Deep hydration of an Li_7–3x La₃Zr₂ M ^III _x O₁₂ solid-state electrolyte material: a case study on Al- and Ga-stabilized LLZO

Hydro­thermally treated Al- and Ga-stabilized stuffed Li garnets, nominally Li₇La₃Zr₂O₁₂, have been investigated by single-crystal X-ray diffraction to determine structural and site-occupation alterations due to Li^I/H^I exchange.

Abstract

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li₇La₃Zr₂O₁₂ (LLZO), have been analysed using single-crystal X-ray diffraction to determine the pristine structural state immediately after synthesis via ceramic sinter­ing techniques. Hydro­thermal treatment at 150 °C for 28 d induces a phase transition in the Al-stabilized com­pound from the commonly observed cubic Ia d structure to the acentric I 3d subtype. Li^I ions at the inter­stitial octa­hedrally (4 + 2-fold) coordinated 48e site are most easily extracted and Al^III ions order onto the tetra­hedral 12a site. Deep hydration induces a distinct depletion of Li^I at this site, while the second tetra­hedral site, 12b, suffers only minor Li^I loss. Charge balance is maintained by the incorporation of H^I, which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with com­plete depletion of Li^I at the 48e site. The Li^I/H^I exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.

Introduction

The garnet family, X ₃ Y ₂ Z ₃O₁₂, has been well described mineralogically and crystallographically in recent decades (Novak & Gibbs, 1971 ▸), and is of inter­est to a range of scientists from the fields of geoscience and technology, due to its thermodynamic stability in a variety of geological environments and its flexible structure, which can host ∼60 different chemical elements as major and minor components (Geiger, 2013 ▸; Baxter et al., 2013 ▸). Furthermore, the so-called Li-stuffed garnets, e.g. Li₄La₃Zr₂Li₃O₁₂, or as sum formula, Li₇La₃Zr₂O₁₂ (LLZO), have raised particular inter­est as promising materials for use as solid-state electrolytes in all solid-state Li batteries due to their superior Li-ion conductivity (Cussen, 2010 ▸; Wang et al., 2020 ▸; Murugan et al., 2007 ▸; Samson et al., 2019 ▸). Pure end-member LLZO is tetra­gonal, has the space group I4₁/acd (Awaka et al., 2009 ▸) and has distinctly lower Li-ion conductivities. The high Li-ion conductivity is associated with the ‘standard’ cubic garnet structure with Ia d symmetry. The latter can be stabilized by various aliovalent substitutions, e.g. by small amounts of Al^III, which – in the first experiments – entered the structure as a contaminant from the corundum crucibles during synthesis (Geiger et al., 2011 ▸; Buschmann et al., 2011 ▸). The incorporation of Ga^III into LLZO also increases the Li-ion conductivity, but induces a reduction of the sym­metry to I 3d (Rettenwander et al., 2016 ▸; Wagner et al., 2016a ▸; Robben et al., 2016 ▸), i.e. the space group of hydro­garnet Ca₃Al₂(O₄H₄)₃ (Lager et al., 1987 ▸). A stabilization of the cubic structure for nominally pure LLZO is also possible by the uptake of H^I, which raises a question about LLZO stability in hydrous environments (Larraz et al., 2013 ▸). Several studies have investigated the role of Li^I/H^I exchange under different environmental conditions and found that LLZO-type materials are distinctly unstable in the presence of moisture (Ma et al., 2015 ▸; Galven et al., 2011 ▸, 2012 ▸, 2013 ▸; Larraz et al., 2015 ▸; Orera et al., 2016 ▸; Liu et al., 2019 ▸). Surfaces quickly degrade with the formation of LiOH and Li₂CO₃, and an increase in unit-cell parameters is observed as Li^I/H^I exchange progresses. Until now, only a few studies have systematically investigated the mechanisms behind this structural degradation. In recent studies, we have shown, using diffraction methods, that significant Li^I is especially lost from the inter­stitial sites of the structure in Al^III-, Ga^III- and Ta^V-substituted LLZOs during ageing at room temperature under high humidity (Hiebl et al., 2019 ▸; Redhammer et al., 2021a ▸,b ▸). In this article, we report on the deep hydration of Al- and Ga-substituted LLZO using hydro­thermal treatment of single-crystalline material.

Experimental methodology

Synthesis and aging of material

Single crystals of Al- and Ga-stabilized LLZO were obtained using a solid-state ceramic sinter­ing method, which has been described in detail elsewhere (Rettenwander et al., 2016 ▸; Wagner et al., 2016a ▸). In brief, Li₂CO₃ (with an excess of 10%), La₂O₃, ZrO₂ and Al₂O₃ or Ga₂O₃ were carefully mixed in the required stoichiometric proportions for the nominal com­positions Li_6.55La₃Zr₂Al_0.15O₁₂ (Al15-LLZO) and Li_5.8La₃Zr₂Ga_0.4O₁₂ (Ga40-LLZO). The mixtures were pressed into pellets and preheated at 850 °C for 4 h for deca­rbonatization. The samples were subsequently milled in a high-energy ball mill under alcohol using ZrO₂ balls, then dried, pelletized and sintered at 1230 °C for 6 h. This yielded a dense ceramic consisting of large individual crystallites of up to 150 µm. Structural refinements were carried out on the fresh LLZOs within 24 h of synthesis to assess the unaltered structural state. Parts of the pellets were crushed carefully and aged in a 45 ml Teflon-lined autoclave (25 mg sample and 250 ml distilled water) at 150 °C for a period of 28 d. The pH value of the liquid, in which the crystals were submerged, was measured after the experiment; the pH value was ∼13 for both com­positions. The hydrated single LLZO crystallites were then filtered off and the remaining liquid was left to evaporate. The resulting precipitate was identified as Li₂CO₃ using powder diffraction (PXRD), i.e. proving that Li^I was extracted from the LLZO material.

Refinement

Information on data collection and refinement results is given in Table 1 ▸. Indexing the diffraction data for pristine Al-LLZO yields the space-group symmetry Ia d. Refining the structure with framework cations and O atoms results in only two strong residual electron-density peaks at the 24d and 96h positions. Assuming that Li atoms occupy only these two positions, then there must be a distinct overpopulation at the 24d site. Thus, Al^III is assigned to the 24d site and its content was fixed using the chemical com­position calculated from energy dispersive X-ray (EDX) analysis on a similar material synthesized using an identical experimental setup (Rettenwander et al., 2016 ▸). The Li content was allowed to refine freely.

Table 1. Experimental details.

For all structures: Z = 8. Experiments were carried out at 298 K with Mo Kα radiation using a Bruker SMART APEX diffractometer. Absorption was corrected for by multi-scan methods (APEX2; Bruker, 2012 ▸). LLZO-Al15-pristine = fresh sample of Al-doped LLZO, measured directly after the end of the synthesis, LLZO-Al15-hydro-150C = Al-doped LLZO aged hydrothermally at 150 °C, LLZO-Ga40-pristine = fresh sample of Ga-doped LLZO, measured directly after the end of the synthesis and LLZO-Ga40-hydro-150C = Ga-doped LLZO aged hydrothermally at 150 °C.

	LLZO-Al15-pristine	LLZO-Al15-hydro-150C	LLZO-Ga40-pristine	LLZO-Ga40-hydro-150C
Crystal data
Chemical formula	Al0.15La2.95Li5.73O12Zr2	Al0.15H5.52La2.88Li1.64O12Zr1.95	Ga0.28La2.94Li6.44O12.00Zr2.00	Ga0.26H3.90La2.96Li1.99O12Zr2
M r	827.89	790.95	847.67	821.46
Crystal system, space group	Cubic, I a\overline{3}d	Cubic, I\overline{4}3d	Cubic, I\overline{4}3d	Cubic, I\overline{4}3d
a (Å)	12.9637 (2)	13.0738 (2)	12.9669 (2)	13.06720 (12)
V (Å3)	2178.65 (10)	2234.63 (10)	2180.26 (10)	2231.25 (6)
μ (mm−1)	13.24	12.61	13.88	13.57
Crystal size (mm)	0.12 × 0.11 × 0.07	0.13 × 0.12 × 0.08	0.13 × 0.13 × 0.10	0.12 × 0.11 × 0.09
Data collection
T min, T max	0.22, 0.39	0.21, 0.36	0.19, 0.25	0.21, 0.36
No. of measured, independent and observed [I > 2σ(I)] reflections	33130, 455, 441	34201, 819, 819	35033, 1054, 1045	35657, 918, 915
R int	0.029	0.026	0.038	0.028
(sin θ/λ)max (Å−1)	0.840	0.804	0.884	0.838
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.016, 0.032, 1.51	0.013, 0.029, 1.27	0.013, 0.026, 1.29	0.018, 0.037, 1.12
No. of reflections	455	819	1054	918
No. of parameters	25	44	48	43
No. of restraints	0	1	2	3
H-atom treatment	–	Only H-atom coordinates refined	–	Only H-atom coordinates refined
Δρmax, Δρmin (e Å−3)	0.71, −0.44	0.58, −0.51	0.53, −0.65	0.54, −0.52
Absolute structure	–	Refined as an inversion twin	Refined as an inversion twin	Refined as an inversion twin
Absolute structure parameter	–	0.50 (3)	0.50 (4)	0.99944 (15)

Computer programs: APEX2 (Bruker, 2012 ▸), SIR2014 (Burla et al., 2012 ▸), SHELXL2014 (Sheldrick, 2015 ▸), ORTEP-3 for Windows (Farrugia, 2012 ▸) and WinGX (Farrugia, 2012 ▸).

For pristine Ga-stabilized LLZO, indexing of the diffraction data yields the space-group symmetry I 3d. Three different sites are identified from residual electron-density maps for the Li^I ions: at Wykoff positions 12a, 12b and 48e. It is evident that Ga^III must be located at the 12a position, as the 12a site becomes distinctly overpopulated when refined with only Li^I. In subsequent refinements, both the Li1 and Li2 sites are assumed to be fully occupied and the electron density is modelled with Li + Ga = 1. The result is that Ga^III almost exclusively resides at the 12a position. Refinement of the anisotropic atomic displacement parameters (adps) is possible for all atoms using the same strategy as that applied by Wagner et al. (2016a ▸,b ▸). The data of the hydro­thermally treated Ga-LLZO sample can also be indexed using I 3d symmetry when the model for untreated material is used as the starting point. The Li1 site is again assumed to be fully occupied and its electron density was modelled with Li + Ga = 1. This approach is considered valid as the resultant Ga^III content is similar (albeit slightly higher) to that obtained for the untreated sample. The Li2 and, in particular, the Li3 sites are distinctly depleted in Li and no anisotropic atomic displacement refinement is possible. Thus, the isotropic adps are adjusted and fixed to the U _eq value refined for the Li1 site; anisotropic adps could be obtained for Li1. Protons are located close to the O1 atom using residual electron-density maps. Fixing the U _eq value of hydrogen yields reliable occupation factors for this site and an almost charge-balanced chemical formula (with a slight surplus of 0.35 negative charges).

For deeply hydrated Al-LLZO, indexing of data yields a change in symmetry from Ia d to acentric I 3d. The structure of this com­pound is refined using the model of the Ga-stabilized LLZO, as described above for the La-, Zr- and the two O-atom positions. The electron densities at Li1 and Li2 were modelled first with only Li⁺ ions. In this case, the Li1 position is distinctly overpopulated, while the occupation of the Li2 position is low, and there is no indication that the Li3 position is occupied at all. Consequently, all Al^III is assigned to the Li1 site and its occupation is fixed to the value used in the unaltered sample, consistent with the assumption that no Al left the structure during hydration. Refinement of the anisotropic adps is not possible for the Li1 site, and the isotropic adp is very small, so the isotropic adp of Li1 was adjusted in such a way that it has a similar value to the U _eq value of the Li2 site (where anisotropic adp refinements were possible) and fixed as such in subsequent refinements. Two distinct residual electron-density peaks are identified in residual electron-density maps: one high (2.5 e Å⁻³), very close to the Zr-atom position, and another at ∼0.8 Å from the O1 atom. This latter (x, y, z) position is close to the proposed H-atom positions given by Larraz et al. (2013 ▸) and Orera et al. (2016 ▸). Hence, this residual density is assigned to the H atom, which is bonded to the O1 atom. Independent refinement of the x, y and z positions, and the occupation of the H atom is possible, whereas the isotropic adp had to be fixed. A residual density close to Zr can be explained by positional disorder at this site, similar to that reported for deeply hydrated Ta-substituted LLZO, which also transformed to the I 3d structure. How­ever, there is a marked decrease in the reliability factors associated with the refinements when Zr^IV disorder is applied, e.g. there is no sign of another site close to O2 that would allow for another H atom to be bonded to the O2 atom. Furthermore, no reliable residual electron-density peaks can be detected.

Two additional crystals were hydrated in the same way and both were then analysed; the results are consistent with those reported in the tables and below.

Results and discussion

In the pristine state, Al-LLZO shows the typical garnet structure with Ia d symmetry. La^III occupies the eightfold-coordinated 24c site with two symmetrically independent La—O bond lengths (see Table 2 ▸). A slight deficit in the La^III ion site occupation is observed, which is in line with previous studies (Hiebl et al., 2019 ▸; Rettenwander et al., 2016 ▸; Wagner et al., 2016a ▸,b ▸). The Zr^IV ions are located at the 16a position with a regular sixfold oxygen coordination and a bond length of 2.076 (16) Å. As depicted in Fig. 1 ▸, the garnet structure com­prises an integrated framework constructed of edge-sharing octa­hedral and dodeca­hedral sites, in which the Li atoms are located at both the regular 24d tetra­hedral site (Li1) and at the inter­stitial 96h position (Li2), often denoted to have a distorted octa­hedral coordination [Li2—O distances range between 1.854 (15) and 2.646 (14) Å, with the average of the four smaller bond lengths being 2.085 Å]. Al^III substitutes into the 24d position and the four equivalent Li—O lengths are 1.9044 (17) Å; both the 24d and the 96h positions show distinct vacancies.

Table 2. Selected geometric parameters (Å).

LLZO-Al15-pristine
La1—O1i	2.5110 (17)	Li2—O1	1.854 (15)
La1—O1	2.5951 (17)	Li2—O1vii	2.085 (14)
Zr1—O1ii	2.1076 (16)	Li2—O1vi	2.159 (15)
Li1—Li2iii	1.616 (14)	Li2—O1viii	2.242 (14)
Li1—O1iv	1.9044 (17)	Li2—Li2viii	2.46 (2)
Li1—Li2v	2.367 (14)	Li2—O1ix	2.646 (14)
Li2—Li2vi	0.79 (3)
LLZO-Al15-hydro-150C
La1—O1i	2.506 (3)	Zr1B—O2xii	2.06 (3)
La1—O1	2.518 (3)	Zr1B—O2iv	2.10 (3)
La1—O2x	2.544 (3)	Zr1B—O1ii	2.14 (3)
La1—O2i	2.607 (3)	Zr1B—O1xiv	2.19 (3)
Zr1A—O1xi	2.007 (4)	Zr1B—O1xi	2.36 (3)
Zr1A—O2xii	2.217 (4)	Li1—O1xv	1.998 (3)
Zr1B—O2xiii	1.85 (3)	Li2—O2xvi	1.976 (3)
LLZO-Ga40-pristine
La1—O2xiii	2.4935 (19)	Li2—O2xvi	1.9246 (19)
La1—O1i	2.5264 (19)	Li2—Li3xviii	2.340 (9)
La1—O2i	2.587 (2)	Li3—O1	1.878 (14)
La1—O1xvii	2.5975 (19)	Li3—O1vii	2.075 (8)
Zr1—O2xiii	2.0823 (18)	Li3—O2i	2.108 (9)
Zr1—O1ii	2.1346 (18)	Li3—O1viii	2.229 (8)
Li1—Li3iii	1.645 (8)	Li3—Li3vii	2.513 (13)
Li1—O1xv	1.8941 (18)	Li3—O2vii	2.639 (8)
LLZO-Ga40-hydro-150C
La1—O1i	2.514 (3)	Li1—Li3iii	1.5 (2)
La1—O1xvii	2.525 (3)	Li1—O1xv	1.987 (4)
La1—O2xiii	2.539 (4)	Li2—O2xvi	1.982 (4)
La1—O2i	2.603 (4)	Li2—Li3xviii	3.2 (2)
Zr1—O1ii	2.031 (3)	Li3—O1vii	2.4 (2)
Zr1—O2xiii	2.187 (4)	Li3—Li3viii	2.6 (4)

Symmetry codes: (i) z, x, y; (ii) x − {1\over 4}, z − {1\over 4}, y − {1\over 4}; (iii) −z + {3\over 4}, y − {1\over 4}, −x + {1\over 4}; (iv) z, −x, −y + {1\over 2}; (v) −y + {1\over 2}, z − {1\over 2}, x; (vi) −x + {1\over 4}, z − {1\over 4}, y + {1\over 4}; (vii) y − {1\over 4}, −x + {1\over 4}, −z + {3\over 4}; (viii) −y + {1\over 4}, x + {1\over 4}, −z + {3\over 4}; (ix) y, −z + {1\over 2}, x + {1\over 2}; (x) −x, y − {1\over 2}, −z + {1\over 2}; (xi) z − {1\over 4}, y − {1\over 4}, x − {1\over 4}; (xii) −y + {1\over 2}, z, −x; (xiii) −x, −y + {1\over 2}, z; (xiv) y − {1\over 4}, x − {1\over 4}, z − {1\over 4}; (xv) −z + {3\over 4}, −y + {1\over 4}, x + {1\over 4}; (xvi) x + {3\over 4}, −z + {1\over 4}, −y + {3\over 4}; (xvii) x, −y, −z + {1\over 2}; (xviii) y + {3\over 4}, −x + {1\over 4}, −z + {3\over 4}.

Figure 1.

The crystal structure of Al-stabilized LLZO (space group Ia d) in a polyhedral format. LaO₈ sites are in light blue and ZrO₆ octa­hedra in sea green, whereas the Li1 (orange) and Li2 sites (yellow) are shown as spheres only for clarity and to highlight their diffusion pathway.

The hydro­thermally altered sample of Al-LLZO shows additional Bragg peaks of type k = odd and l = odd that obey Ia d symmetry. Calculated precession images of the hk0 plane for unaltered and altered Al-LLZO are com­pared in Fig. 2 ▸, where some Bragg peaks that obey Ia d symmetry are marked. Indexing of the observed data definitively yield the space-group symmetry I 3d, in accordance with the findings of Larraz et al. (2013 ▸) and Orera et al. (2016 ▸) for Li^I/H^I-exchanged LLZO. The symmetry reduction is associated with several rearrangements in the structural architecture. The oxygen site at 96h in Ia d splits into two different 48e positions, i.e. O1 and O2, in I 3d, thus allowing for two different sets of Zr—O and four different La—O bond lengths. The Zr^IV ion shifts from (0, 0, 0) with site symmetry (Ia d) to the 16c position (x, x, x) with site symmetry 3 (I 3d). Some positional disorder is observed at the Zr position, where around 20% of the Zr^IV is displaced to a general 48e position, with a Zr—Zr offset of ∼0.39 (3) Å. Free refinement of the site occupancies of these two Zr positions total 1.96 Zr^IV atoms per formula unit, i.e. very close to the expected value of 2.0. Whereas in Ia d, the six Zr—O bond lengths are equivalent to a value of 2.1076 (16) Å. In the hydro­thermally altered structure, Zr^IV is in a very distorted octa­hedral environment at 16c, with Zr1A—O1^xi bond lengths of 2.007 (4) Å (×3), and Zr1A—O2^xii of 2.217 (4) Å (×3) (see Table 2 ▸ for symmetry codes). For the general position, the Zr1B—O bonds range between 1.85 (3) and 2.36 (3) Å (Table 2 ▸). It is inter­esting to note that a similar behaviour was found during hydro­thermal alteration of Li₆La₃ZrTaO₁₂ (LLZTO). Pristine LLZTO shows Ia d space-group symmetry but deep hydration induces a symmetry reduction to I 3d. A disorder at the Zr/Ta site is also observed in hydrated LLZTO similar to that in hydrated Al-stabilized LLZO in this study. It would appear that the symmetry reduction, induced by Li^I/H^I exchange, causes a large distortion of the O-atom environment around the 16c position and induces some positional disorder.

Figure 2.

Reconstructed precession images of the hk0 plane of (a) a single crystal of Al-stabilized LLZO directly after synthesis with some selected Bragg reflections indexed and (b) a single crystal after hydro­thermal alteration at 150 °C for 28 d. Note the presence of sharp superstructure reflections, which obey Ia d symmetry. In part (b), the yellow encircled Bragg reflections are the same as in part (a), while the blue encircled reflections in the rectangular box correspond (from left to right) to the 0, 0, 0,… Bragg reflections.

The regular 24d tetra­hedral site of the Ia d garnet structure splits into two different sites, 12a (Li1) and 12b (Li2), upon symmetry reduction to I 3d. Al^III is ordered onto the 12a (Li1) site but a distinct number of vacancies are observed on both sites. While ∼4.5 apfu Li^I occupy the inter­stitial 96h position in the pristine state, this position (Li3 at 48e) is com­pletely unoccupied in the deeply hydrated form, i.e. all the Li^I ions have vacated the inter­stitial octa­hedral site.

Recently, Redhammer et al. (2021b ▸) observed a progressive increase of the tetra­hedral site (24d and 12a) occupation in Al-stabilized LLZO during continuous Li^I/H^I exchange in a humid atmosphere and under mild hydro­thermal conditions. The shift and ordering of Li^I from the inter­stitial site to the regular tetra­hedral site, and the preference of Li^I and Al^III for the 12a position, are described as triggers for the symmetry reduction from Ia d to I 3d (Redhammer et al., 2021b ▸). A study of the structure of the deeply hydrated samples here shows that, after com­plete recovery of Li^I from the inter­stitial site, the tetra­hedral 12a and 12b sites also take part in the Li^I/H^I exchange. When com­pared to the data of Redhammer et al. (2021b ▸), it is obvious that, in this study, more Li^I is extracted from the 12a site [∼1.4 to 0.64 (2) apfu], but there is only a moderate change in site occupation at 12b [∼1.1 to 0.82 (2) apfu]. As Li^I is extracted from the structure, H^I is incorporated and bonds with the O1 atom. The Li^I ions in the 12a tetra­hedron are thus coordinated by four OH groups, with the O—H vector pointing towards the empty Li3 site (com­pare with Fig. 3 ▸). The proposed position of H^I is in line with that found by Orera et al. (2016 ▸) for pure undoped LLZO based on neutron diffraction on polycrystalline powders. Refinements indicate that the deeply hydrated Al-LLZO has a com­position of La_2.88Zr_1.95Al_0.15Li_1.64H_5.52O₁₂. It is also worth noting that a significantly lower number of La^III ions are found at the 24d site, so it would appear that La^III also leaves the structure under hydro­thermal conditions. This is in line with the observations of Redhammer et al. (2021a ▸,b ▸), who observed an instability of LLZO powders in highly humid air, with decom­position of LLZO, leading to the formation of lanthanite La₂(CO₃)₃·8H₂O within ∼30 d of exposure.

Figure 3.

Polyhedral representations of part of the crystal structure of Ga-sta­bil­ized LLZO (space group I 3d) in the (a) pristine and (b) hydro­ther­mally altered state viewed on the (12, ,3) plane. LaO₈ sites are in light blue, ZrO₆ octa­hedra in green and Li1, Li2 and Li3 sites in orange, yellow and grey, respectively. The dashed grey bonds around the Li3 site are the two most distant. [Symmetry codes: (i) −z +  , −y +  , x −  ; (ii) −z + 1, −x +  , y; (iii) −x + 1, −y +  , z; (iv) −x +  , y, −z; (v) −x +  , −y, z +  ; (vi) −z +  , −y +  , x +  .]

The Li^I/H^I exchange is accom­panied by a large increase in the a unit-cell parameter from 12.9637 (2) to 13.0738 (2) Å, which is among the largest values yet recorded for hydrated LLZO, cf. 13.06245 (4) Å at 77 K for hydrated LLZO with an Li_2.3H_4.7La₃Zr₂O₁₂ com­position (Orera et al., 2016 ▸) or 13.0530 (8) Å for Li_3.08H_3.52La₃Zr₂O₁₂Ta_0.4 (Yow et al., 2016 ▸). A replacement of stronger Li—O bonds by weaker O—H bonds and the creation of a large number of vacant sites, especially around the empty 48e (Li3) site, both require more space and are considered responsible for the lattice expansion.

Pristine unaltered Ga-stabilized LLZO shows I 3d sym­metry. A section of this crystal structure is illustrated in Fig. 3 ▸, together with that of the hydro­thermally treated sam­ple. Both the tetra­hedrally coordinated 12a and 12b positions appear to be fully occupied, with the Ga^III ions ordered onto the 12a position. In contrast to the Li-stuffed garnets with Ia d symmetry, the decreased number of vacancies at the regular tetra­hedral sites are considered to be a characteristic feature of the I 3d garnet structure. The 48e inter­stitial site [Li3 in Fig. 3 ▸(a)] is occupied by 3.71 Li^I apfu, equivalent to being ∼62% full. All of the tetra­hedral faces of the fourfold oxygen coordination around the Li1 and Li2 sites are shared with neighbouring Li3 sites, with inter­atomic contacts of 1.645 (8) (Li1—Li3) and 2.340 (9) Å (Li2—Li3), thereby forming a three-dimensional network that is responsible for the good Li-ion conductivity in this com­pound. The dodeca­hedral site has a small number of vacancies, while the regular octa­hedral positions are fully occupied with Zr^IV, resulting in a com­position of La_2.94Zr_2.00Ga_0.28Li_6.43O₁₂ for the unaltered Ga-stabilized LLZO material.

A smaller and larger set of La—O1/O2 bonds are observed at the dodeca­hedral site [2.4935 (19)–2.5264 (19) and 2.587 (2)–2.5975 (19) Å] and the octa­hedron is regular with two inde­pen­dent Zr—O bonds ranging between 2.0823 (18) and 2.1346 (18) Å. Therefore, the octa­hedra are much less distorted than those in the altered Al-LLZO with the same symmetry (Table 2 ▸). The Li1 (12a) site, which hosts Ga^III, is slightly smaller than the Li2 (12b) site, whereas the Li3 site, as in Al-LLZO, shows a very distorted 4 + 2-fold coordination, with bond lengths between 1.878 (4) and 2.639 (8) Å; the average of the four shorter bonds is 2.073 Å, i.e. slightly smaller than in the pristine Al-LLZO [see Fig. 3 ▸(a)].

As in Al-LLZO, hydro­thermal treatment of Ga-LLZO induces Li^I/H^I exchange. Again, a large increase of the lattice parameter to 13.06720 (12) Å is close to the value in hydrated Al-LLZO, suggesting that values around 13.07 Å represent an upper limit for lattice-parameter increase due to hydration. From site-occupation refinements, very minor Li^I ions are found at the inter­stitial 48e position, i.e. this site is almost com­pletely depleted. One key difference with Al-LLZO is that no Li^I is lost from the 12a position. It seems that Ga^III ions pin Li^I at this site. However, Li^I ions are extracted from the 12b site where the amount of Li^I is reduced from 1.49 apfu in the pristine to 0.50 apfu in the altered sample. Protons are again located close to the O1 atom [Fig. 3 ▸(b)], giving rise to a fully hydrated Li1(OH)₄ coordination around the 12a site. Some differences to Al-LLZO are, however, evident: there is no significant reduction in the La-site occupation and no residual electron density is observed close to the Zr^IV ions, i.e. there is no indication of disorder at the 16c position in altered Ga-LLZO. Nevertheless, the ZrO₆ octa­hedron is much more distorted in the altered sample, with Zr—O bond lengths ranging between 2.03 (3) and 2.187 (4) Å; the difference between the two independent Zr—O bonds increases with pro­longed Li^I/H^I exchange. This was outlined by Redhammer et al. (2021b ▸) and the data of this study fit the extrapolated trends observed there. A distinct alteration is also observed within the coordination sphere of the 24d site due to H^I incorporation. The most prominent effects include the reduction of the longer La1—O1^xvii distance by ∼0.073 Å, as well as changes in the three other La—O bond lengths. In addition, Li1—O1^xv and Li2—O2^xvi are extended in the altered sample (see Table 2 ▸).

Supplementary Material

CCDC references: 2122845, 2122844, 2122843, 2122842