Instability of the Ionic Conductor Li6BaLa2B2O12 (B = Nb, Ta): Barium

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Instability of the Ionic Conductor Li6BaLa2B2O12 (B = Nb, Ta): Barium Exsolution from the Garnet Network Leading to CO2 Capture Cyrille Galven, Gwenael̈ Corbel, Françoise Le Berre, and Marie-Pierre Crosnier-Lopez* Université Bretagne Loire, Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, avenue O. Messiaen, 72085 Le Mans, France ABSTRACT: The instability of the two garnets Li6BaLa2B2O12 (B = Nb, Ta) has been studied on samples prepared in powder form by solid-state reaction. For this study, we coupled different techniques: powder X-ray diffraction, IR spectrometry, thermal analysis, transmission electron microscopy, and complex impedance spectroscopy. We showed that in ambient air and at low temperature ( 500°C) Ba4Sb2 O9(s) + 3CO2(g) → 3BaCO3(s) + BaSb2 O6(s) (T > 600°C)

As observed for the other oxides mentioned above, the absorption is reversible, since the mother compounds can be regenerated upon heating. © XXXX American Chemical Society

Received: September 16, 2016

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DOI: 10.1021/acs.inorgchem.6b02238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. IR spectrum (a) and PXRD pattern (b) evolution of Li6BaLa2Nb2O12 maintained for one night at 100 °C. Stability toward Humidity. The stability was checked by exposing the compound to humid air in a closed reactor. For this, a sample of about 0.5 g of Li6BaLa2Nb2O12 was introduced inside a platinum crucible placed in a reactor (125 mL) containing 10 drops of H2O. After the reactor was closed, a CO2 pressure of 10 bar was fixed before heating at 140 °C for one night. According to studies on the RP Li2SrTa2O71 and on some lithium garnets,2 these conditions allow an increase in the kinetics of the Li+/H+ exchange reaction and a quick determination of whether or not the garnet is stable against humid air. In the case of instability, the PXRD pattern analysis performed on the sample after the test revealed the simultaneous presence of Li2CO3 (spontaneous carbonation of the LiOH by the atmospheric CO2) and of a garnet phase with a larger cell parameter according to the reaction

lithium occupancy. The two garnets studied in this work Li6BaLa2B2O12 (B = Nb, Ta) belong to this garnet class.

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EXPERIMENTAL SECTION

Synthesis. Li6BaLa2B2O12 (B = Nb, Ta) was obtained in powder form from solid-state reactions using the dehydrated carbonates Li2CO3 (Merck 99%) and BaCO3 (Prolabo 99.5%), dehydrated La2O3 (Chempur 99.9%, treated at 1000 °C for one night), and Nb2O5 (Strem Chemicals 99.9%) or Ta2O5 (Alfa Aesar 99.85%). All of the starting materials were mixed in a stoichiometric ratio except for Li2CO3 (an excess of 20% must be used to obtain the pure garnet phase). The resulting mixture was pressed into pellets (∼1 g) which were heated for 6 h at 500 °C and 6 h at 700 °C. After it was ground, the powder was compacted again and heated for 6 h at 800 °C. A third step of 24 h at 900 °C was required to obtain the desired pure garnet. A last brief heating at higher temperature (1 or 2 h at 950 °C) allowed us to improve the crystallization of the product. Characterizations. Powder X-ray Diffraction (PXRD). The phase purity and the cell parameters were determined from PXRD patterns. These patterns were recorded in air with a PANalytical X’pert Pro diffractometer equipped with the X’Celerator detector and using Cu Kα radiations. The experimental conditions for the data collection corresponded to 10−130° 2θ angular range and a 0.017° step scan increment (acquisition time 4 h). For the temperature measurements, the diffractometer was equipped with a high-temperature Anton Paar HTK12 chamber. The patterns were collected in air or under a N2 flow with the following conditions: 15−60° 2θ angular range, 0.017° step scan increment (acquisition time 40 mn). The nitrogen used was U quality with a water content ≤5 ppm. The refinements were performed with Fullprof16 using the whole pattern fitting mode. The background points were manually picked up before being refined, while the peak shape was modeled by a pseudoVoigt function. IR Spectroscopy. A Bomem Michelson MB120 FTIR spectrometer with a diamond-anvil cell as a microsampling device was used for infrared spectroscopy. The spectral resolution was 4 cm−1 in the 650− 4000 cm−1 range. Thermal Analysis. The carbonation reaction of Li6BaLa2Nb2O12 was performed on about 150 mg of sample using a thermogravimetric analyzer (TGA Jupiter STA 449 F3 Netzsch) under a CO2/synthetic air gas flow (1.25% CO2; 1 mL min−1) with a heating rate of 5 °C/min from room temperature to 950 °C (Al2O3 crucible). Transmission Electron Microscopy (TEM). The TEM study was carried out using a JEOL 2100 electron microscope operating at 200 kV and equipped with a side-entry ±35° double tilt specimen holder. Chemical analyses were done on at least 15 crystallites with a JEOL JED-2300T energy dispersive X-ray (EDX) spectrometer coupled with the microscope. The sample was prepared as follows: small crystallites were crushed and ultrasonically dispersed in ethanol; one drop of this suspension was then deposited on a carbon-coated copper grid. For this study, two samples were analyzed: the mother form Li6BaLa2Nb2O12 and the sample Li6BaLa2Nb2O12 heated for 48 h at 650 °C.

Li6BaLa 2Nb2O12 + x H 2O → Li6 − xHxBaLa 2Nb2O12 + x LiOH Conductivity Measurements. Impedance spectroscopy measurements were carried out on a Li6BaLa2Nb2O12 pellet freshly annealed at 975 °C for 30 min. The pellet (∼4.9 mm in diameter and ∼1.7 mm in thickness) was obtained with a relative density of 75% (with respect to the theoretical absolute value calculated from X-ray diffraction data). This relatively low compacity is due to the difficulty in sintering this garnet, as has already been observed by Awaka et al. on the tantalum variety.14 Thin Pt film electrodes were deposited by RF sputtering on both flat faces of a “sintered” pellet. Conductivity was measured by using a classical symmetric two-electrode electrochemical cell connected to a Schlumberger Solartron 1296 dielectric interface and a Schlumberger Solartron 1260 frequency response analyzer. Complex impedance spectra were recorded over the 10 MHz to 0.05 Hz range (ac signal amplitude of 50 mV, 40 points/decade) under a dry nitrogen flow at 35, 300, and 500 °C (thermal equilibration for 35 min at each temperature) as a function of the annealing time every 40 min (the time t = 0 corresponds to the beginning of the experiment). The nitrogen used (U quality) for conductivity measurements was dehumidified before being placed into contact with the sample (circulated on a desiccant, silica gel).

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RESULTS AND DISCUSSION Powder X-ray Diffraction. The powder X-ray diffraction pattern of fresh Li6BaLa2Nb2O12 sample reveals that the compound is well crystallized and pure. Indeed, all the diffraction lines of the PXRD pattern are thin and can be indexed in the cubic Ia3̅d space group with the refined cell parameter a = 12.9941(1) Å (Rp = 11.0; χ2 = 1.68). Surprisingly, this cubic cell parameter is larger than the values found in the literature, which show in addition an important discrepancy: 12.868(1), 12.8893(2), and 12.927(3) Å, respectively, in refs 12, 15, and 17. However, our value is in good agreement with the result found by O’Callaghan et al.18 for Li6BaLa2Ta2O12 (13.0229(3) Å). This agreement is generally observed for isostructural phases containing tantalum or niobium, since the ionic radii of the two cations Ta5+ and B


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Inorganic Chemistry Nb5+ are equal (0.64 Å in an octahedral environment19). For example, this is the case for the two garnets Li5La3Nb2O12 and Li5La3Ta2O12 (a = 12.7943(1) and 12.8065(1) Å,20 respectively). Stability of Li6BaLa2Nb2O12 at Low Temperatures. As performed for Li7La3Sn2O12,21 we decided to regularly record IR spectra and PXRD patterns on a fresh Li6BaLa2Nb2O12 sample in order to check its instability versus time of exposure to ambient air. In contrast to what was observed in the case of Li7La3Sn2O12,21 the IR spectra and the PXRD pattern do not show evolution even after 1 month of ambient air exposure. This means that, if an ionic Li+/H+ exchange takes place, it is slower in Li6BaLa2Nb2O12 than in Li7La3Sn2O12. In order to accelerate the exchange, the sample was placed in an oven at 100 °C in ambient air for only one night. The IR spectra recorded after the night shows that the characteristic bands of a CO32− group (1440 and 865 cm−1) have significantly increased in intensity (Figure 1a) while the hkl lines of the PXRD pattern were become wider and shifted toward low 2θ values (Figure 1b). In addition, we have observed the presence of new hkl lines which have been unambiguously attributed to Li2CO3. These results show that a spontaneous Li+/H+ exchange is possible in Li6BaLa2Nb2O12 but with a kinetics slower than that in Li7La3Sn2O12. This spontaneous exchange has been also confirmed by placing the product under a humid CO2 pressure at 140 °C for one night (see the Experimental Section for details). The PXRD pattern recorded after the test shows the typical hkl lines corresponding to the garnet structure with small additional lines due to the presence of Li2CO3 (Figure 2).

Stability of Li6BaLa2Nb2O12 at High Temperature. Considering the two papers concerning the CO2 absorption and desorption of two oxides based on barium,9,10 we have decided to test the stability of the garnet under a dry CO2/air flow. A fresh sample of Li6BaLa2Nb2O12 was heated under a synthetic air flow containing 1.25% CO2. Figure 3 shows the

Figure 3. Mass variation of a fresh Li6BaLa2Nb2O12 sample heated under CO2/synthetic air flow showing CO2 absorption (600 °C < T < 800 °C) and CO2 desorption (T > 800 °C).

TGA curve registered during the heating stage: one can see that, below 600 °C, the mass of the sample is nearly constant (except for a very slight increase around 500 °C) and strongly increases above 600 °C until 750 °C. This can be unambiguously attributed to a CO2 absorption, since no weight variation is observed for the same experiment performed under a synthetic air flow. Above 750 °C (Figure 3) and still under the CO2/air flow, the weight decreases due most probably to CO2 desorption, thus meaning that Li6BaLa2Nb2O12 garnet can reversibly absorb or desorb CO2 depending on the temperature. This absorption reaches its maximum around 700 °C. A PXRD pattern was then recorded on the sample just after the TGA measurement and compared with that of the mother phase (Figure 4). We observed first that the hkl lines characteristic of a garnet phase were still present with, however, a widening of the peaks leading to the disappearance of the Kα2 contribution. This observation means that the crystalline quality of the compound declined during the heating under a CO2 atmosphere. Furthermore, the PXRD pattern revealed several additional lines which were attributed both to BaCO3 and to a 3D cubic perovskite. A full pattern matching refinement of the PXRD pattern with the three phase options (garnet, perovskite, and BaCO3) led to the cell parameter a = 12.886(2) Å for the garnet and a = 4.084(1) Å for the perovskite (Rp = 22.8; χ2 = 2.71). Concerning BaCO3, the refined cell parameters are in agreement with those in ref 26. A decrease in the garnet cell parameter was thus observed: 12.886(2) Å in comparison to 12.9941(1) Å for the mother form. At this stage, though the formulation of this new garnet phase remains unknown, we can conclude unambiguously that the reduction of the cell parameter results at least in part from a decrease in the barium content inside the garnet framework. This means that the barium expulsion of the structure is possible, as observed in Ba 2 Fe 2 O 5 9 and Ba 4 Sb 2 O 9 , 10 and consequently that

Figure 2. Zoom of the observed, calculated and difference PXRD patterns of Li6BaLa2Nb2O12 after exposition to humid air in a close reactor showing the presence of Li2CO3. The vertical bars are related to the calculated Bragg reflection positions corresponding to the garnet form (top) and Li2CO3 (bottom).

As observed in Figure 1b, all of the garnet hkl lines are wider after the exchange than before. The refinement of the PXRD pattern (Rp = 15.3; χ2 = 1.80) reveals an increase in the cell parameter (a = 13.0635(2) Å) in comparison with that of the mother phase. This is the direct signature of the replacement of strong Li−O bonds by weak hydrogen H−O···H bonds associated with a Li+/H+ exchange, as noted in the literature for garnets or perovskites. 1,2,22−25 The instability of Li6BaLa2Nb2O12 in ambient air is consistent with our conclusion made in ref 2: if the lithium quantity is greater than what can be accommodated on the tetrahedral site (24d site in the Ia3̅d space group), the garnet is sensitive to humidity. This corresponds to more than three lithium ions per formula unit, which is the case for Li6BaLa2Nb2O12. C


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Inorganic Chemistry

quantity of the mother garnet Li6BaLa2Nb2O12 (denoted G1) decreases in favor of G2 until G1 has completely disappeared (∼500 min). In addition, we observe the appearance of the 3D perovskite previously mentioned (asterisk). BaCO3 becomes visible only at 30 °C after cooling. The cell parameters of the two garnets have been refined and followed versus time at 650 °C (Figure 6). This figure shows

Figure 4. PXRD patterns of Li6BaLa2Nb2O12 before (a) and after the TGA measurement performed under CO2/synthetic air (b), showing the presence of three phases: garnet, perovskite (★), and BaCO3 (□) .

Li6BaLa2Nb2O12 is unstable at high temperature between 600 and 750 °C in the presence of CO2. Knowing now the temperature interval corresponding to the instability of the garnet Li6BaLa2Nb2O12, we have followed the PXRD pattern evolution of a sample maintained at 650 °C under an ambient air flow (Figure 5). Rapidly (after only 15 min), we observe the appearance of a second garnet named G2 (closed circles) of smaller cell parameter. Progressively, the

Figure 6. Cubic cell parameter evolution of the two garnets formed on heating of Li6BaLa2Nb2O12 at 650 °C (G1, ▲; G2, ●) (corresponding values at room temperature are shown as an open triangle and circle, respectively).

first that the cell parameter of the mother form Li6BaLa2Nb2O12 (closed triangles) progressively decreases until the compound completely disappears (t ≈ 500 min). This means that its composition varies continuously on heating at 650 °C. In the same time, the G2 cell parameter slightly increases at the beginning (t ≤ 200 min) and remains constant until the end of the experiment. This suggests that G2 must rapidly reach a fixed composition during the recording. After cooling, the refined G2 cell parameter at room temperature corresponds to a = 12.8637(4) Å (Rp = 16.8; χ2 = 2.47). At this stage, it is possible to propose a G2 formulation from its cell parameter. Indeed, O’Callaghan et al.18 have showed the existence of the solid solution Li5+xBaxLa3−xTa2O12, (0 ≤ x ≤ 1.6) with a cell evolution perfectly modeled at room temperature by the linear equation a (Å) = 0.2164xLi + 12.805 for 0 ≤ x ≤ 1 (linear correlation coefficient R2 = 0.9997). Thus, if we consider also a perfect linear cell parameter evolution for the niobium counterpart between Li5La3Nb2O12 (12.7943(1) Å20) and Li6BaLa2Nb2O12 (a = 12.9941(1) Å, this work), we obtain the following equation: a (Å) = 0.1998xLi + 12.7943. From this law, we can then reasonably propose the G2 formulation Li5.34Ba0.34La2.66Nb2O12 at room temperature. It is also possible to approach the G1 and G2 compositions evolution versus time at 650 °C. For this, we assume that the G1 formulation does not change between room temperature and the first point of the G1 graph (t = 15 min, Figure 6) at 650 °C. The same assumption is made for G2: its formulation deduced at room temperature (Li5.34Ba0.34La2.66Nb2O12) is the same as that at 650 °C for the last point of the G2 graph (t = 695 min, Figure 6). These hypotheses allow us to find the linear equation a (Å) = f(xLi) at 650 °C, a (Å) = 0.1883xLi + 12.937, and to deduce the G1 and G2 formulation evolution. We thus obtain that xLi for the G1 garnet regularly decreases from 1 to

Figure 5. PXRD pattern evolution of Li6BaLa2Nb2O12 annealed at 650 °C under ambient air flow and showing the appearance of a new G2 garnet (●), a perovskite (★), and BaCO3 (□) phases. D


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Inorganic Chemistry

lanthanum slightly increase. When we used the mother form to calibrate the molar ratio La/Nb (theoretical value equal to 1), we found for the new G3 garnet an La/Nb ratio equal to 1.4 ± 0.1 (average calculated on 18 crystallite analyses), corresponding then to the formulation Li5.2Ba0.2La2.8Nb2O12. This formulation is in very good agreement with that deduced from the cell parameter (Li5.18Ba0.18La2.82Nb2O12), thus confirming the possibility to use the cell parameter evolution between Li5La3Nb2O12 and Li6BaLa2Nb2O12 to determine the formulation of the new garnet obtained after barium expulsion. It is important to specify here that the observation of a phase mixture (here, three phases) is complicated in a TEM. Consequently, each crystallite analyzed by EDX was also observed by using diffraction mode (selected area electron diffraction, SAED), allowing us to associate unambiguously the chemical analysis results with the structural type, garnet or cubic perovskite. Finally, to regenerate the mother garnet Li6BaLa2Nb2O12, the sample heated for 48 h at 650 °C was then annealed at 900 °C for 24 h. This temperature was chosen from the TGA experiment, which revealed that, above 750 °C, the weight of the sample decreased due to CO2 desorption. The PXRD pattern recorded subsequently shows the presence of only two phases: a garnet and the 3D cubic perovskite. The disappearance of BaCO3 and the increase again of the refined cell parameter for the garnet (a = 12.9428(2) Å, Rp = 12.4; χ2 = 1.80) indicate that it is possible to reenter the barium in the structure. However, the mother garnet cannot be restored, since the cell parameter stays smaller than that of the mother garnet. The formulation calculated from the cell parameter corresponds to Li5.74Ba0.74La2.26Nb2O12. This allows us to conclude that the reaction (CO2 absorption/CO2 desorption correlated to Ba expulsion/Ba reintroduction) is not completely reversible. All of these results are gathered in Figure 8.

0.69 (from Li6BaLa2Nb2O12 to Li5.69Ba0.69La2.31Nb2O12) while the G2 composition varies from Li5.20Ba0.20La2.80Nb2O12 to Li5.34Ba0.34La2.66Nb2O12. Concerning the perovskite phase, we observed a constant cell parameter during the entire experiment (a ≈ 4.139 Å), thus meaning that its composition does not change. As it is possible to expell barium of the mother garnet Li6BaLa2Nb2O12, we have tried to completely extract the barium by heating at 650 °C in air a fresh sample for a longer duration (48 h). The PXRD pattern performed after this experiment reveals that the mother compound Li6BaLa2Nb2O12 is entirely transformed into a garnet of lower cell parameter, the 3D cubic perovskite previously mentioned, and BaCO3. However, though the refined garnet cell parameter (a = 12.8299(6) Å; Rp = 12.6; χ2 = 1.85) is lower than the previous value (a = 12.8637(4) Å), it remains still slightly higher than that of Li5La3Nb2O12 (12.7943(1) Å).20 This means that some barium is still present in the structure. As performed for the previous G2 garnet, we can propose a chemical formulation for this new garnet (called G3 hereafter) from the refined cell parameter: Li5.18Ba0.18La2.82Ta2O12. Additional heating at 650 °C did not allow us to obtain a full barium exsolution. To confirm unambiguously the previous formulation, we decided to perform a TEM study of the mother form Li6BaLa2Nb2O12 and of the sample heated for 48 h at 650 °C (mixture of this new garnet G3, a 3D cubic perovskite, and BaCO3). First, in the mother form Li6BaLa2Nb2O12, all of the crystallites observed revealed systematically the simultaneous presence of barium, lanthanum, niobium, and oxygen (Figure 7a). In the sample heated at 650 °C, EDX analyses revealed

Figure 7. EDX analysis of two garnet crystallites corresponding to (a) fresh Li6BaLa2Nb2O12 and (b) Li6BaLa2Nb2O12 heated for 48 h at 650 °C and showing the strong decrease in the barium content due to the annealing. Figure 8. Cubic cell parameter evolution of Li6BaLa2Nb2O12 versus temperature annealing, showing the partial barium exsolution (Li5La3Nb2O12: synthesized by us).

three different chemical phases: the first containing only barium and oxygen and corresponding then to BaCO3, the second containing barium, niobium, and oxygen which corresponds most probably to the perovskite phase, and the third containing lanthanum, niobium, oxygen, and only a small amount of barium (Figure 7b). Unfortunately, as EDX does not allow depiction of the presence of atoms as light as lithium, we cannot confirm the presence of lithium in the perovskite phase as in the garnet phase. As one can see by comparing simply parts a and b of Figure 7, the intensities of the characteristic peaks of barium decrease strongly while those characteristic of

This instability at high temperature is an important point, as we know that before electrical characterization pellets are generally painted with Au paste on both sides and then annealed in air around 700 °C for 1 h, as noted in ref 17. Then, we performed a last heating at 700 °C for 1 h on a fresh sample of Li6BaLa2Nb2O12. As one can see in Figure 9, a second garnet phase with a smaller cell parameter has already appeared. Under E


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Inorganic Chemistry

Figure 9. PXRD pattern of (a) a fresh sample of Li6BaLa2Nb2O12 and (b) the same sample just after an annealing of 1 h at 700 °C. (b) shows the presence of a second garnet G2 (arrows), BaCO3 (□), and a cubic perovskite (★).

these conditions, the electrical conductivity measurements performed on such pellets are not characteristic of the Li6BaLa2Nb2O12 garnet, since the sample is in fact a mixture of several phases. Knowing now the high instability in air of this garnet, we then chose to perform ionic conductivity measurements under an inert atmosphere such as N2. Ionic Conductivity and Stability Study under a Dry N2 Atmosphere. In order to complete the stability study of Li6BaLa2Nb2O12, the effect of isothermal annealing under a flowing dry nitrogen atmosphere on both conductivity and structure was carried out by complex impedance spectroscopy and temperature-controlled X-ray diffraction. In most studies on the lithium ion conductor Li6BaLa2Nb2O12, the cationic conductivity is frequently measured in the temperature range room temperature to 300 °C in air atmosphere and never above 300 °C without any explanation given.12 In addition, studies concerning the ionic conductivity of Li6BaLa2Nb2O12 found in the literature show a log σT = f(1000/T) evolution which is not linear over this temperature interval.12,15,17 We also note that the same behavior has already been observed for the tantalum analogue Li6BaLa2Ta2O12.13,27 Moreover, before the measurements, the pellets were painted with Au paste on both sides and then annealed at 700 °C for 1 h to remove organic solvents.12,13,17 Taking into account our observations on the stability of Li6BaLa2Nb2O12, we have chosen to perform our study under a dry N2 atmosphere for T ≤ 500 °C and to deposit thin platinum film electrodes by RF sputtering, thus avoiding heating the compound in the instability temperature range. The measurements were made at three temperatures (35, 300, and 500 °C), and for each temperature, the evolution of the ionic conductivity versus time was followed. As shown in the Nyquist plot of the complex impedance spectrum recorded at 35 °C (Figure 10), a depressed semicircle in the frequency range 106−104 Hz and a small arc at lower frequency are noted. The presence of a straight line with a slope of 45° at frequency lower than 10 Hz was ascribed to the

Figure 10. Nyquist representation of observed complex impedance spectrum (dots) collected on a ceramic of Li6BaLa2Nb2O12 under dry nitrogen at 35 °C (numbers correspond to frequency logarithms) and comparison with the spectrum calculated (gray line) from the equivalent electrical circuit.

polarization phenomenon associated with lithium ion conduction between the sample and the platinum electrodes. The whole complex impedance spectra recorded at 35 °C as a function of time were satisfactorily fitted with a series combination of two R//CPE elements (where R is a pure resistance and CPE is a constant phase element), a Warburg diffusion element, and the wire inductance L. Fits were performed by using the “calc-modulus” data weighting mode (each data point weight is normalized by its magnitude) of the Z-view 3.4e software.28 The depressed semicircle was assigned to the bulk response on the basis of the value of capacitance obtained by fitting (∼1.7 × 10−11 to 2.4 × 10−11 F). The small arc with a capacitance of ∼1−34 μF is ascribed to phenomenon at the Li6BaLa2Nb2O12 electrolyte/platinum electrode interface. However, no grain boundary contribution was detected over the whole complex impedance spectra. At 35 °C, the bulk conductivity σ of Li6BaLa2Nb2O12 garnet sample at t = 0 is 1.08 × 10−5 S cm−1. This value is surprisingly in good agreement with the value deduced from the temperature dependence of the conductivity measured in air by Thangadurai et al.12 and Zhong et al.15 F


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Inorganic Chemistry The same experiment under dry nitrogen was performed at 300 and 500 °C. At 300 °C, the bulk conductivity σ measured at t = 0 is 3.65 × 10−3 S cm−1, which is 5 times lower than the conductivity σ measured in air in ref 12 but slightly higher than that in ref 15. At 500 °C, the bulk conductivity σ(t = 0) is 2.2 × 10−2 S cm−1. From these three σ(t = 0) values, the conventional relation log([σ(t = 0)]T) versus 1000/T has been plotted (not given here). The evolution is linear in this temperature range (correlation coefficient r2 of 0.9997). This would mean that, in the temperature range 35−500 °C, the conductivity obeyed the Arrhenius equation [σ(t = 0)]T = σ0 exp[−Ea/RT] with activation energy Ea = 0.37 eV and pre-exponential log(σ0) = 3.66 S K cm−1. The value of activation energy we have determined under an inert atmosphere is lower than the value calculated in air (0.44 eV) for the same compound Li6BaLa2Nb2O12 in ref 12. In addition, in this reference, the conductivity of Li6BaLa2Nb2O12 in air obeys the Arrhenius equation only in the temperature range 20−200 °C and deviates negatively from this Arrhenius regime above 200 °C. The study of the ionic conductivity versus time (Figure 11) shows that the annealing of the pellet at 35 and 300 °C for 20 h

Figure 12. PXRD pattern evolution of Li6BaLa2Nb2O12 annealed at 500 °C under N2 flow and showing the formation of the G2 garnet (●) due to barium exsolution.

these operating conditions, the modification of the PXRD pattern is much less pronounced than in the case of the previous study performed in ambient air at 650 °C (Figure 5). Nevertheless, small additional hkl lines are clearly visible for annealing times t ≥ 15 min. These lines are characteristic of a second garnet and of the 3D cubic perovskite. This means that, even under an inert atmosphere, Li6BaLa2Nb2O12 is unstable, this instability resulting most probably because of the temperature than because of the atmosphere. We can then conclude that the strong decrease in the bulk conductivity observed at 500 °C results in fact from the transformation of the mother garnet due to its instability at this temperature. We can also note that impedance spectroscopy measurements are very sensitive to the sample Li6BaLa2Nb2O12 transformation even at low barium exsolution rate. Results on the Tantalum Analogue Li6BaLa2Ta2O12. As most of the studies on the Li+ ionic conduction on the Li6BaLa2M2O12 concern the tantalum species, we have performed the same tests on Li6BaLaTa2O12. All of our results confirm that this compound is also unstable to humid air at low temperature with a spontaneous Li+/H+ exchange and the formation of Li2 CO 3 with the lithium released. The thermodiffraction experiment performed in air at 650 °C (Figure 13) has also revealed that the exsolution of the barium from the garnet network is possible, this leading, as for the niobium counterpart, to the formation of a second garnet (called previously G2), a cubic 3D perovskite, and BaCO3. While as soon as the temperature reaches 650 °C hkl lines associated with the G2 garnet appear, the kinetics of the reaction are however lower for the tantalum analogue. Indeed, 22 h is necessary for the garnet G1 to totally disappear, vs ∼8 h for the niobium garnet.

Figure 11. Time evolution of the conductivity of Li6BaLa2Nb2O12 during annealing under dry nitrogen at 35, 300, and 500 °C. The inset displays the time evolution of the relative conductivity.

leads to a slight decrease in the conductivity (respectively 6.5% and 1.7%). After 85 h at 300 °C, the decrease is equal to 4.3%. These variations are relatively small in comparison to that observed at 500 °C. Indeed, a strong decrease by 60% of the starting bulk conductivity occurs within the first 5 h of annealing at 500 °C. After 85 h of isothermal annealing at 500 °C, the conductivity σ is 4.75 × 10−3 S cm−1 (reduction by 78.4% of the starting σ(t = 0) conductivity), which is close to the value measured at only 300 °C under the same inert atmosphere. This huge decrease in conductivity suggests that even under a dry inert (or CO 2 -free) atmosphere, Li6BaLa2Nb2O12 also becomes unstable at high temperature. To determine if the conductivity decrease at 500 °C under a flowing nitrogen atmosphere originated from progressive barium depletion with the annealing time, the effect of this isothermal annealing on the structure of Li6BaLa2Nb2O12 (freshly prepared raw powder) was studied by in situ temperature-controlled X-ray diffraction. The diffraction patterns were collected at 500 °C for 60 h every 15 min. The evolution with time of X-ray diffraction patterns recorded at 500 °C is displayed in Figure 12. We observe first that, with

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CONCLUSION This study has revealed that the ionic garnet conductors Li6BaLa2M2O12 (M = Nb, Ta) are unstable at low temperature in the presence of humidity, since they undergo a spontaneous ionic Li+/H+ exchange. This behavior is common to all lithium garnets for which the lithium quantity is greater than what can G


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Inorganic Chemistry

oven may be an accelerating factor of modification). In addition, it shows that when measurements require a specific preparation of the sample (pelletization and sintering for example), an analysis of the sample is necessary before measurements to confirm that the mother form is still present.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.-P.C.-L.: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 13. PXRD pattern evolution of Li6BaLa2Ta2O12 annealed at 650 °C in ambient air showing the formation of the G2 garnet (●) due to barium exsolution.

be accommodated on the tetrahedral site. The thermal X-ray diffraction experiments performed on Li6BaLa2Nb2O12 have also shown that the compound is unstable at high temperature (T > 500 °C), regardless of the atmosphere. This instability results in a barium expulsion of the garnet framework followed by a carbonation of the barium released in BaCO3 form. Thanks to the PXRD experiments, we were able to identify the phases formed upon carbonation, leading to an understanding of the CO2 absorption mechanism: the partial barium exsolution lead to the coexistence of two garnets of formulation type Li5+xBaxLa3−xNb2O12, one being rich in barium and the second rich in lanthanum. In addition to these two garnets, the carbonate BaCO3 and a 3D perovskite phase are formed. Unfortunately, the determination of the exact compositions of the garnets via a PXRD structural study is impossible due to the presence of light atoms (lithium) and the overly close X-ray scattering factors of lanthanum and barium. Nevertheless, chemical formulations of the garnets have been proposed according to the cell parameter evolution in the solid solution Li5+xBaxLa3−xNb2O12 and EDX results. A neutron diffraction study could overcome this problem and confirm these chemical formulations. Finally, even if this instability leads to a CO2 absorption behavior, it seems unreasonable to consider this garnet as a CO2 absorbent. Indeed, we have shown that if the CO2 desorption was complete, it was impossible to entirely regenerate the mother compound. This indicates the lack of total reversibility of the behavior. From a more general point of view, this work allows highlighting the importance of sample storage conditions (an H


Article

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Instability of the Ionic Conductor Li6BaLa2B2O12 (B = Nb, Ta): Barium

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