Symmetry reduction due to gallium substitution in the garnet Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12

Gallium-substituted lithium lanthanum zirconate (LLZO; Li6.62La2.65Ga0.49Zr2O12) belongs to the family of garnets and shows a reduction of the symmetry to space group I 3d compared to Ia d typically observed for these structures.


Chemical context
Garnets can be described with the ideal formula A 3 B 2 (XO 4 ) 3 in space group Ia3d, with different coordination polyhedra of the respective elements with oxygen, resulting in a distorted cube for A (e.g. Ca), an octahedron for B (e.g. Al) and a tetrahedron for X (e.g. Si). The variability of the elements on the crystallographic sites (thereby keeping the high symmetry) gives rise to interesting material properties like ferrimagnetism (Geller, 1967). In recent years, garnet-type compounds containing Li have gained considerable interest as promising electrolyte materials for all-solid-state Li-ion batteries. The so-called 'Li-stuffed' garnets, which contain more Li than available on tetrahedral sites (X), meaning that excess Li occupies other sites as well, show an increase in Liion mobility. An exhaustive overview of these compounds was recently given by Thangadurai et al. (2014). The garnet-type fast lithium ion conductor Li 7 La 3 Zr 2 O 12 , abbreviated as LLZO, is such an 'Li-stuffed' garnet. Awaka et al. (2009) described the crystal structure of pure LLZO at ambient conditions in space group I4 1 /acd. Even a small amount of Al in the structure (Al-LLZO) stabilizes the cubic garnet symmetry described in space group Ia3d by Geiger et al. (2011). These authors reported that Al could be found on two different tetrahedral sites using 27 Al MAS NMR spectroscopy but a final analysis was not possible due to the minor Al content. Rettenwander et al. (2014) reported on 71 Ga MAS NMR spectroscopy measurements on gallium substituted Li 7-3x Ga x La 3 Zr 2 O 12 (Ga-LLZO) indicating a fourfold coordination of the gallium atoms. The authors excluded the presence of Ga at the 24d position (Ia3d) and assumed that the local symmetry could be lower than indicated by diffraction methods. In principle, the following exchanges are possible: (i) 3 Li + $ Ga 3+ + 2 voids, which is the most probable one and yields a good explanation for the higher conductivity due to the higher lithium atom jump probability to empty positions as ISSN 2056-9890 discussed (Rettenwander et al., 2014); (ii) La 3+ $ Ga 3+ , a valence-neutral exchange which should lead to a dynamical disorder of the gallium atoms in order to lower the coordination number and shorten the Ga-O bond lengths for bondvalence balance, taking the different radii into account. The valence-neutral exchange should finally lead to higher displacement parameters of the atoms on the lanthanum position compared to that of the lighter zirconium atoms. (iii) Zr 4+ $ Ga 3+ + Li + , which needs slightly more lithium for charge balance and could therefore be of minor probability.

Structural commentary
The unit cell of the obtained single crystals could be well indexed using a body-centered cubic lattice with lattice parameter a = 12.9681 (15) Å . The space group determination with XPREP (Bruker, 2014) leads at once to the highest possible space group Ia3d. However, a satisfactory structure solution or refinement with published structural data (Geiger et al., 2011) in this space group type was not possible. Consequently, structure solutions by charge flipping (Bruker, 2009) were tried in all possible subgroups of Ia3d and the lowest R-values were obtained for the charge-flipping run in space group I43d. Subsequent refinements lead to the present structure model and clearly indicate the substitution of Ga 3+ on the former 24c La 3+ site as well as 24d Li + site in the aristotype in space group Ia3d. The latter site splits into two sites due to the symmetry reduction as indicated by the Bä rnighausen tree (Bä rnighausen, 1980) given in Fig. 1. The deviation from six symmetry-equivalent Zr-O distances in LLZO (Ia3d) results in a distortion of the ZrO 6 octahedron with Zr-O distances of 3 Â 2.095 (2) and 3 Â 2.113 (2) Å in Ga-LLZO. Another significant reduction of the highest possible symmetry for LLZO is the distortion of the eightfold coordinate La position (Fig. 2), for which distances between 2.496 (2) and 2.595 (2) Å are found in Ga-LLZO. This distortion results from the splitting of the 96h position of the oxygen atom in Ia3d into two 48e positions in I43d (Fig. 1). Because the two lithium positions (Li22 and Li32) occupied by gallium are in principle identical to those positions of the higher symmetry structure (but with slightly shorter bond length due to the gallium Bä rnighausen tree (Bä rnighausen, 1980) of the group-subgroup relation between cubic LLZO and the symmetry-reduced cubic Ga-LLZO.

Figure 2
Crystal structure of Li 6.43(2) Ga 0.52(3) La 2.67(4) Zr 2 O 12 (Ga-LLZO) with all substitution, viz. 4 Â 1.916 (1) Å in LLZO and 4 Â 1.908 (2) Å in Ga-LLZO), and the Li1 and Li2 positions are not occupied by gallium, the symmetry reduction is a confirmation of gallium atoms to be found also on the lanthanum position. This is also supported by the higher displacement parameter of the La site compared to the Zr site, as explained previously.

Synthesis and crystallization
The synthesis was configured to yield a compound with nominal composition Li 6.25 Ga 0.25 La 3 Zr 2 O 12 . 2 g of a stoichiometric mixture of the pre-dried (30 h at 373 K in vacuum) educts Li 2 O (with an excess of 10% wt to compensate the lithium loss due to thermal treatment), La 2 O 3 , ZrO 2 and Ga 2 O 3 was weighted into a WC milling beaker (45 ml, 100 WC milling balls of 5 mm diameter, Fritsch, Germany) under inert conditions (glovebox) and high-energy ball-milled in a planetary ball mill (Pulverisette 7 premium line, Fritsch, Germany) under argon atmosphere for 8 h at a rotational speed of 10 s À1 as reported previously (Dü vel et al., 2012) for Al-substituted LLZO. The obtained powder was pressed to a pellet using a uniaxial pressure of 0.8 GPa. A stack of three pellets was placed on a platinum ring seated on a corundum plate, covered with a corundum crucible and heated for 12 h at 1323 K in a muffle furnace before cooling to room-temperature. The middle pellet from the stack had smooth green color and showed visible grains. The surface of the pellet was grey and brittle and consisted mainly of lanthanum zirconates due to Li loss. From this pellet single crystals were extracted using a polarization microscope. Rietveld refinement of X-ray powder diffraction data of the green product shows a mixture of 96.8 (9)% wt cubic garnet-type Ga-LLZO and 3.2 (9)% wt Li 2 ZrO 3 with a lattice parameter of a = 12.9738 (19) Å for its garnet-type structure. Energy dispersive X-ray analysis of the single crystal gave a tentative formula of Li 6.5 (1) Ga 0.5 (1) -La 2.8 (1) Zr 2.0 (1) O 12 , in good agreement with the refined formula Li 6.43 (2) Ga 0.52 (3) La 2.67 (4) Zr 2 O 12 determined from single crystal X-ray diffraction data.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Structure refinement was carried out as a two-component (merohedral) twin. Sites showing a statistical occupancy were constrained with respect to positions and anisotropic displacement parameters. An independent refinement of the anisotropic displacement parameters of Ga and Li on the Ga2/Li22 and Ga3/Li33 sites was not possible, although the reflection-to-parameter ratio is rather high. To ensure charge neutrality during the refinement of the Ga and Li occupancies on the Ga2/Li22 and Ga3/Li33 sites, the occupancies were restrained to exchange three Li atoms against one Ga atom.  (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and publCIF (Westrip, 2010).

Lithium gallium lanthanum zirconate
Crystal data