Crystal structure of Li3Ga(BO3)2

The title substance of lithium gallium borate, Li3Ga(BO3)2, crystallizes in a triclinic cell and is isotypic with the aluminium analog. The structure is composed of lithium- and gallium-centered tetrahedra and boron-centered triangles.


Chemical context
We are examining the alkali metal/gallium/borate phase diagrams, investigations of which have revealed to date, among others, the homologous series A 2 Ga 2 O(BO 3 ) 2 , in which A = Na, K, Rb, and Cs (Corbel & Leblanc, 2000;Smith, 1995;Smith et al., 1997Smith et al., ,2008 and the homologous series A 3 Ga(BO 3 ) 2 , in which A = Li, Na, K, Rb, and Cs. We report herein the crystal structure of the lithium analog ( Fig. 1) of the latter series, which is the only one which melts congruently, which has a unique structure among the series, and which is isotypic with Li 3 Al(BO 3 ) 2 (He et al., 2002); the other analogs have yet to be crystallized in the form of single crystals, but are structurally distinct from the lithium analog and isotypic with each other based on their powder X-ray diffraction patterns.
A crystal structure for this compound was previously reported by Abdullaev & Mamedov (1972) in the same triclinic space-group type P1, and with the same galliumborate polyhedral pattern but with important differences with the structure reported herein, to wit: slightly different cell parameters and a different reduced cell, a significantly smaller cell volume (i.e., 3% smaller), less regular bond-valence sums (BVS), greater deviations from expected interatomic distances, and irregular, five-and six-coordinate lithiumcentered polyhedra. Table 1 compares interatomic distances from the structure reported by Addullaev & Mamedov (1972) and this report, with expected distances using Shannon's radii (Shannon, 1976); it also lists bond-valence sums for each structure. We have considered as bonds all Li-O distances under 3 Å from the 1972 report because doing so produces more reasonable BVS values, thus rendering some of the lithium atoms as being five-or six-coordinate in the previous structure report. It should be noted that the authors, however, reported all lithium atoms as tetrahedrally coordinated. The present structure model clearly differs from the 1972 structure ISSN 2056-9890 model and hence indicates a second possible modification for this composition. Whether a polymorphic relation exists between the two phases remains unknown and needs additional proof by using complementary methods such as thermal analysis.

Structural commentary
The crystal structure of the title compound consists of lithiumand gallium-centered tetrahedra and boron-centered triangles, all of which share oxygen vertices (Fig.1). Each GaO 4 tetrahedron is linked to four BO 3 triangles and six LiO 4 tetrahedra. The gallium-centered tetrahedra and boron-centered triangles adjoin through shared vertices to form infinite chains of composition [Ga 2 (BO 3 ) 4 ] 6À , with the chains extending parallel to the a axis; lithium cations interleave the chains in tetrahedral interstices. Fig. 2 shows a comparison of the galliumborate chains in both the previously reported structure (Abdullaev & Mamedov, 1972)

Figure 2
A comparison of the infinite [Ga 2 (BO 3 ) 4 ] 6À chains in the structures of Li 3 Ga(BO 3 ) 2 with respect to the model by Abdullaev & Mamedov (1972) (top) and the structure model presented in this manuscript. The connectivity of these two chains are the same but there are important differences in the actual bonding. In our structure model, these chains run down the c axis. Here the GaO 4 tetrahedra are light green and the BO 3 triangles are blue-grey (which are shown in light blue-grey in the 1972 structure). Corner O atoms have been omitted for clarity. differences in bond lengths, bond angles and bond-valencesum values (see Table 1). Averaged interatomic distances for the title structure are consistent with those determined from the ionic radii reported by Shannon (1976), viz. 1.97 (5), 1.85 (2), and 1.39 (4) Å for the experimentally determined Li-O, Ga-O, and B-O distances, respectively. We also calculated the bond-valence-um values for each element using the values provided by Brese & O'Keeffe (1991). The results (Table 1) are in good agreement with the expected values of 1, 3, 3 and 2 for Li, Ga, B and O atoms, respectively. Lastly, Fig. 3 displays the anisotropic displacement parameters of the atoms within the asymmetric unit of the title structure.

Synthesis and crystallization
Powder samples were made by solid-state reactions starting with stoichiometric proportions of lithium nitrate, gallium(III) nitrate, and boric acid. We first ground the starting materials and fired them in an alumina crucible at 573 K for two h to decompose them to finely divided oxides, after which we progressively heated the samples to 973 K at 50 to 100 K and 24-hour increments, grinding the samples between each successive heat treatment. Samples were single-phase as revealed by powder X-ray diffraction.
Single crystals were grown from the melt. About 500 mg of sample were placed in a platinum dish, heated to 1033 K in a box oven, slow-cooled at 10 K h À1 to about 470 K, and then air-quenched. Several small, clear, colorless crystals were physically removed from the platinum crucible and mounted on a goniometer for a preliminary scan in order to find one of suitable quality.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Crystal data
Li 3

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Crystals of Li 3 Ga(BO 3 ) 2 were mounted on MiTeGen Microloop with non-drying immersion oil. The crystal was then optically aligned on the Rigaku SCX-Mini diffractometer using a digital camera. Initial matrix images were collected to determine the unit cell, validity and proper exposure time. Three hemispheres (where φ= 0.0, 120.0 and 240.0) of data were collected with each consisting 180 images each with 1.00° widths and a 1.00° step. The structure of Li 3 Ga(BO 3 ) 2 was refined using SHELXT (Sheldrick, 2015) Intrinsic Phasing and SHELXL (Sheldrick, 2015). Olex2 (Dolomanov et al., 2009) was used as a graphical interface. Images of the above compound were made using CrystalMaker for Windows, version 9.2.8. The refinement proceeded without any incidents and without any need for modelling disorder or twinning or any constraints or restraints. Refinement of the structure was based on F 2 against all reflections. The R-factor R is based on F 2 >2?(F 2 ), but is not relevant to the choice of reflections for refinement; whereas the weighted R-factor wR and goodness of fit S are based on F 2 . The maximum electron denisty is 1.035 and is located 1.629 Å from Li(2), 1.960 Å from Li(1) and 2.169 Å from a different Li(1), which leads to nothing reasonable. All other maximum peaks are under 0.600.