ 8 ω/2θ Psi-scan ; 0.752 ; 0.807 2 ; 120 ; 1 1619 857 [Rint.= 0.037] 779
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Crystal data Empirical formula Crystal system ; space group Unit cell dimensions Volume; Z Formula weight; ρcalc. Absorption coefficient (µ) Crystal sharp ; Color Crystal size Data collection Diffractometer Wave length ; Temperature Theta range for data collection Limiting indices Scan mode Absorption correction ; Tmin ; Tmax Standards; frequency (min); decay (%) Reflections collected Independent reflections Observed reflections [I > 2σ(I)] Refinement Refinement method Final R indices [I > 2σ(I)] Reflections ; parameters ∆ρmax ; ∆ρmin (e.Å−3) Goodness of fit (S)
recording
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Summary of crystallographic Na2.77K1.52Fe2.57(AsO4)4.
Full-matrix least-squares on F2 R= 0.043; wR= 0.113 857 ; 89 2.30 ; −1.43 1.24
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Atom Wyck. x y z Ueqa (Å2) Occ. (<1) 8f 0 0.15537 (4) 0.06542 (12) 0.0131 (3) As1 8e 1/4 −0.04501 (4) 1/4 0.0105 (3) As2 4a 0 0 0 0.0080 (4) 0.79 Fe1 8e 1/4 0.09220 (6) 1/4 0.0097 (3) 0.89 Fe2 8f 0 0.2176 (3) −0.0895 (11) 0.0294 (15) O1 8f 0 0.0878 (3) −0.0755 (8) 0.0158 (11) O2 16g 0.1237 (4) 0.15448 (19) 0.2246 (6) 0.0186 (9) O3 16g 0.2119 (4) −0.09030 (17) 0.4482 (5) 0.0152 (8) O4 16g 0.1378 (3) 0.01189 (17) 0.2058 (5) 0.0120 (7) O5 8f 0 0.1633 (4) −0.4357 (10) 0.0419 (15) 0.733 (7) Na1 16g −0.0774 (18) 0.1801 (9) −0.423 (2) 0.0419 (15) 0.206 (8) Na2 8f 0 0.059 (2) 0.508 (6) 0.0419 (15) 0.115 (11) Na3 8f 0 0.129 (2) −0.443 (6) 0.0419 (15) 0.127 (14) Na4 16g 0.2266 (10) 0.2719 (4) 0.132 (3) 0.077 (5) 0.311 (13) K1 8e 1/4 0.2731 (19) 0.2500 0.077 (5) 0.14 (2) K2 a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
ACCEPTED MANUSCRIPT Table3 Main interatomic distances (Å) in the coordination polyhedra for Na2.77K1.52Fe2.57(AsO4)4. As2 tetrahedron As2—O4iv As2—O4 As2—O5iv As2—O5 Fe2 octahedron Fe2—O3iv Fe2—O3 Fe2—O4xiii Fe2—O4vi Fe2—O5 Fe2—O5iv
1.901 (4) 1.901 (4) 2.016 (4) 2.016 (4) 2.100 (4) 2.100 (4)
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As1 tetrahedron As1—O1 As1—O2 As1—O3i As1—O3 Fe1 octahedron Fe1—O2 Fe1—O2vii Fe1—O5vii Fe1—O5i Fe1—O5 Fe1—O5xi Na1 polyhedron Na1—O1 Na1—O3xxi Na1—O3xii Na1—O1xviii Na1—O4vii Na1—O4xi Na1—O2 Na3 polyhedron Na3—O4v Na3—O4xiii Na3—O5i Na3—O5 Na3—O2ii Na3—O5v Na3—O5xiii K1 polyhedron K1—O3 K1—O4xxiv K1—O1xix K1—O1xvi K1—O1 K1—O3iv K1—O3xix
2.77 (1) 2.94 (1) 2.989 (10) 3.068 (11) 3.07 (2) 3.098 (14)
Na2 polyhedron Na2—O4vii Na2—O3xxi Na2—O1 Na2—O1xviii
2.39 (2) 2.42 (2) 2.47 (2) 2.55 (2)
Na4 polyhedron Na4—O4vii Na4—O4xi Na4—O2 Na4—O3xxi Na4—O3xii Na4—O1
2.439 (18) 2.439 (18) 2.55 (4) 2.61 (3) 2.61 (3) 2.97 (4)
K2 polyhedron K2—O3 K2—O3iv K2—O1xvi K2—O1xix K2—O4xxvi K2—O4xxiv
2.85 (4) 2.85 (4) 2.916 (4) 2.916 (4) 3.17 (4) 3.17 (4)
3.23 (1)
Symmetry codes: (i) −x, y, z; (ii) x, y, z+1; (iv) −x+1/2, y, −z+1/2; (v) −x, −y, −z+1; (vi) −x+1/2, −y, z−1/2; (vii) x, y−1/2, −z+1/2; (xi) x, −y, −z; (xii) x, y, z−1; (xiii) x, −y, −z+1; (xvi) x, −y+1/2, z+1/2; (xviii) −x+1/2, −y+1/2, −z; (xix) x−1/2, −y+1/2, −z; (xxi) −x, y, z−1; (xxiv) x, y+1/2, −z+1/2; (xxvi) −x+1/2, y+1/2, z.
ACCEPTED MANUSCRIPT Table 4 CHARDI and BVS analysis of cation polyhedra in Na2.77K1.52Fe2.57(AsO4)4. Q(i) 0.701 0.195 0.119 0.123 0.292 0.132 2.639 2.622 4.872 5.139
V(i).sof(i) 0.746 0.161 0.107 0.107 0.225 0.098 2.533 2.743 4.982 4.987
CN(i) 7 4 7 6 7 6 6 6 4 4
ECoN(i) 6.542 4.028 6.017 5.168 7.080 5.882 5.790 5.563 3.981 3.935
dar(i) 2.685 2.458 2.653 2.603 2.967 2.979 1.987 2.006 1.686 1.695
dmed(i) 2.653 2.451 2.562 2.525 2.985 2.946 1.974 1.979 1.684 1.690
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Notes: q(i) = formal oxidation number; sof(i) = site occupation factor; dar(i) = arithmetic average distance; dmed(i) = weighted average distance; CNs= coordination number; ECoN(i)= effective coordination number; σ = dispersion factor on cationic charges measuring the deviation of the computed charges (Q) with respect to the formal oxidation numbers; σ = [Σi(qi-Qi)2/N-1]1/2=0.10.
ACCEPTED MANUSCRIPT Table 5 Distortion indices (DI) for the coordination polyhedra around Fe and As in Na2.77K1.52Fe2.57(AsO4)4. Fe1
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0.041 0.048 0.019 0.050 ∑ |d d | ⁄n d ; ∑ |a a | ⁄n a and ∑ |o o | ⁄n o . d, a and o signify Fe/As-O bond distance, O-Fe/As-O angle and O-O edge within the relevant polyhedron; index i indicates individual values, index m the mean value for the polyhedron. n1and n2 are 4 and 6 for the arsenate tetrahedral; 6 and 12 for the iron octahedral.
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ACCEPTED MANUSCRIPT Table 6: Refined electrical parameters of Na2.77K1.52Fe2.57(AsO4)4 obtained from the equivalent circuit at different temperatures T (K)
R (Ω)
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298 326 346 366 401 442 465 509
571 599 619 639 674 715 738 782
12100 7940 6242 5200 3363 2126 1650 1050
0.722 1.101 1.400 1.681 2.598 4.110 5.297 8.323
1.751 1.669 1.616 1.565 1.484 1.399 1.355 1.279
-5.491 -5.022 -4.748 -4.534 -4.045 -3.527 -3.242 -2.732
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Fig. 1. EDS spectrum and SEM micrograph (inset of the figure) of Na2.77K1.52Fe2.57(AsO4)4. Fig.2. Rietveld reffinement patterns of Na2.77K1.52Fe2.57(AsO4)4 sample. Fig. 3. The asymmetric unit with atom-labelling scheme. Displacement ellipsoids are drawn at the
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50% probability level. [Symmetry codes: (i) -x, y, z; (iv) ½-x, 1+y, ½-z; (vii) -x, -y, -z; (xi) x, -y, -z; (xiii) x, -y, 1-z]
Fig. 4. Projection of the structure of Na2.77K1.52Fe2.57(AsO4)4 along the c axis.
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Fig. 6. The infrared spectrum of the Na2.77K1.52Fe2.57(AsO4)4.
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Fig. 5. Layer of the Na2.77K1.52Fe2.57(AsO4)4 structure viewed along [010] showing the hexagonal
Fig.7. Impedance spectrums of Na2.77K1.52Fe2.57(AsO4)4 temperatures rang.
recorded in air in the 298°C–509°C
Fig.8. Arrhenius plot of conductivity of Na2.77K1.52Fe2.57(AsO4)4 sample.
|∆V(Na)|=0.2 v.u.
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Fig.9. 2D pathways link Na+ atoms perpendicular to c-axis with bond valence mismatch
Fig.10. 1D pathways link K+ along c-axis with bond valence mismatch |∆V(K)|=0.1 v.u.
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Fig.11. Dimension of the hexagonal windows sections.
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Fig.12. Dimension of interlayer space along c-axis.
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A new single crystal Na2.77K1.52Fe2.57(AsO4)4 was grown by solid state reaction.
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The structure was determined by single-crystal X-ray diffraction.
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The electrical properties were determined complex impedance spectroscopy.
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The conduction pathways for the Na+ and K+ cations are simulated using the BVSM model.
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