Syntheses, Structures, and Characterization of Quaternary Tellurites

Apr 25, 2017 - Crystallographic Data for Li3MTe4O11 (M = Al, Ga, and Fe) ...... Brese , N. E.; O'Keeffe , M. Bond-valence parameters for solids Acta C...
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Syntheses, Structures, and Characterization of Quaternary Tellurites, Li3MTe4O11 (M = Al, Ga, and Fe) Minfeng Lü,† Hongil Jo,† Seung-Jin Oh,† Suheon Lee,‡ Kwang-Yong Choi,‡ Yang Yu,§ and Kang Min Ok*,† †

Department of Chemistry and ‡Department of Physics, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea § State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: Three new quaternary lithium metal tellurites, Li3MTe4O11 (M = Al, Ga, and Fe), have been synthesized through hydrothermal and solid-state reactions by heating a mixture of LiOH·H2O, TeO2, and M2O3. The structures of the title compounds have been determined by single-crystal and powder X-ray diffraction. Li3MTe4O11 reveal three-dimensional (3D) frameworks that consist of MO6 octahedra, TeO3 trigonal pyramids, and TeO4 polyhedra. The variable coordination mode of Te4+ within the framework leads to the formation of 1D channels that host Li+ cations on both tetrahedral and octahedral sites. The bulk and grain boundary Li+ ion conductivities for a Li3FeTe4O11 pellet in open air are estimated to be 1.0 × 10−4 and 2.7 × 10−6 S cm−1, respectively, at room temperature from the impedance profile analysis. A lower activation energy of 19.9 kJ mol−1 is obtained for the system, which is similar to that of Li10GeP2S12 (24 kJ mol−1). Detailed characterizations such as thermal, spectroscopic, and magnetic properties for the reported materials are also reported.

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through a second-order Jahn−Teller (SOJT) effect,21−29 force the cation to have unsymmetrical polyhedral units.14,20 The asymmetric polyhedra found in tellurites with extended structures often result in a variety of framework dimensions. In addition, tellurium(IV) dioxide, the starting Te4+ source, is one of the most versatile reagents for crystal growth of novel compounds. TeO2, with an accessible melting point of 733 °C, exhibits not only an excellent solubility in many solvents but a superior reactivity with other oxides.30,31 Our consistent synthetic endeavors in the Li+−M3+−Te4+− oxide (M = Al, Ga, and Fe) system led us to find three novel quaternary lithium tellurium oxides. Initially, p-block metal cations such as Al3+ and Ga3+ were utilized to fulfill the rich structural chemistry. Further structural analysis for the newly synthesized materials led us to study the ionic conductivities by introducing the transition metal cation Fe3+, which can change the oxidation state reversibly. In this paper, we present the synthesis, structure determinations, and thorough characterization of three new lithium metal tellurites, Li3MTe4O11 (M = Al, Ga, and Fe). For Li3FeTe4O11, detailed ionic conductivity and magnetic properties are also presented. From the impedance profile analysis, we were successfully able to measure the bulk and grain boundary Li+ ion conductivities

INTRODUCTION Developing an excellent sustainable system with adequate electrical-energy storage and generation is one of the most urgent current research topics. Among many, lithium ion and lithium−air batteries have attracted huge attention because of their high power and energy densities.1,2 Limitations of the flammable organic liquid-carbonate electrolytes of the Li+ ion battery,3,4 however, promote a great demand for rechargeable (secondary) all-solid-state lithium batteries, in which the solid electrolyte is one of the most important key components that could improve electric performance as well as security.5 For example, Li3N,6 NASICON-type Li1+xTi2−xAlx(PO4)3,7,8 LISICON-type Li3+xZn1−xGeO4,9 garnet-related Li7La3Ta2O12,3,10 and Li10GeP2S1211 reveal high lithium ion conductivities at room temperature. Specifically, Li3N6 and Li10GeP2S1211 exhibit ionic conductivities as high as 10−3 S cm−1 and 12 mS cm−1, respectively, at room temperature. Recently, anisotropic Li+ ion conductivity has been clearly observed in a huge single crystal of a cobalt coordination compound, LiCo(NC5H3(CO2)2)2(H2O)2.125.12 Tellurites, i.e., oxides containing Te4+ cations, often show rich structural chemistry. The polyhedra of Te4+ in mixed metal tellurium oxides are of particular interest. Te4+ having 3-, 4-, and 5-oxide ligands normally exhibits TeO3 trigonal pyramids, TeO4 seesaws, and TeO5 polyhedra, respectively.13−20 The nonbonded lone pairs on Te4+, which are considered to be the result of mixing between the metal s and oxygen p orbitals © 2017 American Chemical Society

Received: February 28, 2017 Published: April 25, 2017 5873

DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

Article

Inorganic Chemistry for Li3FeTe4O11. Detailed structural analysis to explain the observed Li+ ion conductivities is provided.

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Table 1. Crystallographic Data for Li3MTe4O11 (M = Al, Ga, and Fe)

EXPERIMENTAL SECTION

fw space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z T (K) λ (Å) R(F)a or Rpb Rw(Fo2)c or Rwpd

Synthesis. Li2CO3 (Hayashi, 98%), LiOH·H2O (Aldrich, 98%), Ga2O3 (Alfa Aesar, 99.999%), Ga(NO3)3·xH2O (Alfa Aesar, 99.9%), Al2O3 (Samchun, 99%), Fe2O3 (Junsei, 98%), and TeO2 (Aldrich, 99%) were employed as received. Single crystals of Li3AlTe4O11 and Li3GaTe4O11 were grown hydrothermally. For Li3AlTe4O11, 3.0 mmol of LiOH·H2O (0.126 g), 0.5 mmol of Al2O3 (0.025 g), 3.0 mmol of TeO2 (0.479 g), and deionized water (1.5 mL) were combined. For Li3GaTe4O11, 4.5 mmol of Li2CO3 (0.333 g), 0.15 mmol of Ga(NO3)3· xH2O (0.330 g), 4.5 mmol of TeO2 (0.718 g), and deionized water (4 mL) were mixed. Each reaction mixture was transferred into the respective 23 mL stainless steel autoclaves with Teflon liners. After tightly sealing, the autoclaves were heated slowly to 230 °C for 48 h. After heating, the reactors were cooled to room temperature at a rate of 5 °C h−1. The autoclaves were opened, and the reaction products were recovered by filtration. Pure colorless, transparent, brick-shaped crystals of Li3AlTe4O11 and Li3GaTe4O11 were isolated in 30% and 38% yields, respectively. Crystal growth experiments of Li3FeTe4O11 with similar hydrothermal conditions were not successful. Therefore, powder X-ray diffraction with the Rietveld method was utilized to determine the crystal structure of Li3FeTe4O11. Polycrystalline Li3FeTe4O11 was prepared through a standard solid-state reaction. An intimate mixture of LiOH·H2O, Fe2O3, and TeO2 with the molar ratio of 6:0.5:4 was pressed into a pellet and heated at 105 °C for 12 h and 450 °C for 48 h under flowing Ar. In order to make up for the volatility of Li+ at high temperature, an excess amount of LiOH·H2O was used. After heating, the pellet was cooled to room temperature. Further heating and regrinding were repeated at a similar heating condition for 84 h until a pure phase was obtained. Powder X-ray diffraction data (PXRD) for the ground sample reveal similar patterns to those of Li3AlTe4O11 and Li3GaTe4O11 (see the Supporting Information). Characterization. Single-crystal X-ray diffraction for Li3AlTe4O11 and Li3GaTe4O11 have been collected using a Bruker SMART BREEZE diffractometer with a 1K CCD area detector and monochromated Mo Kα radiation (λ = 0.710 73 Å) at room temperature. To acquire the data, a narrow-frame method was used with scan widths of 0.30° in ω and an exposure time of 10 s/frame. For integration of the obtained data, the program SAINT was used.32 The intensities for collected data were corrected for Lorentz factor, polarization, air absorption, and absorption attributed to the deviation through the detector faceplate in the path length. Absorption corrections have been performed by the multiscan method using the program SADABS.33 The crystal structure was solved using the charge flipping program34 and refined using Jana 2006.35 Powder X-ray diffraction data were obtained on a Bruker D8-Advance diffractometer (Cu Kα radiation) with 40 kV and 40 mA at room temperature. For Li3FeTe4O11, the diffraction pattern was analyzed and refined using the Rietveld method. Structural refinement of Li3FeTe4O11 was performed in the space group C2/c with a starting model of the crystal data for Li3GaTe4O11. The crystallographic data and selected bond lengths for all the reported materials are gathered in Tables 1 and 2. Energy dispersive analysis by X-ray (EDX) was performed using a Horiba Energy EX-250 scanning electron microscopy (SEM) instrument attached to a Hitachi S-3400N. EDX analyses for Li3MTe4O11 revealed proper M:Te ratios of 1:4. Infrared (IR) spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer in the spectral range 400−4000 cm−1 with a resolution of 4 cm−1. All of the reported materials were ground and contacted intimately by a diamond attenuated total reflectance (ATR) crystal. UV−vis diffuse reflectance spectra were collected by a Varian Cary 500 scan UV−vis−NIR spectrophotometer at room temperature. The collected reflectance data was converted to the absorbance spectra via the Kubelka−Munk function.36,37

Li6Al2Te8O22

Li6Ga2Te8O22

Li6Fe2Te8O22

1468.40 C2/c (No. 15) 18.9467(4) 5.13400(10) 10.6284(2) 109.632(12) 973.75(8) 2 298.0(2) 0.71073 0.0340 0.0320

1553.88 C2/c (No. 15) 18.7906(10) 5.1841(2) 10.7786(5) 109.726(4) 988.35(8) 2 298.0(2) 0.71073 0.0234 0.0305

1545.10 C2/c (No. 15) 18.9693(5) 5.18465(9) 10.93452(16) 109.7835(9) 1011.93(3) 2 298.0(2) 1.5406 0.0386 0.0490

R(F) = ∑∥Fo| − |Fc∥/∑|Fo|. bRp = ∑|Io − Ic|/∑Io. cRw(Fo2) = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. dRwp = [∑w|Io − Ic|2/∑wIo2]1/2.

a

Thermogravimetric analysis (TGA) was performed using a highresolution PerkinElmer TGA 7 thermal analyzer. Polycrystalline samples of the title compounds were mounted in alumina crucibles and heated to 900 °C at a rate of 10 °C min−1 under flowing argon. Electron paramagnetic resonance (EPR) measurements were performed at an X-band (ν = 9.4444 GHz) by using a JEOL JESFA200 spectrometer at room temperature. The dc magnetic susceptibility for polycrystalline Li3FeTe4O11 was measured using a Quantum Design Magnetic Properties Measurement System (MPMS) SQUID magnetometer. Zero-field-cooled (ZFC) and field-cooled (FC) data were collected in a temperature range 2− 300 K and in an external field of H = 1000 Oe. An isothermal magnetization curve was measured using a pulsed magnet with a 30 ms duration induction method with a pick-up coil up to 35 T (Dresden High Magnetic Field Lab) as well as using a SQUID magnetometer in a field range of −7 to 7 T at 2 K. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB MKII spectrometer with an Al Kα X-ray excitation source. Conductivity measurements were performed on Li3FeTe4O11 by impedance spectroscopy using a Solartron 1260 impedance analyzer with a 10 mV amplitude signal over a 0.1 Hz to 10 MHz frequency range. Polycrystalline Li3FeTe4O11 was uniaxially compacted into a 13 mm diameter and ∼0.7 mm thick pellet at 600 MPa. The pellet was annealed at 450 °C for 12 h. A silver paste was applied to both sides of the pellet.

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RESULTS AND DISCUSSION Structures. All three reported tellurites, Li3MTe4O11 (M = Al, Ga, and Fe), are isostructural and crystallize in the monoclinic space group, C2/c; thus, only the detailed structural description of Li3AlTe4O11 is provided here. Li3AlTe4O11 is a new quaternary tellurite exhibiting a three-dimensional (3D) structure. The anionic backbone of Li3AlTe4O11 is composed of AlO6 octahedra, TeO3 trigonal pyramids, and TeO4 seesaws (see Figure 1). The unique Al3+ cation is linked by six oxygen ligands in a slightly distorted octahedral moiety with Al−O bond distances of 1.883(5)−1.935(5) Å. Two unique Te4+ cations existing in an asymmetric unit are in an unsymmetrical coordination environment owing to the stereoactive lone pairs. Interestingly, the two unique Te4+ cations reveal different coordination modes. Te(1)4+ is bonded by three oxide ligands in a trigonal pyramidal geometry, whereas Te(2)4+ forms a seesaw environment with four oxygen atoms. In addition, while Te(1)4+ reveals normal Te−O bond distances of 1.862(4)− 1.881(6) Å, Te(2)4+ exhibits two short [1.847(5)−1.871(6) Å] 5874

DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

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Inorganic Chemistry Table 2. Selected Bond Distances (Å) for Li3MTe4O11 (M = Al, Ga, and Fe) Li3AlTe4O11 Al(1)−O(1)× 2 Al(1)−O(2) × 2 Al(1)−O(3) × 2 Te(1)−O(2) Te(1)−O(3) Te(1)−O(4) Te(2)−O(1) Te(2)−O(4) Te(2)−O(5) Te(2)−O(6) Li(1)−O(1) Li(1)−O(2) Li(1)−O(2) Li(1)−O(3) Li(1)−O(4) Li(1)−O(5) Li(2)−O(4) Li(2)−O(5) Li(2)−O(5) Li(2)−O(6)

Li3GaTe4O11 1.935(5) 1.883(5) 1.919(5) 1.864(6) 1.862(4) 1.881(6) 1.871(6) 2.279(5) 1.847(5) 2.038(2) 2.026(17) 2.152(16) 2.103(17) 2.160(18) 2.295(16) 2.312(19) 2.18(4) 1.90(3) 1.94(3) 2.12(3)

Ga(1)−O(1)× 2 Ga(1)−O(2) × 2 Ga(1)−O(3) × 2 Te(1)−O(2) Te(1)−O(3) Te(1)−O(4) Te(2)−O(1) Te(2)−O(4) Te(2)−O(5) Te(2)−O(6) Li(1)−O(1) Li(1)−O(2) Li(1)−O(2) Li(1)−O(3) Li(1)−O(4) Li(1)−O(5) Li(2)−O(4) Li(2)−O(5) Li(2)−O(5) Li(2)−O(6)

Li3FeTe4O11 1.985(4) 1.939(3) 1.978(3) 1.861(4) 1.862(3) 1.887(4) 1.857(4) 2.272(4) 1.844(4) 2.0271(18) 2.022(10) 2.110(11) 2.078(12) 2.148(12) 2.443(11) 2.226(13) 2.16(2) 1.926(18) 1.937(16) 2.072(17)

Fe(1)−O(1)× 2 Fe(1)−O(2) × 2 Fe(1)−O(3) × 2 Te(1)−O(2) Te(1)−O(3) Te(1)−O(4) Te(2)−O(1) Te(2)−O(4) Te(2)−O(5) Te(2)−O(6) Li(1)−O(1) Li(1)−O(2) Li(1)−O(2) Li(1)−O(3) Li(1)−O(4) Li(1)−O(5) Li(2)−O(4) Li(2)−O(5) Li(2)−O(5) Li(2)−O(6)

2.109(8) 2.009(11) 1.978(9) 1.874(15) 1.849(8) 1.855(8) 1.847(9) 2.258(8) 1.840(10) 2.007(4) 1.86(5) 2.24(4) 2.10(5) 2.20(5) 2.52(4) 2.10(5) 2.11(12) 1.88(9) 1.94(8) 2.40(8)

Figure 1. ORTEP (50% possibility ellipsoids) drawings for AlO6, TeO3, and TeO4 polyhedra found in the framework of Li3AlTe4O11.

and two long [2.038(2)−2.279(5) Å] Te−O bond lengths. A similar flexible coordination environment of Te4+ with oxide ligands has been previously observed.13,16,38 Li(1)+ and Li(2)+ cations interact with six and four oxygen atoms, respectively, with Li−O contact lengths of 1.90(3)−2.312(19) Å. TeO3 and TeO4 polyhedra share their corners through oxygen atoms and form a Te4O11 tetramer (see Figure 2a). Two Te4O11 tetramers are linked by two AlO6 octahedra through oxide ligands, and an eight-membered ring (8-MR) is obtained (see Figure 2b). Further connections of the 8-MRs through oxygen atoms complete an anionic 3D framework of Li3AlTe4O11, where 3-, 4-, and 8-MR channels are observed along the [010] direction (see Figure 2c). Li+ cations reside in the 8-MR channels. In connectivity terms, Li3AlTe4O11 may be recorded as an anionic

Figure 2. Ball-and-stick models of (a) a Te4O11 tetramer, (b) an eightmembered ring (8-MR), and (c) a 3D framework of Li3AlTe4O11 in the ac plane (blue, Al; green, Te; yellow, Li; red, O). 3-, 4-, and 8-MR channels are observed along the [010] direction. Li+ cations reside in the 8-MR channels.

framework of {[AlO6/2]−3 [Te(1)O3/2]+ [Te(2)O3/2O1/1]−}−3, and the overall charge balance is retained by three Li+ cations. It should be noticed that the site of Li(2)+ is disordered, and only half of the site is occupied by lithium cations. Bond valence sum calculations39 for Li+, Al3+, and Te4+ in Li3AlTe4O11 reveal values of 0.91−0.92, 2.73, and 4.02−4.04, respectively. 5875

DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

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Inorganic Chemistry IR Spectroscopy. IR spectra of the reported materials reveal all the vibrations of Te−O and M−O (M = Al, Ga, and Fe) bonds. Multiple peaks occurring at 742 and 700 cm−1 for Li3AlTe4O11, 740 and 695 cm−1 for Li3GaTe4O11, and 750 and 703 cm−1 for Li3FeTe4O11 may be attributed to the Te−O vibrations.40 Vibrational bands found at around 451−487 cm−1, 478−486 cm−1, and 470−480 cm−1 should be attributable to Al−O, Ga−O, and Fe−O bonds, respectively.15,38,41−44 The IR spectra for the title compounds can be found in the Supporting Information. UV−Vis Diffuse Reflectance Spectroscopy. Band gaps extracted from the K/S versus E plots of Li3AlTe4O11, Li3GaTe4O11, and Li3FeTe4O11 are calulated to be 3.63, 3.85, and 1.98 eV, respectively (see the Supporting Information). The observed band gaps should be mainly attributed to the extent of Al/Ga/Fe orbital participation in conduction bands as well as the deformation arising from the TeO3 trigonal pyramid and TeO4 seesaw. An abrupt change of the band gap found in Li3FeTe4O11 may be due to the existence of 3d orbitals in Fe3+. TGA. The TGA data suggest that Li3AlTe4O11, Li3GaTe4O11, and Li3FeTe4O11 are thermally stable up to about 450, 560, and 600 °C, respectively. PXRD patterns measured on the calcined products at 600 °C show that Li3AlTe4O11 and Li3GaTe4O11 decomposed to Li2TeO3 and the corresponding metal oxides. The decomposition products at 900 °C turned out to be amorphous, attributed to the complete melting and sublimation of tellurium oxide. The TGA diagrams for all the title compounds were deposited in the Supporting Information. EPR Measurements. To determine the crystal environments of Fe3+ ions, the EPR spectrum of the polycrystalline Li3FeTe4O11 sample was measured at a frequency of ν = 9.4444 GHz and at T = 295 K. As shown in Figure 3, we observe a

induced by an exchange interaction. The three different gfactors are determined to be gx = 2.37(1), gy = 2.28(8), and gz = 1.78(4), which deviate from what is expected for a high-spin state of Fe3+ (d5; S = 5/2, g ≈ 2.003) ions in an octahedral environment. The substantial deviation from the typical g-factor of Fe3+ may be due to the site disorders of the Li(2)+ ion, which slightly modify the crystal environments of Fe3+. Magnetization Measurements. The temperature dependence of the magnetic susceptibility, χ(T), was measured for Li3FeTe4O11 in the temperature range of 2 to 300 K under an external field of H = 1000 Oe and in ZFC and FC processes. As shown in Figure 4a, we find no difference between the ZFC and

Figure 4. (a) Magnetic susceptibilities measured in an external field of H = 1000 Oe in the temperature range from 2 to 300 K. The solid line is a Curie−Weiss fit. The vertical bars mark a magnetic transition temperature. Inset shows the derivative of temperature dependence of the magnetic susceptibility, allowing identification of a magnetic transition at TN = 12 K. (b) High-field magnetization curve (M versus H) measured at 2 K in an external field up to 35 T. The open circle is the SQUID data. Inset: Zoom-in of the magnetization curve in low magnetic fields.

FC data except for the small splitting below 12 K. The former feature is typical for an antiferromagnetic system, while the latter feature may be caused by the presence of the tiny site disorders of the Li(2)+ ion. At low temperatures, a small kink of χ(T) is observed, indicative of an antiferromagnetic ordering. Taking the derivative of the magnetic susceptibility dχ/dT allows identification of an antiferromagnetic transition at TN = 12 K (see the inset of Figure 4a). For temperatures above 70 K, χ(T) follows the Curie−Weiss behavior. Fitting to the Curie−Weiss law χ(T) = C/(T − ΘCW) + χ0 yields C = 4.4(0.9) mol−1·K, ΘCW = −72(21) K, and χ0 = 0.014(2) emu/mol·Oe. The effective magnetic moment is estimated to μeff = 5.96 μB per Fe3+, which is close to the spinonly value of μtheory = 5.92 μB. The sizable negative value of ΘCW indicates a dominant antiferromagnetic coupling between the Fe3+ ions. For temperatures below 70 K, however, χ(T)

Figure 3. Derivative of the EPR absorption of the Li3FeTe4O11 powder sample measured at an X-band frequency and T = 295 K. The solid red line is a fit to a single Gaussian profile, and the solid black line represents a fit to a powder sum with three different gfactors, gx = 2.37(1), gy = 2.28(8), and gz = 1.78(4).

single EPR absorption line, while showing no additional peaks related to impurities or structural defects. The EPR signal is well described by a powder sum of three Lorentzian profiles rather than a single Gaussian profile (compare the solid lines in Figure 3). This means that the EPR signal of Fe3+ is exchangenarrowed due to fast electronic fluctuations of Fe3+ ions 5876

DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

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Inorganic Chemistry exhibits a strong enhancement, suggesting the presence of additional ferromagnetic exchange interactions. From the crystal structure, Li3FeTe4O11 can be viewed as a two-dimensional S = 5/2 square lattice, in which the Fe3+ ions are coupled to each other via an Fe−O−Li−O−Fe superexchange path along the b-axis and via Fe−O−Te−O−Fe along the c-axis. In mean-field approximation, the nearest neighbor exchange interaction (J) is given by J = −3ΘCWkB/[zS(S + 1)] = 6.17 K, where z = 4 is the number of nearest neighbors and kB is the Boltzmann constant. Isothermal magnetization curve, M(H), was measured at 2 K using both a pulsed and static magnetic field. As evident from Figure 4b, M(H) increases quasilinearly with increasing field without showing hysteric behavior. A close inspection unveils the deviation from a linear dependence. The small convex curvature of M(H) confirms the existence of the ferromagnetic exchange interactions in addition to the dominant antiferromagnetic interaction. The magnetization reaches a saturated magnetic moment of Ms = 4.5 μB at 31 T. We note that the saturated moment is smaller than the theoretical magnetic moment, Ms = gSμB = 5.3 μB, and the saturation field is also lower than the estimated value of Hs = gSJ = 47 T. The former is related to the dilution of the Fe3+ ions induced by the Li(2)+ disorders, while the latter is caused by the presence of the ferromagnetic exchange interactions. XPS Measurements. The XPS measurements on Li3FeTe4O11 reveal Fe 2p, Te 3d, and Li 1s patterns. The absolute binding energy of the first 2p3/2 peak at ca. 711.7 eV is consistent with the presence of Fe3+ in Fe2O3.45 The chemical shift of the second Fe 2p1/2 peak centered at ca. 725.23 eV is far from that for Fe 2p1/2 in Fe2O3 (724 eV). The presence of Fe2+ in Li3FeTe4O11 is excluded because the binding energy for the Fe 2p1/2 peak in Fe3O446 is nearly 0.5 eV lower than that in Fe2O3. The binding energy of the Te 3d5/2 level peak situated at ca. 575.8 eV is consistent with the data reported for the related Te4+-containing oxides.47 The Li 1s peak centered at ca. 54.1 eV suggests the presence of Li+ cations in Li3FeTe4O11. XPS data for Li3FeTe4O11 can be found in the Supporting Information. Conductivity Measurements. Since Li3FeTe4O11 contains Li+ ions inside the channels and Fe3+ transition metal ions in the framework, the ionic conductivity of Li3FeTe4O11 was examined in open air. The electrochemical impedance spectrum measured using the sintered pellet of Li3FeTe4O11 revealed that the bulk and grain boundary conductivities for Li3FeTe4O11 are 1.0 × 10−4 and 2.7 × 10−6 S cm−1, respectively (see Figure 5). Thus, the total conductivity of Li3FeTe4O11 at room temperature is estimated to be 2.3 × 10−6 S cm−1. This value is approximately 2 orders lower than that of Li1.3Ti1.7Al0.3(PO4)3 (7 × 10−4 S cm−1) that was prepared by a conventional solidstate reaction at room temperature.8 As we described in the Structures section, Li+ cations in Li3FeTe4O11 reside in the channel along the [010] direction. The Li+ cations on both tetrahedral and octahedral sites within the channels are responsible for the observed lithium ion conductivity. Specifically, Li+ cations on the edge-sharing octahedral sites reveal a Li−Li distance of about 2.7 Å. The distance is in fact associated with the dominant activation energy for the motion of Li+ cations. The connectivity between the Li+ sites gives rise to a one-dimensional Li+ ion pathway. The lower conductivity of Li3FeTe4O11 may be attributed to the lower pellet density (∼70%) and the interference of lone pairs for the traveling of Li+ ions through the channels. In order to determine an

Figure 5. Experimental (open circles) and simulated (solid curves) electrochemical impedance profile of a Li3FeTe4O11 pellet measured in open air. Inset represents an Arrhenius plot of the conductivity for a Li3FeTe4O11 pellet.

activation energy required for the transport of Li+ ions in the channels of Li3FeTe4O11, the temperature dependence of total conductivity was examined using a Li3FeTe4O11 pellet in the temperature range of 24 to 60 °C (see the inset of Figure 5). As seen in the plot, the conductivity change was fitted well to an Arrhenius plot, and the activation energy for the conductivity of Li3FeTe4O11 was calculated to be 19.9 kJ mol−1. Although the activation energy is higher than those of the superionic conductors such as Rb4Cu16I7Cl13 (7.0 kJ mol−1)48 and RbAg4I5 (7.1 kJ mol−1),49 the value is quite similar to that of Li10GeP2S12 (24 kJ mol−1).11

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CONCLUSIONS Single crystals and pure polycrystalline samples of three new lithium metal tellurites, Li3MTe4O11 (M = Al, Ga, and Fe), have been successfully prepared. The isostructural materials reveal 3D frameworks consisting of MO6 octahedra, TeO3 trigonal pyramids, and TeO4 polyhedra. The UV−vis diffuse reflectance spectra of Li3AlTe4O11, Li3GaTe4O11, and Li3FeTe4O11 show the estimated band gaps of 3.63, 3.85, and 1.98 eV, respectively. Magnetic measurements indicate that Li3FeTe4O11 has a weak antiferromagnetic coupling. The framework structures suggest that the Li+ ion mobility may occur along the channels that contain Li+ ions on both tetrahedral and octahedral sites. Impedance spectroscopy measurements on a Li3FeTe4O11 pellet revealed moderate bulk and grain boundary conductivities of 1.0 × 10−4 and 2.7 × 10−6 S cm−1, respectively, at room temperature and a lower activation energy of 19.9 kJ mol−1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00530. IR spectra, TGA diagrams, XPS data, and PXRD data (PDF) X-ray crystallographic file for Li 3 AlTe 4 O 11 and Li3GaTe4O11 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Tel (K. M. Ok): +82-2-820-5197. Fax: +82-2-825-4736. Email: [email protected]. 5877

DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

Article

Inorganic Chemistry ORCID

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Kang Min Ok: 0000-0002-7195-9089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant 2016R1A2A2A05005298). Y.Y. thanks the National Natural Science Foundation of China (21671185) for support.

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DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879

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DOI: 10.1021/acs.inorgchem.7b00530 Inorg. Chem. 2017, 56, 5873−5879