Instability of Lithium Garnets against Moisture. Structural

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Instability of Lithium Garnets against Moisture. Structural Characterization and Dynamics of Li7‑xHxLa3Sn2O12 and Li5‑xHxLa3Nb2O12 Cyrille Galven,† Jens Dittmer,† Emmanuelle Suard,‡ Françoise Le Berre,*,† and Marie-Pierre Crosnier-Lopez† †

LUNAM Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France ‡ Institut Laue-Langevin, 6 rue G. Horowitz, 38042 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: We have recently demonstrated the instability of Li7La3Sn2O12 garnet in humid atmosphere: a spontaneous and reversible ionic exchange Li+/H+ occurs rapidly, leading to the protonated garnet Li7‑xHxLa3Sn2O12. In the present article, we show that this instability cannot be generalized to all lithium garnets. We have tested different garnets with various cell parameters, lithium quantity, and lithium distribution and have observed that the Li+/H+ exchange feasibility is directly connected to the lithium stoichiometry: if the concentration of Li+ ions is greater than what can be accommodated on the tetrahedral site commonly occupied, meaning more than three lithium ions per formula, the garnet is sensitive to humidity. Structures determined by powder neutron diffraction are presented for two exchanged garnets: Li7‑xHxLa 3Sn2O12 and Li5‑xHxLa3Nb2O12 obtained from ionic exchange in ethanol and benzoic acid. For the Sn one, the ionic Li+/H+ exchange is associated with a transition from a tetragonal (I41/acd) to a cubic (Ia3̅d) cell, while for the Nb phase, the use of a noncentrosymmetric space group (I213), confirmed by second harmonic generation (SHG) test, is essential to describe the structure. The work is completed by 6Li, 7Li, and 119Sn solid state Nuclear Magnetic Resonance (NMR) of the tin compounds. The impact of the Li+/H+ exchange on the dynamics of the lithium ions has been investigated by 7Li relaxation, and the dynamics of protons and lithium ions in the exchanged phase have been compared. KEYWORDS: garnet oxide, neutrons powder diffraction, ionic exchange, proton, lithium, solid state NMR

1. INTRODUCTION The term “garnets” originally corresponds to natural silicates, and their general formula is A3B2C3O12 where A, B, and C refer, respectively, to 8-coordinate, octahedral and tetrahedral cation sites. Such compounds crystallize most often in a cubic body centered cell (a parameter close to 13 Å) which contains eight formula units. Garnets are well-known to exhibit various beneficial properties for some areas in the field of materials science as solid-state lasers1 or magnetic materials.1 This results from the structural flexibility which allows various chemical substitutions giving rise to a large number of phases. Among them, lithium garnets are of great interest since they exhibit a high Li+ conduction2−5 making them good candidates for electrolytes in batteries. Until now, all lithium garnets were considered chemically stable when being exposed to moisture and air,6,7 a desirable characteristic for a potential use as electrolytes in rechargeable lithium batteries.8 However, we have recently published a study showing the instability of the Li7La3Sn2O12 garnet in air.9 This behavior consists in fact of a spontaneous Li+/H+ ionic exchange leading to the protonated © 2012 American Chemical Society

garnet Li7‑xHxLa3Sn2O12, while the lithium released under LiOH form is combined with the atmospheric CO2 giving Li2CO3. This ionic exchange, which works through a topotactic reaction, is linked to a structural transition from tetragonal (I41/acd) to cubic (Ia3̅d). We also showed that this reaction is reversible since the lithium garnet can be restored by a heating at 900 °C for 30 min. Interestingly, Wang et al. have tested the stability of another garnet, Li5La3Ta2O12, in a humid environment and observed it was also unstable in air at room temperature.10 In the present article, we demonstrate that the instability under humid atmosphere cannot be generalized to all lithium garnet-type oxides and determine the parameters responsible of this behavior. In addition, we present structural studies performed by powder diffraction experiments (neutron and X-ray) on two protonated forms: Li7‑xHxLa3Sn2O12 - for which we recently published our first results from X-ray Received: March 27, 2012 Revised: June 7, 2012 Published: June 12, 2012 3335

dx.doi.org/10.1021/cm300964k | Chem. Mater. 2012, 24, 3335−3345

Chemistry of Materials

Article

Table 1. Tested Garnets for the Li+/H+ Exchangea cell parameter (Å)

a

compound

before

after

Li+/H+ exchange

Li3Gd3Te2O12 Li6CaSm2Nb2O12 Li3Nd3Te2O12 Li5Nd3Ta2O12 Li5Nd3Sb2O12 Li6CaLa2Nb2O12 Li5La3Nb2O12 Li5La3Ta2O12 Li6SrLa2Nb2O12 Li7La3Sn2O12* Li7La3Zr2O12

12.3937(1) 12.5495(1) 12.5621(5) 12.6268(1) 12.6710(2) 12.7232(2) 12.7972(2) 12.8104(1) 12.8360(2) 13.1281(1) ; 12.5540(1) 12.964(1)

12.3931(1) 12.5931(2) 12.5590(1) 12.6580(1)

no yes no yes

12.8076(2) 12.8603(1) 12.8546(1) 12.9142(4) 12.9892(1) 13.0667(2)

yes yes yes yes yes yes

Li sites

mean Li−O distances (Å)

ref

24d (100%) 24d (26%) + 96h (44%) 24d (100%) 24d (unknown) + 48g (unknown) 24d (62%) + 96h (26.2%) 24d (61%) + 96h (34.8%) 24d (83.6%) + 48g (11%) + 96h (15.2%) 24d (80.2%) + 48g (13.9%) + 96h (14.7%) 24d (59%) + 96h (35.2%) 8a (100%) + 32g (100%) + 16f (100%) 24d (94%) + 96h (34.9%)

1.8813 1.843−2.22 1.9286 1.898−2.225 1.9087−2.25 1.8953−2.25 1.9364−2.269−2.298 1.9344−2.265−2.298 1.9112−2.28 1.897−2.304−2.264 1.9077−2.290

16 17 16 18 19 20 6 6 20 21 22

S.G.: Ia3̅d except for * (I41/acd). 2.2.2. Second Harmonic Generation Test. Second Harmonic Generation (SHG) test has been performed at room temperature on a polycrystalline sample with the use of the fundamental wave (1064 nm) emitted by a Q-switched laser (JDS Uniphase). 2.2.3. Chemical Analysis. The chemical formula of the protonated garnets has been determined, in addition to the TGA measurement, from the Li content analysis of the filtrate obtained from the washing stage. This was performed using the flame emission spectrometer Sherwood 410 and from standard solutions prepared with the same ratio water/ethanol and the same benzoic acid concentration as the analyzed solution. 2.2.4. Powder Diffraction Experiments. Powder X-ray diffraction (PXRD) was used to check the purity and the quality of the samples (PANalytical X’pert Pro diffractometer equipped with the X’Celerator detector using CuKα radiation), while powder neutron diffraction (PND) was used to determine the structure of the partially protonated garnets. The neutron diffraction data have been recorded at ILL Grenoble on the D2B high-resolution neutron diffractometer with the following experimental conditions: λ = 1.59543(1) Å; angular range (°2θ) = 0.10−159.95; step scan increment (°2θ) = 0.05; counting time = 5 h. The data were analyzed by means of the Rietveld method14 using FullProf software15 with a pseudo-Voigt function to describe the peak shape. The background level was determined manually before being refined. In the case of Li5‑xHxLa3Nb2O12, the PXRD data used with the PND ones for the structural study have been recorded from 15 to 130° 2θ with a step of 0.008° 2θ and a counting time by step of 260 s. 2.2.5. Solid-State NMR. All NMR experiments have been performed on a Bruker Avance III spectrometer with 300 MHz proton frequency, equipped with a 4 mm WVT probehead with X and H channel. 6Li and 119Sn spectra were acquired under MAS with 10 kHz spinning rate. For a 1H−6Li HETCOR correlation spectrum the contact time was 1 ms, and homonuclear FSLG decoupling was applied during the 1H chemical shift evolution. All 7Li experiments were done under static conditions. 7Li spectra were acquired with a 90°−τ−90° echo sequence, while the signals of the relaxation experiments were recorded without echo. For the rotating frame (R1ρ) relaxation experiments a spin lock of ω1 = 2π × 32.5 kHz was applied. 6/7Li spectra were referenced to a aqueous solution of 1 M LiCl.

diffraction data - and Li5‑xHxLa3Nb2O12. In this particular case, the ionic Li+/H+ exchange is associated with a transition toward a noncentrosymmetric space group for the resulting lithium hydrogarnet. Nuclear Magnetic Resonance (NMR) is a complementary technique for both the investigation of structural features that do not necessarily follow the periodicity of the lattice and the investigation of ionic motion in an ion conductor.11 It allows us to complement the previous study of the conductivity of the Sn phases9 from a different point of view. In addition, there are already studies of the dynamics of other garnets and related compounds, in particular on Li7La3Zr2O12,12,13 which help with the assessment and comparison of the results. In our previous study we have observed that the conductivity does not change upon Li+/H+ exchange but were not able to determine if the protons contributed to the ionic conductivity.9 As one can study separately the dynamics of the Li and the H atoms by 7Li and 1H NMR, it is the method of choice for answering this question.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. 2.1.1. Mother Forms Li7La3Sn2O12 and Li5La3Nb2O12. The two garnet phases have been synthesized under powder form by a solid state reaction. The starting products − SnO2 (Johnson Mattey, 99.9%), Nb2O5 (Strem Chemicals, 99.9%), dehydrated La2O3 (Chempur, 99.9%), and Li2CO3 (Merck, 99%) − were ground together in ethanol and in stoichiometric ratio with an excess of Li2CO3 (15% for Li7La3Sn2O12 and 20% for Li5La3Nb2O12). The resulting mixture was placed in an alumina crucible and heated at 700 °C for 12 h and 900 °C for 20 h with two intermediate grindings for Li7La3Sn2O12. Concerning Li5La3Nb2O12 a pure sample was obtained after 12 h at 800 °C followed by 12 h at 900 °C and finally 1 h at 1000 °C with intermediate grindings. The purity and the crystallinity of the two samples are checked after each step by X-ray diffraction analysis. 2.1.2. Exchanged Li+/H+ Form. In order to perform the structural study from neutron diffraction experiments on the exchanged forms, we prepared a large quantity of the two protonated garnets by ionic exchange from the mother phases by following the procedure we used in ref 9: 7.5 g of the mother form were placed in a round-bottom flask containing 750 mL of ethanol and a large excess of benzoic acid (75 g). The solution was heated at reflux for one week. The resulting product was washed with ethanol, filtered, and dried at 60 °C. The filtrate was preserved in order to determine the exchanged lithium rate. 2.2. Sample Characterization. 2.2.1. Thermal Analysis. Thermal analysis is performed with a TGA-TA Instruments SDT Q600 between 25 and 900 °C under N2 flow with a heating rate of 5 °C min−1.

3. RESULTS AND DISCUSSION 3.1. Stability of Lithium Garnets. Our first results concerning the reactivity in ambient atmosphere of some lithium garnet phases pushed us to believe that such instability could be generalized to all lithium garnet-type oxides. In order to check this assumption, we have tested two garnets with lithium ions only located in a tetrahedral oxygen environment (corresponding to the 24d site in the usual centric Ia3̅d space 3336



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Figure 1. TGA curves for Li7‑xHxLa3Sn2O12 and Li5‑xHxLa3Nb2O12 showing a single weight loss due to the departure of proton as H2O.

group): Li3Gd3Te2O12, and Li3Nd3Te2O12.16 In order to study the stability, we chose the following conditions: a platinum crucible with 0.5 g of the garnet was placed in a reactor containing 20 drops of water. A CO2 pressure of 10 bar was applied, and the reactor (125 cm3) heated at 140 °C for one night. After drying at 60 °C for one hour, a PXRD pattern was registered. The advantage of such a test is that we quickly obtain the response on the Li+/H+ exchange feasibility and in addition from two different ways. Indeed, in case of a successful exchange, the PXRD pattern study must first reveal an increase of the cell parameter due to the replacement of strong Li−O bonds by weak hydrogen O−H···O bonds as already observed9,10 and second the presence of Li2CO3 due to combination of the lithium released with CO2. However, no trace of Li2CO3 and no variation of the cell parameter were observed indicating that no Li+/H+ exchange has occurred and that the two tested garnets are stable toward moisture. Taking into account our results concerning the Li+/H+ exchange on the Li7La3Sn2O12 garnet,9 we know however that such lithium ions in a tetrahedral oxygen environment can take part in this reaction since the population of the corresponding site is affected during the exchange. At this stage, if we consider now some characteristics of these two compounds as the cell parameter or the transport properties,16 we observe that they are vastly different from those of other lithium garnets: their cell parameters are small (≈12.5 Å), while their ionic conductivity is low.16 In order to see if the lattice size could play a role in the lithium garnet instability, we have tested other lithium garnets with various cell parameters. Using the conditions previously described, we obtained the results gathered in Table 1. In this table, all cell parameters have been refined by pattern matching mode from PXRD data on compounds we prepared, while the lithium distribution and the distances are taken from the indicated references. We observed first that most of the tested garnets are unstable under our operating conditions and that this behavior cannot be directly connected to the lattice size. Indeed, Li6CaSm2Ta2O12 which can be exchanged presents a smaller cell parameter than Li3Nd3Te2O12 (a = 12.5495(1) and 12.5621(5) Å, respectively).17 Second, it seems that the exchange feasibility cannot be correlated to the transport property since Li7La3Sn2O12 which can be exchanged is a poor conductor 21 as well as the stable Li 3 Gd 3 Te 2 O 12 and Li3Nd3Te2O12.16 A careful examination of Table 1 reveals that the Li+/H+ exchange occurs only for garnets with a lithium concentration larger than permitted by the conventional garnet

stoichiometry. This corresponds to more than three lithium ions per formula and to a distribution of these ions over two or three sites depending on the lithium content (24d, 48g, and 96h in the Ia3̅d space group). In such cases, lithium ions are located in two different environments: tetrahedral and octahedral. We also notice that Li5Nd3Sb2O12 is slightly different from the others since after reaction, the garnet was highly decomposed in hydroxides and carbonates while a broadening of the remaining (hkl) garnet lines was observed. It is then difficult to conclude on the Li5Nd3Sb2O12 stability due to the poor refinement quality. Lastly, numerous studies6,22−24 have shown that the ability of Li+ ions to move inside the garnet structure was due to the existence of a continuous threedimensional framework constituted by lithium polyhedra according to the model of Cussen and Yip.7 Two routes are possible for the jump of lithium ions: either between two neighboring octahedral and tetragonal sites and/or between two neighboring octahedral sites. In both cases, the lithium displacement is impeded by oxide ions: a triangular window in the first case and an O−O pair in the second case lie almost perpendicularly to the Li−Li axis. Consequently, we can easily imagine that an increase of these O−O distances would facilitate the lithium movement between the different crystallographic sites, as shown by Murugan24 on Li5La3Sb2O12 and Li5SrLa2Sb2O12. In our case, the garnet instability cannot be correlated to such O−O distances (Table 1). 3.2. Stoichiometry of the Protonated Phases Li7‑xHxLa3Sn2O12 and Li5‑xHxLa3Nb2O12. For the structural study of the protonated garnets, the exchange was performed via a benzoic acid/ethanol treatment as described in the Experimental Section. The formulation of the two resulting compounds was first approached by the determination of the exchanged lithium quantity in the filtrate by flame photometry. For the two garnets, the analysis reveals a partial exchange leading to the following formulations: Li2.1H4.9La3Sn2O12 and Li0.9H4.1La3Nb2O12 (exchange Li rate equal, respectively, to 70 ± 1% and 82 ± 1%). These results have been confirmed by TGA measurements (see Figure 1) since the experimental weight loss observed for each compound is in agreement with the formulations proposed above and with a dehydration process: theoretical/experimental weight % is equal to 5.09/ 5.02 for Li2.1H4.9La3Sn2O12 and 4.59/4.43 for Li0.9H4.1La3Nb2O12. Owing to the good agreement between these two characterization techniques, we have retained these two formulations for the two exchanged compounds. 3337



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However, we have also noticed that the Li+/H+ exchange rate was not reproducible. Indeed we have performed several experiments in the same conditions and starting from the same sample Li5La3Nb2O12. The obtained formula were always different, suggesting the existence of a solid solution. The cell parameter evolution confirms this hypothesis and shows a regular increase with the exchange rate (Figure 2 and Table 2).

Li+/H+ exchange, (ii) all lithium sites are affected by this exchange independently of their oxygen environment, and (iii) the remaining lithium ions are completely redistributed even in sites not occupied in the mother form. In the following we attempt to complete this structural study with a new data set from neutron diffraction. By this way, we hope to confirm this structural approach and to render more precisely the location of light cations, as lithium and protons. We first observed that all the diffraction lines of the PND pattern are indexed in the Ia3d̅ as for the PXRD diffraction pattern. We started then the structural refinement with the model of the [La3Sn2O12]7‑ framework, and we applied Fourier difference calculations when their corresponding positions were judged stabilized. Two possible cationic positions were evidenced: the first one (24d) being attributed to the lithium site in an oxide tetrahedral environment with four distances Li−O close to 2 Å and the second one (96h) corresponding to a proton with one distance H−O close to 1.1 Å. Allowing the occupation factors of these two sites to vary, the reliability factors decreased significantly, while approximately one-third of the lithium and proton expected quantity still misses. Subsequent Fourier difference map revealed two other proton sites (96h). Taking into account these new sites in the refinement, with no constraint on the occupation factors, we noted a significant improvement of the reliability factors. With this structural model, no other lithium site could be found on Fourier difference maps. The refinement converged to satisfactory values with a single isotropic temperature factor for the three proton sites: Rp = 10.5, Rwp = 8.83, Rexp = 3.28, χ2 = 7.23, and RBragg = 4.05 (Table 3).

Figure 2. Evolution of the cubic cell parameter (Å) versus x in the solid solution Li5‑xHxLa3Nb2O12 from PXRD data (S.G.: I213).

Table 2. Evolution of the Cubic Cell Parameter (Å) versus x in the Solid Solution Li5‑xHxLa3Nb2O12 from PXRD Data (S.G.: I213) x

formulation

a

0 1.2 2.7 3.0 3.2 3.7 4.1

Li5La3Nb2O12 Li3.8H1.2La3Nb2O12 Li2.3H2.7La3Nb2O12 Li2.0H3.0La3Nb2O12 Li1.8H3.2La3Nb2O12 Li1.3H3.7La3Nb2O12 Li0.9H4.1La3Nb2O12

12.7972(1) 12.8176(1) 12.8333(1) 12.8326(1) 12.8365(1) 12.8444(1) 12.8605(1)

Table 3. Structure Refinement Results of Li7‑xHxLa3Sn2O12 from PND Data and of Li5‑xHxLa3Nb2O12 from PND (Left) and PXRD (Right) Data space group refined parameters peak shape, η cell parameters a / Å halfwidth parameters u v w x asymmetry parameters P1 P2 Reliability factors RBragg Rp Rwp Rexp χ2

This increase is due to the replacement of strong Li−O bonds by weak hydrogen O−H···O bonds as already observed.9,10 Recently, L. Truong and V. Thangadurai have published a study on Li+/H+ exchange in Li5La3Nb2O12.25 Surprisingly, they observed an inverse evolution of the cell parameter with the Li + /H + exchange rate (Li 5 La 3 Nb 2 O 12 : 12.7943(1) Å; H5La3Nb2O12: 12.735(6) Å in ref 6). However, the chemical formulations were only determined by TGA measurements, and no structural refinement was performed on the exchanged compounds. A solid solution Li7‑xHxLa3Sn2O12 has also been evidenced (four formulations prepared with 4.2 ≤ x ≤ 5.3) with, in this case, a very slight cell parameter evolution (Δa ≈ 0.004 Å). The existence of the two solid solutions compelled us to prepare 7.5 g of the protonated sample for collecting PND data in only one exchange step to obtain the most homogeneous possible sample. 3.3. Structure Determination of the Exchanged Phases. 3.3.1. Li7‑xHxLa3Sn2O12. Our first structural results obtained on Li7‑xHxLa3Sn2O12 from powder and single crystal XRD data9 revealed that (i) there is a topotactic structural transition from tetragonal (I41/acd) to cubic (Ia3̅d) during the

Li7‑xHxLa3Sn2O12

Li5‑xHxLa3Nb2O12

Ia3̅d (no. 230) 50 0.63(2) 12.9892 (1)

I213 (no. 199) 100 0.42(2) 0.73(2) 12.8604 (1) 12.8604 (1)

0.113(3) −0.250(6) 0.272(4) −0.0012(3)

0.138(4) −0.268(7) 0.266(4) 0.0007(3)

0.044(1) −0.014(1) 0.0056(2) −0.0053(5)

0.01(1) −0.002(5)

−0.01(1) −0.002(5)

0.156(3) 0.018(1)

4.05% 10.50% 8.83% 3.28% 7.23

2.63% 7.58% 7.03% 2.57% 7.49

4.22% 10.60% 8.59% 1.52% 4.22

Figure 3 shows the observed, calculated, and difference plots of the PND pattern. Table 4 gathers atomic coordinates, isotropic temperature factors, and bond valence sums, while selected interatomic distances are reported in Table 5. 3.3.2. Li5‑xHxLa3Nb2O12. As for the exchanged Sn garnet, the PXRD pattern of the exchanged Nb product is related to a wellcrystallized sample with thin (hkl) lines. In addition, it reveals that the garnet type structure is preserved, implying thus a topotactic reaction. All lines can be indexed in the cubic system 3338



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Table 5. Selected Interatomic Distances (Å) for Li7‑xHxLa3Sn2O12 from PND Data (S.G.: Ia3d̅ ) La polyhedron

Sn octahedron

La−O: 4 × 2.531(1) La−O: 4 × 2.569(1) Li tetrahedron Li−O: 4 × 1.970(1)

Sn−O: 6 × 2.063(1) H···O distances H1−O: 1.13(1) H2−O: 1.23(4) H3−O: 1.07(1)

Figure 3. Observed, calculated, and difference PND patterns of Li7‑xHxLa3Sn2O12 in the Ia3̅d space group. Vertical bars are related to the calculated Bragg reflection positions.

as for the mother form with, as expected, a larger cell parameter (12.86038(8) Å compared to 12.7986(1) Å) corresponding to a slight increase (1%). However, three lines with weak intensity at 2θ values close to 54.26° (the most intense), 65.72° and 86.11° (Figure 4) cannot be indexed in the centric Ia3̅d space group (no. 230) of the mother phase, implying thus the search for a new space group. This new space group must be noncentrosymmetric since we obtained a positive response with SHG test performed at room temperature on the powder sample. Only six groups are possible in the cubic symmetry with an I mode: I23 (no. 197), I213 (no. 199), I432 (no. 211), I4132 (no. 214), I4̅3m (no. 217), and I4̅3d (no. 220). We first chose to test the noncentrosymmetric subgroups of Ia3̅d present in the six possibilities listed above: I4132 (no. 214) and I4̅3d (no. 220). For these two space groups, we built the [La3Nb2O12]5‑ framework from that described in the centric space group, but the reliability factors remain high (respectively close to 19 and 12% for RBragg) while isotropic temperature factor values became negative. We saw then that these two space groups have a common subgroup which moreover belongs to the six groups previously presented: I213 (no. 199). As for I4132 and I43̅ d, we built an atomic model (two sites 12b for La, two sites 8a for Nb, and four sites 24c for O), and the refinement converged quickly from this starting point to reasonable reliability factors: Rp = 9.6% and RBragg = 3.7%). With the use of the Gfourier procedure in the WinPLOTR package, Fourier difference calculations were then performed to complete the structure. Only one site 12b for lithium ions was revealed, corresponding to half of the lithium 24d site of the mother form in the Ia3d̅ space group. We then used PND data to complete this structural study since neutron diffraction is

Figure 4. Powder X-ray diffraction pattern of Li5La3Nb2O12 before and after exchange. The stars indicate diffraction peaks which are not indexed in the Ia3̅d space group.

clearly more sensitive for the location of light atoms as lithium and hydrogen. Starting in the I213 space group from the [La3Nb2O12]5‑ framework refined from X-ray data, the reliability factors converged quickly to Rp = 13.4% and RBragg = 8.3%. Fourier difference maps revealed then four negative peaks: two 12b sites for lithium and two 24c sites for proton, taking into account reasonable Li−O and H−O distances. At this stage, we used the two data sets, PXRD and PND, to refine the structure as the information seems complementary. Indeed, among the three lines nonindexed of the PXRD data in the Ia3̅d space group, only the last one is clearly visible in the PND data, the two others being in the background. In these conditions, we started the calculations with applying a global constraint to the occupation factors of the four cationic sites Li1/Li2 and H1/H2 in agreement with the formulation obtained from the chemical and thermal analyses (RB = 4.59% for PXRD and 2.80% for PND data). In a second step, we allowed the lithium and proton distribution to vary with no constraint for the four crystallographic sites but with only one isotropic temperature factor for the two lithium sites as for the two proton sites. Attempts to refine independently isotropic temperature parameters for each lithium and proton sites led

Table 4. Atomic Coordinates, Biso, and Bond Valence Sums (Σ) for Li7‑xHxLa3Sn2O12 from PND Data (S.G.: Ia3̅d) La Sn O Li H1 H2 H3

site

s.o.f.

x

y

z

B(Å2)

Σexp

Σtheo

24c 16a 96h 24d 96h 96h 96h

1 1 1 0.61(2) 0.203(5) 0.047(3) 0.101(4)

1/8 0 0.1043(1) 3/8 0.6721(6) 0.655(3) 0.649(1)

0 0 0.1944(1) 0 0.6364(6) 0.585(2) 0.564(1)

1/4 0 0.2799(1) 1/4 0.1063(8) 0.239(3) 0.300(1)

1.25(3) 1.06(3) 1.49(3) 1.7(2) 0.6(2) 0.6 (2) 0.6(2)

2.89(1) 3.92 (1) 1.83 (1) 1.02 (1) 0.86 (1) 0.82 (2) 0.87 (1)

3 3 2 1 1 1 1

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systematically to a destabilization of the refinement. This can be due not only to the very low lithium and proton occupancy of the sites, as noticed in the refs 7 and 8, but also to an inhomogeneous distribution of these two cations over their respective available sites. This is also an indication that the structural refined model proposed here is an average since PXRD or PND data give us a global sight which is, as usual, not representative of the local lithium and proton distribution. However, these refinement conditions, which progressively empty one Li site, led to a slight improvement of the fit quality as shown by the reliability parameters values (Table 3) and the satisfactory match between observed and calculated intensities (Figure 5). Atomic coordinates, isotropic temperature factors, and bond valence sums are reported in Table 6, while selected interatomic distances are given in Table 7.

Table 7. Selected Interatomic Distances (Å) for Li5‑xHxLa3Nb2O12 from Coupled X-ray and Neutron Data (S.G.: I213) La polyhedron

Nb octahedra

La1−O1: 2 × 2.54(1) La1−O1: 2 × 2.62(1) La1−O3: 2 × 2.54(1) La1−O3: 2 × 2.56(1) La2−O2: 2 × 2.51(1) La2−O2: 2 × 2.54(1) La2−O4: 2 × 2.45(1) La2−O4: 2 × 2.53(1) H···O distances H1−O1: 0.95(5) H2−O3: 1.00(5)

Nb1−O3: Nb1−O4: Nb2−O1: Nb2−O2:

3 3 3 3

× × × ×

1.95(2) 2.06(2) 2.12(1) 1.93(1)

Li tetrahedron Li−O2: 2 × 1.92(3) Li−O3: 2 × 2.00(3)

averaged models, which do not represent the local distribution of lithium and proton over their respective available sites which are, in addition, all partially occupied. If the cationic sites highlighted on the Fourier difference maps are clearly those which are the most occupied, we think that lithium ions and protons can also be located on other sites with respect to the Li−O and H−O usual distances. Moreover, the solid solutions evidenced for the two garnets can led to chemically inhomogeneous samples. This is not the first time that the noncentrosymmetric space group I213 is proposed for a garnet compound. Indeed, this space group was found by Hyooma et al.26 who published in 1988 the crystal structures of Li5La3M2O12 (M = Nb, Ta) performed on single crystal. The reflection conditions observed were clearly inconsistent with the classical Ia3̅d space group. In 1991, I213 was also proposed for the Li5Ln3Sb2O12 (Ln = Pr, Nd, Sm) garnets from a powder X-ray study, but, as in the previous one, no SHG tests were performed.27 Later, the structure of all these garnets has been reinvestigated from powder neutron data, and it was clearly shown that they crystallize in the Ia3̅d space group.6,19 Li5‑xHxLa3Nb2O12 is then the first noncentrosymmetric garnet. It is interesting to note that such transition toward a noncentrosymmetric space group during a Li+/H+ ionic exchange has already been observed for a lithium Ruddlesden−Popper phase28 meaning that the ionic Li+/H+ exchange is perhaps a good way to obtain noncentrosymmetric compounds. Whatever the space group (Ia3̅d or I213), the classical garnet network La3M2O12 (M = Sn or Nb) is preserved with isolated MO6 octahedra and LaO8 polyhedra connected by edges. As usual, the polyhedra in the noncentrosymmetric space group

Figure 5. Observed, calculated, and difference PND patterns of Li5‑xHxLa3Nb2O12 in the I213 space group. Vertical bars are related to the calculated Bragg reflection positions.

3.3.3. Discussion. First, one can remark that the two chemical formulations calculated from these structure refinements, Li1.8H4.2La3Sn2O12 and Li1.3H3.7La3Nb2O12, are not exactly consistent with those obtained from chemical and thermal analyses (Li2.1H4.9La3Sn2O12 and Li0.9H4.1La3Nb2O12). In addition, the electrical neutrality is preserved only for the niobium-containing phase. However, we can notice that the formulations are not so different if we consider the estimated standard uncertainty of the occupancy rate for the lithium and proton sites obtained from diffraction data refinements. As stated above, these refined structures are probably only

Table 6. Atomic Coordinates, Biso, and Bond Valence Sums (Σ) for Li5‑xHxLa3Nb2O12 from PXRD and PND Data (S.G.: I213) La1 La2 Nb1 Nb2 O1 O2 O3 O4 Li H1 H2

site

s.o.f.

x

y

z

B(Å2)

Σ

Σexp

12b 12b 8a 8a 24c 24c 24c 24c 12b 24c 24c

1 1 1 1 1 1 1 1 0.88(4) 0.98(4) 0.25(3)

0.1324(8) 0.6157(7) 0.0075(8) 0.7592(7) 0.2886(6) 0.7269(7) 0.852(1) 0.1381(9) 1/2 0.211(1) 0.569(4)

0 0 0.0075(8) 0.7592(7) 0.1089(9) 0.8982(9) 0.5279(7) 0.4727(7) 1/4 0.355(1) 0.352(4)

1/4 1/4 0.0075(8) 0.7592(7) 0.1983(7) 0.8046(9) 0.0532(8) 0.9436(7) 0.132(3) 0.079(1) 0.104(4)

0.9(1) 0.7(1) 1.6(3) 1.4(4) 1.2(2) 0.4(2) 0.4(2) 1.5(2) 0.4(4) 4.3(4) 4.3(4)

2.79(3) 3.26(4) 4.68(8) 4.59(7) 1.94(3) 2.02(4) 1.98(4) 1.78(3) 1.06(4) 0.93(1) 0.92(4)

3 3 5 5 2 2 2 2 1 1 1

3340



Article

Figure 6. Partial projection of Li7‑xHxLa3Sn2O12 structure showing the LiO4 tetrahedra and SnO6 octahedra and the three H+ sites (LaO8 polyedra have been omitting for more clarity).

Figure 7. Partial projection of Li5‑xHxLa3Nb2O12 structure showing the LiO4 tetrahedra and NbO6 octahedra and the two H+ sites (LaO8 polyedra have been omitting for more clarity).

Li+/H+ exchange. This result for Li7‑xHxLa3Sn2O12 is slightly different from what we observed from the single crystal study.9 Indeed, we detected three Li sites corresponding to those observed by Cussen in Li5La3M2O12 (M = Nb, Ta): 24d, 48g, and 96h.6 From the neutron data, only the 24d site was depicted, but the repartition of the lithium can vary from one experiment to another one and lithium still misses in our structural refinement. In the case of Li5‑xHxLa3Nb2O12, all

are less regular, but, in both cases, the M−O and La−O interatomic distances (Tables 5 and 7) are in agreement with the sum of the ionic radii from Shannon’s table. 29 Consequently, the calculated bond valence sums30 are close to the expected values (Tables 4 and 6). In the two structures, all the remaining lithium ions are in a tetrahedral environment in one site partially occupied. This means that the two octahedral or pseudo-octahedral sites have been emptied under 3341



Article

protons rather tend to correlate to the upfield part of the 6Li signal. The 7Li signal of the parent compound (Figure 8B) shows distinct features of quadrupolar coupling, which allow for distinguishing at least two sites, a dominating one with a coupling constant ωQ1 = 2π × 58 kHz and low asymmetry and a smaller one with ωQ2 about 2π × 90 kHz (arrow in Figure 8B). One would assign the former signal to a tetrahedral site, but the stoichiometry of the tetrahedral Li1-site 8a is only 1/7 of all Li. However, one of the two octahedral sites (Li2-site 32g) can also be understood as nearly tetrahedral with two additional oxygens at a longer distance.21 Therefore, the 58 kHz signal is assigned to Li1−8a and Li2−32g (altogether 5/7), while the 90 kHz is assigned to the distorted octahedral Li3− 16f (2/7). The existence of pronounced satellite peaks indicates homogeneity within each site, which is due to the full population of the available Li sites, and it indicates a long lifetime of the lithium ions on their site at the experimental condition (room temperature). The 7Li signal of the exchanged phase Li7‑xHxLa3Sn2O12 is in contrast relatively featureless (Figure 8B). A variable environment by different occupations of the neighbored Li and secondary neighbored H sites lead to a distribution of the electric field gradient and accordingly the quadrupolar coupling. The average coupling is however of the same order as for the mother phase. There is additional broadening by dipolar couplings with the protons, as decoupling narrows the signal by 4 kHz. 3.4.2. 119Sn NMR. The local environment of Sn can easily be probed by NMR of the spin-1/2 isotope 119Sn. The mother phase Li7La3Sn2O12 yields a relatively narrow signal at −600 ppm, while the signal of the exchanged phase exhibits a significant broadening compared to the mother phase, which again reflects the distribution of H and perhaps also Li on the available sites (Figure 8C). Again, there is a maximum at the chemical shift of the mother phase and an additional maximum of comparable intensity upfield at −610 ppm. There is thus, on a different scale, an analogy to 6Li: there seem to be two chemically different populations within the (only) crystallographic site Sn. While one of them is not so much different from the mother phase, the other has more hydrogen in its environment. 3.4.3. Li and H Dynamics. In order to study the dynamics of Li ions and protons, the temperature dependence of the 7Li longitudinal relaxation rate R1 = 1/T1 and the rate of the relaxation in the rotating frame, R1ρ = 1/T1ρ, of parent and exchanged Li7‑xHxLa3Sn2O12 as well as the 1H line width of the exchanged phase have been collected. Following the BPP theory,32 the review of Böhmer et al.,11 and the detailed study of van der Maarel and co-workers33−35 the relaxation rates have the general form

lithium ions are located in a 12b site which corresponds to the half of the 24d site occupied by these ions in the mother phase. In both structures, the isolated LiO4 tetrahedra share all their corners with two LaO8 and one MO6 polyhedra thus simultaneously creating a three-dimensional network (Figures 6 and 7). The Li−O distances (1.92 Å in Li7‑xHxLa3Sn2O12 and ranging from 1.92 to 2.00 Å in Li5‑xHxLa3Nb2O12) agree well with the sum of the ionic radii leading to satisfying bond valence sums for the lithium ions (1.02 and 1.06, respectively). Concerning the location of protons, the evidenced sites are compatible with classical H−O distance (ranging from 1.07 to 1.23 Å in Li7‑xHxLa3Sn2O12 and equal to 0.95 and 1.00 Å in Li5‑xHxLa3Nb2O12). If we compare these sites to the deuterium 96h observed in the hydrogarnet Sr3Al2(O4D4)3,31 we notice that the three proton sites in Li7‑xHxLa3Sn2O12 are located around the deuterium position, while in Li5‑xHxLa3Nb2O12 each 24c site corresponds exactly to the fourth of the 96h site of the deuterium. 3.4. Solid State NMR on Li 7 La 3 Sn 2 O 1 2 and Li7‑xHxLa3Sn2O12. 3.4.1. 6Li and 7Li Signals. The change in the environment of the Li sites upon proton exchange of Li7La3Sn2O12 can be followed by 6Li NMR. In the parent compound, three fully occupied crystallographic sites, i.e. the tetrahedral Li1 (8a), and two distorted octahedral Li2 (32g), Li3 (16f) were identified.21 Given the low chemical shift dispersion of lithium, their environment is so similar that they coincide to one peak at 1.7 ppm (Figure 8A). Upon Li+/H+

Figure 8. (A) 6Li MAS NMR spectra of Li7La3Sn2O12 (red) as well as Li7‑xHxLa3Sn2O12 acquired by single pulse (blue) and cross-polarization (violet). Below a 1H−6Li HETCOR of Li7‑xHxLa3Sn2O12. (B) 7 Li static and (C) 119Sn MAS spectra of Li7La3Sn2O12 (red) and Li7‑xHxLa3Sn2O12 (blue). All spectra are normalized to the same integral.

exchange, there is only one crystallographic Li site as well as three H+ sites, and they are not fully occupied. The 6Li NMR signal broadens into the upfield direction (Figure 8A). A significant part coincides with the signal of the parent compound. Omitting 1H decoupling does not change the signal. The 6Li signal obtained by means of cross-polarization is, in contrast, rather centered on the upfield part. This behavior reflects the distribution of hydrogen on the second-neighboring sites, and it indicates that there is certain heterogeneity in this distribution. Due to the low population of the 16 potential hydrogen positions in a radius of 2.6 and 3.5 Å around Li, some Li ions are few influenced, while for others the chemical shift is affected. The HETCOR 1H−6Li correlation spectrum in Figure 8A emphasizes the observation by 1D cross-polarization: the

R=

1 ⟨ΔωQ ⟩2 ∑ ci J(ωi) 5 i

(1)

as relaxation is induced by fluctuations of the (most likely) quadrupolar couplings for example by translational motion whose root-mean-square amplitude is ⟨ΔωQ⟩ and whose spectral composition is given by J(ωi) = 3342

τc 1 + (ωiτc)1 + β

(2)



Article F− dominating contributions to RF+ 1ρ and R1ρ in the experimental S temperature range. The rate R1ρ depends only on J(ω0) and J(2ω0) and is therefore in theory very small under the present conditions (10−4 to 0.1 s−1). In the experiment it will therefore be superimposed by other effects as dephasing due to the imperfection of the spin lock irradiation, and the experimental rates (10−30 s−1) probably do not have a physical meaning. For lower temperatures (295 to 355 K) the RF1ρ± rates become so small that they are not separable from the slow rate. For the sake of consistency in the data treatment, the slow rate and initial intensity were fixed to the values observed for higher temperatures. The two fast rates exhibit the same temperature dependence and reach a maximum at the same temperature, as expected (approximately) by the theory for relaxation of spin 3/2 nuclei with a static quadrupolar field. According to eqs 1-3 the F− maximum of both rates RF+ 1ρ and R1ρ is reached in a temperature range where the correlation time τc corresponds roughly to the inverse of λ1 or λ2. Compared to the longitudinal rate R1, it therefore characterizes relatively slow motions. The maxima for the exchanged phase differ compared to the mother phase; they are shifted toward higher temperatures. In intensity and slope the curves are quite similar, indicating that the Li dynamics does not change fundamentally upon Li+/H+ exchange. By fitting a model with a nonzero average quadrupolar coupling as described above to the two fast R1ρ rates, we find Ea = 530 ± 100 meV for the mother phase and Ea = 440 ± 100 meV for the exchanged phase, respectively. The values for the root-mean-square of the fluctuations ⟨dωQ⟩ are 2π × 33 kHz and 2π × 29 kHz, respectively, thus in the order of the difference of the couplings that are observed for the mother phase. With the corresponding attempt times τc0 = 0.9 × 10−12 s and 2.4 × 10−11 s, respectively, the mobility as a function of the temperature according to eq 3 is as shown in Figure 9C. For both phases, the correlation time of motions is then in the order of 1 ms at room temperature, which is not sufficient to suppress the satellite peaks (2π/ωQ1 ≈ 0.02 ms). Only from Tonset = 355 K (τc = 0.025 ms) the satellite peaks begin to vanish (Figure 10). This starting temperature of “narrowing” (here: suppression of the satellites) can be used to countercheck the result for the activation energy with the empirical estimation of Waugh and Fedin36

The frequency components that coincide with transition energies ℏωi lead to noncoherent transitions and thus relaxation. The ci are coefficients, and the stretch parameter β is ideally 1. The motion is commonly characterized by a correlation time τc , which follows an Arrhenius relation

⎛ E ⎞ τc = τc 0 exp⎜ a ⎟ ⎝ kBT ⎠

(3)

Ea represents here the activation energy needed by the Li ion to move from one site to another, and τc0 is the corresponding attempt time. In the experiments, we observe for both phases in particular at higher temperatures a triexponential decay of the coherence Ta11 under spin lock. A mono- and even biexponential model function are for all decay curves above 355 K not sufficient to describe the relaxation behavior. The two faster rates are increasing with the temperature over 2 orders of magnitude until a maximum about 425 K (Figure 9A). R1ρ relaxation under

Figure 9. (A) The two fast modes of the rotating frame relaxation F± of Li7La3Sn2O12 (red squares and solid lines) and rates R1ρ Li7‑xHxLa3Sn2O12 (blue dots and dashed lines) as a function of temperature. Symbols: experimental rates. Lines: curve fits of RF± 1ρ . (B) 1 H signal line width of Li7‑xHxLa3Sn2O12. Symbols: experimental widths. Line: model. (C) Motional correlation times of Li in Li7La3Sn2O12 (red, solid) and Li (blue, short dashed) and H (dark blue, long dashed) in Li7‑xHxLa3Sn2O12, as determined from the relaxation rates and line widths.

a, on the time scale of the inverse Larmor frequency, nonzero average (“static”) quadrupolar interaction ω̃ Q which is in the order of magnitude of the spin lock frequency ω1 is also in theory triexponential35 with a slow rate RS1ρ and two fast rates F− RF+ 1ρ and R1ρ . The two observed faster rates correspond to the two rates RF1ρ± that consist of a linear combination of the spectral densities J at ω0, 2ω0, and two smaller frequencies λ1 and λ2 that are in the order of magnitude of ω1 and ω̃ Q (see eqs 41−42 in ref 33). The transitions at λ1 and λ2 give the

Figure 10. 7Li NMR spectra of Li7La3Sn2O12 as a function of the temperature. 3343


Chemistry of Materials Ea = 1.617 meV/K·Tonset ≈ 570 meV

Article

shift and the other one is shifted upfield, a difference in the number of hydrogens in the Li or Sn environment can be made responsible. The NMR studies show that in the tin phases, compared to other Li ionic conductors, the Li ions move relatively slowly from site to site, which can mainly be attributed to the high activation energy. This is in agreement with the previous study of the conductivity: it is relatively low.9 Whether, in the mother phase, there are Li jumps only between two neighboring octahedral sites or between two neighboring octahedral and tetragonal sites can be answered by means of the evolution of the 7Li spectra as a function of the temperature (Figure 10). If the Li ions on the tetrahedral site did not move, the spectral features of their signal would be essentially preserved. There is however no evidence for this, as the featureless spectra at high temperatures appear to be an average of all sites. Strictly speaking, there is a set of activation energies for the different types of jumps. This might be a reason why the energy found by ion conductivity experiments (790 meV)9 is higher than those found here for Li and for H. This discrepancy has already been observed for many other compounds, and different explanations have been given.11,37 Previous studies9 have shown that conductivity does not change much either upon Li+/H+ exchange which posed the question whether the protons in the exchanged phase can contribute to cationic conduction. In ref 9, we showed by impedance spectroscopy measurements that the ionic conductivity of Li7‑xHxLa3Sn2O12 was equivalent to that of Li7La3Sn2O12 (σ200 °C = 4.8 and 2.7 × 10−6 S cm−1, respectively). This observation meant either that the protons participate in conduction, so that the total number of mobile species is preserved (with the same mobility), or that the Li+ mobility is increased thanks to the formation of vacancies, and protons do not take part in the conduction. The dependence of 7 Li R1ρ relaxation and 1H line width on the temperature reveals that the Li mobility does not change much upon Li+/H+ exchange and that the H+ mobility is similar. It is therefore likely that the protons participate in the conduction to a similar extent as the Li+ ions. In the majority of the studies of R1ρ relaxation, monoexponential decays are reported although in principle a triexponential decay is predicted by the theory, if the time averaged quadrupolar coupling does not vanish. The example for which the theory was developed is 23Na in liquid crystals.35 As an example for polycrystalline systems, the layered perovskite Li2xCa0.5‑xTaO3 exhibits a biexponential decay.38 The resolvable triexponential behavior of the tin phases studied here indicates a particularly strong average (“static”) quadF+ rupolar coupling which separates RF− 1ρ and R1ρ . For further discussion see the Supporting Information.

(4)

A similar behavior, albeit, due to the lack of explicit satellite features not as striking, is observed for the exchange phase, confirming that the line shape at room temperature is due to the inhomogeneous distribution of sites. But also in other aspects, there are not many differences in the dynamics after 1H exchange. The maximal rates and thus the amplitude of the fluctuations of the interactions differ by only 10%, and the difference in the activation energies is in the order of the uncertainty by the parameter correlation. The observed shift of the maximum toward higher temperatures is just as much due to an increase in the attempt times, τc0, which is surprising in the view that there are more free sites available. In the used temperature range the correlation times are not very different. In order to increase the precision, we have acquired an additional temperature series for the longitudinal relaxation R1. We find values between 0.14 s−1 at 295 K and 3.7 s−1 at 475 K for the mother compound and about the double for the exchanged phase. In theory, the rates depend on J(ω0) and J(2ω0), and with parameters consistent with the calculation for R1ρ the theoretical values are between 10−5 and 0.1 s−1. They are thus in a range where quadrupolar relaxation by translational motion is superimposed by other relaxation mechanisms as paramagnetic impurities, vibrations,13 or electronic motion.37 The relaxation is induced by fluctuations of interactions which are in general not related to translational motion and therefore can have a different correlation time or, in the case of vibrations, a completely different dynamics. Therefore neither an analysis of the slope of the temperature dependence (ignoring the absolute intensities), as frequently done, is meaningful for the studied tin phases. We study the mobility of the protons by means of the line width δω of the 1H signal. Faster motion at higher temperatures leads to a sudden narrowing with respect to the rigid lattice line width δω0 when the motion is canceling the effect of the dipolar coupling, described by the expression33 δω 2 =

2 2 δω0 arctan(δωτc) π

(5)

Deriving τc from the experimental data according to this model and fitting it by an Arrhenius model (eq 3) yields an activation energy of 480 meV and an attempt time of 1.1 × 10−11 s (Figure 9B), both quite similar to the values found for the Li ions. The mobility of the protons is therefore not very different from that of the Li ions (Figure 9C), and this suggests that both contribute to cationic conduction. 3.4.4. Discussion. In contrast to crystallographic techniques, NMR signals reflect the local environment of a nucleus, independently of any lattice periodicity. The 7Li, and 119Sn NMR signals of Li7La3Sn2O12, in which there are no partly occupied Li sites, correspond to the crystallographic sites, albeit the quadrupolar coupling of 7Li is (obviously) a less discriminating criterion than crystal diffraction. The chemical shift of Li typically has particularly little dispersion, so that there is only one 6Li signal to be expected. In the exchanged phase the crystallographic Li and H positions are only partly occupied, and therefore the 7Li signals lose their satellite features and the 6Li and 119Sn signals are inhomogeneously broadened. However, the distribution appears to be not fully random, as the chemical shifts of both 6Li and 119Sn resolve two subgroups of Li and Sn environment. As for both nuclei the center of one of the signals corresponds to the mother phase

4. CONCLUSION In this paper, we have demonstrated that many lithium garnets are unstable under ambient air. Indeed, a Li+/H+ ionic exchange occurs under such conditions, leading to a partially protonated garnet phase. In the presence of CO2, the lithium released under LiOH form turns into Li2CO3. We have also observed that the exchange feasibility seemed directly connected to the lithium content: for a greater concentration than can be accommodated on the tetrahedral site commonly occupied in garnets, the Li+/H+ exchange occurs if the garnet is stored in humid atmosphere. We have completed this work by the structural studies performed by powder neutron diffraction 3344



Article

Duppel, V.; Kienle, L.; Janek, J. Phys. Chem. Chem. Phys. 2011, 13, 19378. (14) M. Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (15) FullProf, version 4.90, July 2010. Reference: RodriguezCarvajal, J. Physica B 1993, 192, 55. (16) O’Callaghan, M. P.; Lynham, D. R.; Cussen, E. J.; Chen, G. Z. Chem. Mater. 2006, 18, 4681. (17) Yoo, C. Y.; Kim, S. C.; Lee, S. S.; Kim, S. J. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, E65, 74. (18) Roof, I. P.; Smith, M. D.; Cussen, E. J.; Zur Loye, H. C. J. Solid State Chem. 2009, 182, 295. (19) Percival, J.; Kendrick, E.; Slater, P. R. Mater. Res. Bull. 2008, 43, 765. (20) Percival, J.; Apperly, D.; Slater, P. R. Solid State Ionics 2008, 179, 1693. (21) Percival, J.; Kendrick, E.; Smith, V.; Slater, P. R. Dalton Trans. 2009, 5177. (22) Awaka, J.; Takashima, A.; Kataoka, K.; Kaijima, N.; Idemoto, Y.; Akimoto, J. Chem. Lett. 2011, 40, 60. (23) O’Callaghan, M. P.; Powell, A. S.; Titman, J. J.; Chen, G. Z.; Cussen, E. J. Chem. Mater. 2008, 20, 2360. (24) Murugan, R.; Weppner, W.; Schmid-Beurmann, P.; Thangadurai, V. Mater. Res. Bull. 2008, 43, 2579. (25) Truong, L.; Thangadurai, V. Inorg. Chem. 2012, 51, 1222. (26) Hyooma, H.; Hayashi, K. Mater. Res. Bull. 1988, 23, 1399. (27) Isasi, J.; Veiga, M. L.; Saez-Puche, R.; Jerez, A.; Pico, C. J. Alloys Compd. 1991, 177, 251. (28) Galven, C.; Fourquet, J. L.; Suard, E.; Crosnier-Lopez, M. P.; Le Berre, F. Dalton Trans. 2010, 39, 1. (29) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751. (30) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, B47, 192. (31) Chakoumakos, B. C.; Lager, G. A.; Fernandez-Baca, J. A. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1992, C48, 414. (32) Bloembergen, N.; Purcell, E. M.; Pound, E. M. Phys. Rev. 1948, 73. (33) van der Maarel, J. R. C. Concepts Magn. Reson., Part A 2003, 19, 97. (34) van der Maarel, J. R. C. Concepts Magn. Reson., Part A 2003, 19, 117. (35) van der Maarel, J. R. C.; Jesse, W.; Hancu, I.; Woessner, D. E. J. Magn. Reson. 2001, 151, 298. (36) Waugh, J. S.; Fedin, E. I. Sov. Phys. Solid State 1963, 4, 1633. (37) León, C.; Lucía, M. L.; Santamaria, J.; París, M. A.; Sanz, J.; Várez, A. Phys. Rev. B 1996, 54, 184. (38) Pham, Q. N.; Bohnké, C.; Emery, J.; Bohnké, O.; Le Berre, F.; Crosnier-Lopez, M.-P.; Fourquet, J.-L.; Florian, P. Solid State Ionics 2005, 176, 495. (39) Paris, M. A.; Sanz, J.; León, C.; Santamaría, J.; Ibarra, J.; Várez, A. Chem. Mater. 2000, 12, 1694.

experiments on two protonated forms, Li7‑xHxLa3Sn2O12 and Li5‑xHxLa3Nb2O12, and have observed that the ionic exchange corresponds to a topotactic reaction and is always associated with an increase of the cell parameter due to the replacement of strong Li−O bonds by weak hydrogen O−H···O bonds. Under exchange, Li7La3Sn2O12 undergoes a structural transition from tetragonal (I41/acd) to cubic (Ia3̅d), while, in the case of Li5‑xHxLa3Nb2O12, we observe a transition toward a noncentrosymmetric space group (I213) (SHG positive answer). We also showed that all the lithium sites in the mother phase are affected by the Li+/H+ exchange and observed that the protons sites are in a good agreement with those found in a previous study performed on the hydrogarnet Sr3Al2(O4D4).31 This work has also revealed the existence of two solid solutions Li7‑xHxLa3Sn2O12 and Li5‑xHxLa3Nb2O12 via the regular increase of the cell parameter versus x. 6Li and 119Sn NMR indicate that the potential hydrogen sites in the exchanged phase are not occupied in a fully random fashion. The comparison of the dynamics shows that there is relatively little change upon exchange and that protons and Li ions exhibit similar mobility and can contribute both to cationic conduction.

■

ASSOCIATED CONTENT

S Supporting Information *

Analysis of a minor upfield 6Li signal, discussions of the frequently used stretching parameter β, the average quadrupolar coupling, and the origin of the interactions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Prof. Joël Emery (IMMM, Université du Maine, Le Mans, France) for many valuable discussions and to Dr. Denis Mounier (IMMM, Université du Maine, Le Mans, France) for his help with SHG test.

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REFERENCES

(1) Hong, H. Y. P. Mater. Res. Bull. 1978, 13, 117. (2) Nicolas, J. Microwave ferrites. In Ferromagnetic Materials; Wohlfarth, E. P., Ed.; North-Holland: Amsterdam, 1982; Vol. 1, p 243. (3) Zaiß, T.; Ortner, M.; Murugan, R.; Weppner, W. Ionics 2010, 16, 855. (4) Murugan, R.; Thangadurai, V.; Weppner, W. Angew. Chem., Int. Ed. 2007, 46, 7778. (5) Thangadurai, V.; Weppner, W. Adv. Funct. Mater. 2005, 15, 107. (6) Cussen, E. J. Chem. Commun. 2006, 412. (7) Cussen, E. J.; Yip, T. W. S. J. Solid State Chem. 2007, 180, 1832. (8) O’Callaghan, M. P.; Cussen, E. J. Solid State Sci. 2008, 10, 390. (9) Galven, C.; Fourquet, J. L.; Crosnier-Lopez, M. P.; Le Berre, F. Chem. Mater. 2011, 23, 1892. (10) Wang, W. G.; Wang, X. P.; Gao, Y. X.; Yang, J. F.; Fang, Q. F. Front. Mater. Sci. China 2010, 4 (2), 189. (11) Böhmer, R.; Jeffrey, K. R.; Vogel, M. Prog. Nucl. Magn. Reson. Spectrosc. 2007, 50, 87. (12) Kuhn, A.; Narayanan, S.; Spencer, L.; Goward, G.; Thangadurai, V.; Wilkening, M. Phys. Rev. B 2011, 83, 094302. (13) Buschmann, H.; Dölle, J.; Berendts, S.; Kuhn, A.; Bottke, P.; Wilkening, M.; Heitjans, P.; Senyshyn, A.; Ehrenberg, H.; Lotnyk, A.; 3345


Instability of Lithium Garnets against Moisture. Structural

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