Superionic Behavior and Phase Transition in a Vanthoffite Mineral

Communication pubs.acs.org/IC

Superionic Behavior and Phase Transition in a Vanthoffite Mineral Vaishali Sharma, Diptikanta Swain, and Tayur N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

powder X-ray diffraction (XRD) and thermal studies.16 Because these compounds are rich in Na content and the coordination around Na+ might aid in the possibility of ion migration in the structure, we have explored the superionic behavior and the prospect of structural phase transition in this family of compounds. Because the phase transition is reversible with temperature, it will aid in the design of a possible switchable superionic material. Single crystals of Na6Mn(SO4)4 were grown from an aqueous solution containing a 3:1 molar ratio of Na2SO4 (MERK; 99%) and MnSO4·H2O (MERK; 99%) in an oven maintained at 80 °C. Single-crystal X-ray diffraction data were collected on an Oxford Xcalibur Eos(Mova) diffractometer with an X-ray generator operating at 50 kV and 0.8 mA, using Mo Kα radiation (λ = 0.7107 Å) at room temperature. The structure was solved by direct methods using the program SHELX-2013 present in the WINGX17 suite of programs. Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) measurements were carried out in the temperature range 30−600 °C at a heating rate of 10 °C/min in a N2 atmosphere. Variable-temperature powder XRD data (from 30 to 600 °C in steps of 50 °C; retained for 1 min at each temperature) were collected on a Rigaku SmartLab X-ray diffractometer, using Cu Kα (1.54059 Å) radiation in the 2θ range of 10−100° with a step size of 0.01°. Single crystals were powdered and pelletized under 15 kN before alternating-current (ac) impedance measurements were carried out. About 100 nm of gold was sputtered onto both sides of the pellet for better ohmic contact. The diameter and thickness of the pellets were approximately 10 and 1.2 mm, respectively. The pellets were then placed between two steel electrodes of a homemade conductivity cell. ac impedance measurements were carried out (Novocontrol Alpha-A) in the frequency range of 106−10−2 Hz (signal amplitude = 0.05 V) from 30 to 600 °C at intervals of 25 °C up to 400 °C and of 5 °C up to 600 °C. The temperature of the sample was allowed to equilibrate at each temperature for 10 min (a programmable Thermolyne furnace) prior to the impedance measurements. The crystals belong to a monoclinic system, space group P21/c, Z = 2. Thus, the asymmetric unit contains half the formula unit with a manganese atom in a special position (Wyckoff 2a) along with two sulfate units and three sodium atoms in the general position (Wycoff 4e; Table S1). Manganese atoms are octahedrally coordinated with symmetrically related oxygen atoms with two short [2.1639(2) Å], two long [2.2010(2) Å], and two medium [2.1745(1) Å] bonds. Sulfate tetrahedra are distorted with bond lengths between 1.4547(2) and 1.4915(2) Å. Bond-length variation in the SO4 units has been extensively

ABSTRACT: Crystals of a Vanthoffite mineral, Na6Mn(SO4)4, grown from an aqueous solution, belong to a monoclinic system, P21/c, Z = 2, at ambient temperature. Thermal analysis indicates a phase transition at 455 °C, which was substantiated by in situ variable-temperature powder X-ray diffraction. The structure is orthorhombic (Pmmm) after the phase transition and reverts to the monoclinic system upon cooling. Variable-temperature ionic conductivity measurements show a significantly higher value (∼10−2 S cm−1) beyond the phase transition temperature.

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olid inorganic electrolytes (fast ion conductors) are materials having high ionic conductivity (approximately 10−3−10−1 S −1 cm ) at relatively modest temperatures (200−500 °C) and, hence, can aid in the design and development of next-generation batteries. Among these, the smaller ionic radius of the Li+ ion works in favor because it assists diffusion in solids, leading to the possibility of using it in the commercialization of lithium-ion rechargeable batteries. However, the rapidly increasing demand for lithium sources is a matter of concern, and alternate ionic sources are in high demand. Sodium-ion (Na+)-based batteries are a promising alternative because of the abundance of Na+ in natural sources (Li:Na = 1:1000)1 and also the nontoxic behavior of this ion.2−4 Some early examples are β/β″-aluminas, which depict high Na+ conduction.5−8 Indeed, a single crystal of β/β″alumina shows a conductivity value of 0.1 S cm−1 at room temperature;9 however, it is highly direction-dependent. Extensive studies are being done on NASICONS (Na superionic conductors),10,11 with compounds like Na3.4Sc0.4Zr1.6(SiO4)2(PO4) showing a conductivity value of 4.0 × 10−3 S cm−1 at 25 °C serving as viable candidates. For example, yet another compound belonging to the NASICON series, Na3Sc0.5Zr1.5Si1.5P1.5O12, has a conductivity value of 10.7 × 10−2 S cm−1 at 300 °C. It is worth noting that an abundant source for Na+ comes from minerals, and several studies have suggested the possible use of minerals as solid inorganic electrolytes. In recent studies in our laboratory,12,13 we have examined several minerals with bimetallic sulfates like Langbeinites, Leonites, Kröhnkites, and Tuton’s salt for their ability to generate fast ion conductors with Na+ participation. In this context, Na2Cd(SO4)2 shows a structural phase transition (at 552 °C) that enhances the conductivity value from 4.6 × 10−5 S cm−1 at 280 °C to 4.6 × 10−2 after the transition temperature.14,15 Vanthoffites, with a structural formula of Na6M(SO4)4 (M = Mn, Co, Ni, Fe, Mg), are minerals occurring in oceanic salt deposits. Several of these minerals were synthesized in the laboratory by solid-state reaction and characterized based on © 2017 American Chemical Society

Received: March 31, 2017 Published: May 11, 2017 6048

DOI: 10.1021/acs.inorgchem.7b00802 Inorg. Chem. 2017, 56, 6048−6051

Communication

Inorganic Chemistry studied,18,19 and the recent article based on careful charge density analysis rules out hypervalent description in the sulfate group.19 The three sodium atoms are differently coordinated (6, 7, and 8) with oxygen atoms (Table S2). The details of data collection are given in Table S3. Figure 1 shows the packing as viewed down the

Figure 1. Structure of Na6Mn(SO4)4 as viewed down the c axis.

c axis, illustrating alternative corner-shared sulfate tetrahedra and manganese octahedra, leading to a chain along the b axis. These chains cross-link with each other through sulfate tetrahedra, forming an infinite two-dimensional framework along the bc plane, with the Na atom nesting between the chains. This structure suggests a possible pathway for Na+ migration between the sheets formed by Mn(SO4)6 akin to several examples in the literature.20,21 Rietveld refinement confirmed the structure in the bulk (Figure S1 and Table S4). Thermal studies establish the reversible phase transition in this material, as seen from the DSC (Figure 2), and the transition

Figure 3. Powder XRD patterns: (a) heating cycle; (b) cooling cycle. Full patterns are given in Figures S4 and S5.

Figure 2. DSC traces (both heating and cooling cycles), with the DTA plot shown as an inset for Na6Mn(SO4)4.

to ensure that there is a complete phase transition, the data collected at 550 °C were considered for indexing of the pattern. Crysf ire22 and CHECKCELL23 programs were used for indexing, and JANA200024 was used for profile fitting (Figure 4). The XRD pattern belongs to an orthorhombic system, space group Pmmm, with a = 5.3685 Å, b = 7.5760 Å, c = 9.2988 Å, and volume V = 378.19 Å3. Figure 4 shows the fitted profile (Rp = 5.62; Rwp= 8.18). Upon cooling to room temperature in situ, the monoclinic phase P21/c is restored (Figure 3b), establishing the reversible phase transition. Attempts to solve the high-temperature structure using ab initio methods were not successful. To evaluate the conductivity properties of the alkaline metal, impedance spectra were carried out with respect to the

temperature is confirmed to be 455 °C, as derived from the DTA trace [Figure 2 (inset)]. An in situ variable-temperature powder XRD study clearly establishes the reversible nature of the transition (Figure 3a,b). It is worth noting that the XRD traces were recorded after allowing a retention time of 1 min at each temperature to obtain the trends; however, if the sample was allowed to have more retention time, there could have been a perfect one-to-one match in the pattern. To identify the structure at high temperature, a variable-temperature powder XRD study was performed. Figure 3a shows the temperature evolution of the powder XRD pattern of Na6Mn(SO4)4. There is a clear indication of the structural phase transition above 500 °C, and 6049

DOI: 10.1021/acs.inorgchem.7b00802 Inorg. Chem. 2017, 56, 6048−6051

Communication

Inorganic Chemistry

combination of covalently bonded octahedra and tetrahedra in the orthorhombic phase. To find the activation energy, a conductivity (σ) plot was fitted to the Arrhenius equation, σ = A exp(−Ea/KT), where A is the preexponential factor, K the Boltzmann constant, T the absolute temperature, and Ea the activation energy. The activation energy was calculated after the phase transition in the temperature range 490−600 °C and estimated to be 0.8 eV. There is a high possibility that Na+ can conduct through a hopping mechanism between different sites. This is supported by the estimated activation energy, which is similar to that of some of the known classes of Na+ conductors.25−27 In summary, we solved the structure of Na6Mn(SO4)4 and described the structure in detail. It shows high Na+ conduction after the phase transition and sets an example for a possible switchable fast-ion-conducting material. This observation is supported by DTA and variable-temperature powder XRD.

Figure 4. Fitted profile of Na6Mn(SO4)4.

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temperature (Figure 5). The bulk ionic conductivity was calculated from the intercept of single semicircle arcs obtained

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00802. Tables S1−S4 and Figures S1−S5 (PDF) Accession Codes

CCDC 1548170 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 5. Conductivity plot of Na6Mn(SO4)4.

ACKNOWLEDGMENTS The authors thank Dr. Aninda J. Bhattacharry, Dr. Dipak Dutta, and Subhra Gope for conductivity measurement and discussion on the ionic conductivity. T.N.G.R. is thankful for funding by the DST through a a J. C. Bose Fellowship, and V.S. thanks the IISc for SRF.

in the complex impedance of Re(Z′)−Im(Z″) plots. The room temperature conductivity value is on the order of 6.41 × 10−8 S cm−1. It is interesting to note that the conductivity does not follow the Arrhenius equation from room temperature to 125 °C. The conductivity follows the Arrhenius equation beyond this temperature until the phase transition, at which a sudden increase in the value of the conductivity is perceived. The possible reason for the non-Arrhenius behavior until 125 °C is suggested to be due to the surface water absorbed by the sample during the experiment (because the experiment was performed in air). Additionally, there are no significant changes in the cell parameters, suggesting no structural involvement in this temperature range (Figure S2; room temperature to 125 °C). However, specific heat capacity studies were carried out to explore the possibility of a second-order phase transition in this range, and the result showed no accompanying phase change (Figure S3). The conductivity value abruptly increases from value 3.5 × 10−11 to 7.4 × 10−3 S cm−1 at 490 °C in accordance with the DSC/DTA data. Indeed, the value of the conductivity increases to 3.9 × 10−2 S cm−1 at 600 °C (Figure 5), indicating possible Na+ migration through the framework generated based on a

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DOI: 10.1021/acs.inorgchem.7b00802 Inorg. Chem. 2017, 56, 6048−6051