Electrical properties and alkali-pathways simulation of new mixed conductor Na4Li0.62Co5.67Al0.71(AsO4)6

Polycrystalline sodium lithium-cobalt-aluminum arsenate, Na4Li0.62Co5.67Al0.71(AsO4)6, was synthesized by solid state reaction route and characterized by infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The title material is a member of β-Xenophyllite family which show 3D anionic framework with interconnecting tunnels where alkali-ion are located. Dense ceramic with relative density of 97% is obtained after planetary grinding of the synthesized powder followed by optimal sintering at 1113 K. The effect of grinding on microstructure of sample is controlled by Scanning Electron Microscope (SEM). The electrical study using impedance spectroscopy, in the 443–773 K temperature interval, show interesting electrical performance of the dense ceramic: σ433K = 1.41 10–5 Scm−1 and the activation energy Ea = 0.449 eV. The alkali migration pathways in the anionic framework were simulated using Bond Valence Site Energy (BVSE) model to correlate structure with electrical properties of the studied material. The calculation results were compared to other β-Xenophyllite materials.


Introduction
Extensive search have focused on developing new materials or modifying existing ones by substitution or doping to study their properties. Phosphates and arsenates of monovalent metals and transitions consider a promising field for various applications [1][2][3][4][5][6][7][8]. Essentially, the introduction of monovalent ions in phosphates and arsenates can lead to materials with interesting properties, in particular ionic conduction [9].
In this context, this paper is dedicated to the synthesis of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 material as polycrystalline powder then as dense ceramic. Qualitative and quantitative analyzes were carried out to confirm the composition and purity of the studied material: x-ray Diffraction, FT-IR, SEM, EDX and ICP-MS. The Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. electrical properties were investigated using impedance spectroscopy. The BVSE simulation of the alkali-ions in the anionic framework of Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 are carried out in order to correlate structure with electrical properties and to compare results to other isostructural arsenates: Na 4 Co 4 (Co 1.63 Al 0.91 )(AsO 4 ) 6 and Na 4 Co 7 (AsO 4 ) 6 .

Synthesis
The title material was synthesized by solid-state reaction method at atmospheric air and pressure. A mixture of Na 2 CO 3 (Sigma-Aldrich, 99.5%); Li 2 CO 3 (Sigma-Aldrich, 99.0%), (Co(CH 3 COO) 2 .4H 2 O (Merck, 99.9%); Al 2 O 3 (Sigma-Aldrich, 99.0%), As 2 O 5 (Merck, 99%) is finely ground in an agate mortar in molar proportions Na: Li: Co: Al: As=4: 0.62 : 5.67 : 6. First, the mixture placed in platinum crucible has been heated at 673 K for 24 h to ensure evaporation of acetate; carbonate and water. After cooling followed by a second grinding, the residue was heated at 1043 K for 3 days. The mixture was cooled slowly to room temperature. Pink polycrystalline powder was obtained. The Fourier-Transform Infrared (FT-IR) spectrum of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 material was recorded using Bruker FT-IR IFS 66 spectrophotometer over the range 500-1200 cm −1 , at room temperature. KBr was added to the sample and shaped as cylindrical pellet.
A qualitative energy dispersive x-ray (EDX) spectroscopy analysis was used to identify the elements present in the polycrystalline sample. The EDX analysis was performed using a JSM 6301 microscopy.
The morphologies of the crystalline powder and pellets were controlled using Scanning Electron Microscopy SEM JEOL 6340.
The Na, Li, Co, Al, As elements of the synthesized powder were quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS; Perkin Elmer model) operate NexION-300X software. The duration of acquisition is 1 000 ms. A Helium collision cell was used to eradicate polyatomic interferences. Impedance spectroscopic measurements were performed via Hewlett-Packard 4192-A automatic bridge supervised by HP workstation. Impedance spectra were recorded with 0.5 V AC-signal in the 5 Hz-13 MHz frequency range. The measurements were preceded by a pretreatment of the sample in order to reduce the mean particle size of the synthesized powder. In fact, mechanical grinding during 100 min was carried out using FRISCH planetary micromill pulverisette 7. The polycrystalline sample is shaped as cylindrical pellet using uniaxial press. The pellet was sintered in air at an optimal temperature of 1113 K for 2 h with 5 K min −1 heating and cooling rates. The geometric factor of the dense ceramic is g=e/S=0.812 cm −1 where e and S are the thickness and surface of pellet, respectively. After the control of the microstructure and determining the density of the pellet, the last step before beginning the electrical measurements is the metallization. To ensure good contact between the two faces of the pellet and the two measurement electrodes, a gold metal layer was deposited using a SC7620 Mini Sputter Coater. The faces of the pellet were previously coated with a metallic layer of gold (about 36 nm). The sample was placed between two platinum electrodes. These are connected by platinum cables (to ensure good electrical contact) to the frequency response analyzer (HP 4192 A) controlled by a microcomputer.
The Bond Valence Site Energy (BVSE) model [26,27] has been used to simulate the alkali migration in the 3D anionic framework. In fact, The BVSE model is the current expansion of BVS model developed by Pauling (1929) [28] to describe the formation of inorganic materials. The BVS model was improved by Brown & Altermatt 1985 [29] followed by Adams 2001 [30], ensuing in the expression (1) for an: s A-X : individual bond-valence; R A-X : distance between counter-ions A and X; R 0 and b: fitted constants, R 0 : the length of a bond of unit valence.
Since 1999, the BVS model was used in the of cation motion simulation in the anionic framework by following the valence unit as a function of migration distance [31]. In 2011, The valence units was related to potential energy scale and electrostatic interactions treated by Adams [26,27]. The BVSE was used with success to simulate the transport pathways of monovalent cations (Na + ; K + and Ag + ) in NaCo 2 As 3 O 10 [32], Na 1.14 K 0.86 CoP 2 O 7 [33] and Ag 3.68 Co 2 (P 2 O 7 ) 2 [34]. The BVSE calculations were performed using the 3DBVSMAPPER code using Na + as a test ion and default settings [35].  [36][37][38], is shown in table 1.

Results and discussion
3.2. X-ray diffraction and structural characteristic of Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 X-ray diffraction (XRD) pattern of the studied material recorded in the 10°-70°range with step of about 0.02°, at room temperature is illustrated in figure 2. The powder diffraction pattern is indexed using dicvol software (Boultif & Louer, 2004) [39]. The Rietveld method has been used to verify the purity of the powder using the crystallographic data previous work [22] operate JANA2006 software (Petricek, Eigner, Dusek, & Cejchan, 2016). The final agreement factors are Rp=0.049, Rwp=0.065, and GOF=1. 16. No additional peaks have been detected. The final Rietveld plot is presented in figure 2.
The structural study conducted in our prevoius work previously showed us that the prepared material Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 has characteristics favorable to the mobility of the alkali-ions such as: cations located in tunnels with large sections, partial occupations of cationic sites, the relatively high thermal agitation of these ions ( figure 3). Therefore, it is interesting to carry out a electrical study of the titled maerial.

Morphology and microstructure control by SEM and EDX analysis
The SEM micrograph of the synthesized powder is shown in figure 2(a). It confirms the good crystallinity of the prepared powder. The mean grain size is about 5 μm. As for the chemical composition, the EDX analysis on SEM confirmed the presence of sodium, cobalt, aluminum, arsenic and oxygen elements ( figure 4(a)). We note that the lithium element does not appear in the EDX spectrum. In this case a quantitative analysis can confirm the material composition like the ICP-MS technique.
After the synthesis of the crystalline powder, and before starting the electrical measurements, the powder has undergone a planetary milling treatment with a duration of 100 min and a sintering at suitable temperature, the relative density of the ceramic increases by 83 % to 97%. The maximum densities obtained were reached for a total grinding time of 100 min There is no more remarkable variation after two other grinding sequences of 10 min The sample densification was controlled by SEM micrographs. Figures 4(b) and (c) show sintered pellet before and after mechanical grinding, respectively. In fact, we move from a relatively porous material ( figure 4(b)) to a dense ceramic ( figure 4(c)).

Quantitative analysis using ICP-MS
After polycristalline powder preparation and XRD caracterization (figure 1), the sample was analyzed by inductively coupled plasma mass spectrometry ICP-MS in order to approve the adopted chemical composition. The measured mass and the mass percentage values of the Na, Li, Co, Al, As elements contents in 0.50000 g of the powder are grouped in table 2. The ICP-MS analysis approves the element contents of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 formula. In fact, the weight percentages of the Na, Li, Co, Al and As elements are 7.167%; 0.335%; 26.042% 1.493% and 35.035%, respectively. The weight and the weight percentage of the oxygen can be deduced as being the remainder of  Consequently, the quantitative analysis by ICP-MS confirmed that the composition of the material is exactly that of the initial fraction of the starting reactants described in the experimental section Na: Li: Co: Al: As=4: 0.62 : 5.67 : 6 (ie the nominal formula Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 ).

Electrical properties
The electrical properties of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 sample were determined via complex impedance spectroscopy technique. The electrical measurements were carried out in the frequency range 5Hz-13 MHz. The  normalized impedance spectra recorded on Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 ceramic at 160°C-445°C are showed in figure 5. Z-View software [40] was used to simulate the Nyquist plots which fitted as an equivalent circuit consisting of R//CPE. R is the resistance while CPE presents an empirical impedance function described in equation (2).    The electrical parameters values achieved from the equivalent circuit in the temperature range 160-445°C are summarized in table 3. Where, the resistivity ρ=R/k is extracted from the refinement of each contribution which the geometric factor of the cylindrical pellet g(cm −1 )=e/S (e=thickness; S=surface).
The Arrhenius plot of the electrical conductivity, log (σT (S.Kcm −1 )) as a function of 1000/T (K −1 ), in the temperature interval 160-445°C is illustrated in figure 6. As a single linear plot and following the Arrhenius law, the activation energy of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 compound determined by linear fit is 0.449 eV. Compared to previous works, the activation energy of the studied material is lower than that of the Na 4 Co 7 (AsO 4 ) 6 (Ea=1.0 eV) and Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 (Ea=0.53 eV).
Investigation of isosurfaces connecting sodium ion sites in the unit cells of isostructural materials show that the sodium can move along the [100] direction and form 1D infinite pathways (figure 7) while the calculated activation energy are 0.583 eV; 0.786 eV and 0.611 eV for Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 ; Na 4 Co 7 (AsO 4 ) 6 and Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 suggest fast ionic conductivity of this type of materials.
The BVS analyzes are in agreement with the suggestions deduced from the structural studies [20,26]: the most likely conduction pathways of the monoarsenates are in the [100] direction which is the direction of the tunnels. The dimensions of the hexagonal sections of tunnels along [100] direction of the three compounds are shown in figure 8(a). These canals are delimited by two CoO 6 octahedra, two MO 6 or CoO 6 octahedra and two AsO 4 tetrahedra. These sections are the largest and therefore more favorable to the movement of sodium cations.
The b direction appears less conducive than [100] direction. In fact, the windows ( figure 8(b) On the other hand, the crystallographic studies [20,25] show that the substitution of cobalt by aluminum allows to increase the unit cell parameters and the dimensions of the tunnel sections. In fact, the volume of the unit cell of Na 4 Co 7 (AsO 4 ) 6 is V=1019.65(16) Å 3 which is lower than those of Na 4   The conduction pathways simulation shown that the Li/Al-sub Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 and the Al-sub Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 materials have electrical performances more interesting than the parent material Na 4 Co 7 (AsO 4 ) 6 . In fact, the substituted materials have activation energies lower than that of Na 4 Co 7 (AsO 4 ) 6 . However, the activation energy of the studied material is relatively higher than that of Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 . While, electrical measurements have shown that the studied material is more conductive than Al-sub material. This can be explained by the fact that the BVSE model does not take into account the electronic conductivity which may be due to the presence of several degrees of oxidation in the same crystallographic site. Indeed, in the studied material, in addition to site occupied by Co 2+ and Al 3+ , another site is occupied by both cobalt and lithium ions Co 2+ /Li + . This double oxidation in the same site can create an electronic conductivity that will increase the value of the total conductivity: ionic and electronic.

Conclusion
The title compound, Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 , has been synthesized as polycrystalline powder by solid-state method. The powder composition has been confirmed by qualitative analysis XRD followed by quantitative analysis, ICP-MS. This structure, isostructural to Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 and Na 4 Co 7 (AsO 4 ) 6 , presents an 3D open anionic framework facilitating one-dimensional Na ionic conductivity. The substitution of cobalt by aluminum reduces the activation energy of Na conductivity, as determined by impedance spectroscopy and supported by BVSE calculations. Overall, our study suggests that tuning of the Na 4 Li 0.62 Co 5.67 Al 0.71 (AsO 4 ) 6 crystal structure geometry by doping can further improve Na ionic conductivity and thus rate capability of Na 4 Co 7 (AsO 4 ) 6 based cathodes.