LiNa3(SO4)2·6H2O: a lithium double salt causing trouble in the industrial conversion of Li2SO4 into LiOH

The crystal structure of LiNa3(SO4)2·6H2O is discussed. In the context of the production of LiOH for batteries, the formation of the double salt should be avoided. Detection of its presence by means of XRD is important.


Chemical context
In the presently preferred process of LiOH production for batteries, an aqueous Li 2 SO 4 solution is reacted with NaOH at temperatures well below 273 K (mostly at 268 K) for separating sodium sulfate in the form of its decahydrate (Glauber salt) according to the equation Li 2 SO 4(aq) + 2 NaOH (aq) ! 2 LiOH (aq) + Na 2 SO 4 Á10H 2 O (s) The sodium sulfate hydrate is removed and from the remaining solution, water is evaporated to crystallize LiOHÁH 2 O. However, during cooling the solution from ambient temperature, the solution passes the stability field of LiNa 3 (SO 4 ) 2 Á6H 2 O, which extends from 271.3 to 321 K (Sohr et al., 2017). Once formed, it will not disappear on further cooling. Rapid and reliable detection of its presence or absence by means of XRD is important. A powder diffraction pattern is available from the PDF database (Powder Diffraction File 33-1258, International Center for Diffraction Data), but no conclusive comment is attached regarding the conditions under which the material was obtained and prepared for powder XRD. It is known that the material loses its water of crystallization very easily. Therefore, in their careful thermodynamic study of the system Li 2 SO 4 -Na 2 SO 4 -H 2 O at 298 K, Filippov & Kalinkin (1989) did not make an attempt to isolate the double salt hydrate because of instability. Ji et al. (2015) include a figure of the PXRD pattern, but only in a mixture with anhydrous LiNaSO 4 . The growth of crystals under defined conditions and deriving the PXRD pattern from single-crystal structure analysis could resolve doubts about the PXRD pattern.
LiNa 3 (SO 4 ) 2 Á6H 2 O was first crystallized by Mitscherlich (1843) and later, preparative conditions were specified (Scacchi, 1867). Early crystallographic characterization is summarized by Groth (1908), where the cited paper of Traube (1894) is of particular interest, since he determined the correct ISSN 2056-9890 polar point group 3m for this compound and the isomorphic compounds LiNa 3 (MO 4 ) 2 Á6H 2 O with M = S, Se, Mo, Cr. Even a mixed compound LiNa 3 {(SO 4 ) 0.5 (CrO 4 ) 0.5 } was described within this series. A first crystal structure of the molybdate was published by Klevtsova et al. (1988). Later, Kaminskii and coworkers grew large crystals of the molybdate (Kaminskii et al., 2009) and selenate (Kaminskii et al., 2007) for studies on the non-linear optical effects of the materials, where they also redetermined and refined the crystal structures at ambient temperature, but without discussion of structural details.

Structural commentary
Single-crystal structure determination was performed at five temperatures between 90 and 293 K. At all temperatures, the structure could be solved in the polar space group R3c H (161). The cell parameters varied continuously with temperature (Table 1 and Fig. 1). Thus, the results confirm the isomorphism to the molybdate LiNa 3 (MoO 4 ) 2 Á6H 2 O (Kaminskii et al., 2009) and no structural change within the investigated temperature range. Fig. 2 shows the asymmetric unit completed with atoms to visualize the coordination of sodium, lithium and sulfur. There is only one crystallographically distinguishable sodium and lithium position, but two for sulfur. Sodium is surrounded by six oxygen atoms, three belong to water molecules (blue) and the remaining three to sulfate groups. The distance of 2.639 Å between Na1 and O4 is quite long. Also, the angle O1-Na1-O4 of 165 deviates considerably from 180 . However, in a first approximation the environment of sodium atoms can be described as a distorted octahedron. The water molecules with O6 bridge three sodium ions to a trimeric unit as shown in Fig. 3. The trimers look like cyclohexane rings (Fig. 3b) in a chair conformation with the water molecules on the upper three points (Fig. 3c) Asymmetric unit plus bonds. Ellipsoids are drawn at the 50% level. Symmetry codes: (I) 1 À y, 1 + x À y, z; (II) Ày À y, 1 À x, z; (III) À 1 3 À x + y, À 2 3 + y, À1/6 +) ; (IV) Àx + y, Àx, z; (V) 1 À y, x À y, z; (VI) 1 À x + y, 1 À x, z; (VII) 1 3 À x + y, À 1 3 + y, 1 6 + z; (VIII) 4/3 À y, 2 3 À x, 1 6 + z; (IX) 1 3 + x, 2 3 + x À y, 1 6 + z.

Figure 3
The trimeric unit Na 3 (H 2 O) 3 viewing from different directions (a, b and c).

Figure 4
Stacking of (a) the trimeric Na 3 (H 2 O) 3 and (b) the Li(H 2 O) 3 (SO 4 ) units along the c axis within a unit cell. Additional SO 4 groups with the S2 sulfur atom are shown (dark yellow).

Figure 1
Variation of lattice parameters with temperature; a axis in red, c axis in blue, values divided by four, volume shown in black; circles: from singlecrystal measurements, stars: data from powder X-ray measurement: a = 8.4552 (7) The lithium cation is coordinated by three water molecules (O5) and the apex (O3) of a sulfate anion containing S1 completes a tetrahedron (Fig. 2). Thus, the trimeric Na 3 (H 2 O) 3 and the double tetrahedron Li(H 2 O) 3 (SO 4 ) form the characteristic structural units of this compound. In Fig. 4, the arrangement of these units is shown within the unit cell separately for Na 3 (H 2 O) 3 (Fig. 4a) and Li(H 2 O) 3 (SO 4 ) (Fig. 4b). In Fig. 4b the sulfate anions with S2 are added as darker colored tetrahedra. The repeat unit requires stacking of six such units along the c-axis direction. The uniform orientation of the units underlines the polar character of the c axis.

Supramolecular features
The overall structure of the compound is polymeric with water and sulfate anions connecting the cations. The three water molecules coordinated at the lithium cation are at the same time coordinated to three sodium cations, each sodium ion belonging to another trimeric sodium ring forming a watercation coordination network, as shown in Fig. 5. When including the entire coordination spheres of sodium, one can describe the trimers as edge-bridged octahedra, as illustrated in Fig. 6a and 6b. Thereby, the O4 oxygen from the sulfate anion of S2 represents a common coordination point from below (Fig. 6a). The height of sulfur S1 along the c axis is near that of Na1. Thus, the three corners of this sulfate tetrahedron connect three trimeric units within a sodium ion layer, as shown in Fig. 7 from two viewing angles. As shown in Fig. 6, the sulfate with S2 is positioned with its oxygen atom (O4) at the center below the trimeric units, and thus the other three O1 atoms of this sulfate anion connect three trimeric sodium units from the adjacent layer below (Fig. 8). In this way, the sulfate with S2 acts as a connector between sodium layers and the sulfate with S1 within one layer. Additional interconnections between layers are realized by the sulfate of S1 as Cation-water coordination network within the ab plane. Sodium = gray, oxygen = blue, lithium = pink, hydrogen = white.

Figure 6
Representation of the sodium ion coordination within a trimeric unit: (a) stick and ball, (b) polyhedrons.

Figure 7
Interconnection of trimeric units within one layer by sulfate tetrahedra with the S1 sulfur atom viewed from two directions (a and b).

Figure 9
Interconnections of sodium layers by sulfate tetrahedra with the S2 sulfur atom (dark yellow).

Figure 8
Interconnections of sodium layers by sulfate tetrahedra with the S2 sulfur atom (dark yellow).
part of the double tetrahedron Li(H 2 O) 3 (SO 4 ), as illustrated in Fig. 9. Investigation of the hydrogen-bond network (Table 1) revealed that, interestingly, the water molecules form hydrogen bonds only to the sulfate groups, but not between themselves as is observed in a channel-like arrangement in Li 2 SO 4 ÁH 2 O (Fig. 10). However, as can be seen from Fig. 11, the hydrogen atoms H1 and H3 share O1 as a common acceptor atom of the sulfate with S1, and H2 and H4 do the same with O2 at the sulfate anion of S2. The bond lengths vary between 1.92 and 2.15 Å . Fig. 12 shows a larger part of the hydrogen-bond network, projected both along the c axis ( Fig. 12a) and perpendicular to the c axis (Fig. 12b). From the latter, it can be recognized that the hydrogen bonds contribute to the bonding strength within a layer, but not between the layers. Connections between the layers are established by cation-anion coordination as shown in Figs. 8 and 9.

Database survey
In the Inorganic Crystal Structure Database (ICSD), only 1164 records with space group R3c (No. 161) can be found. Most of them belong to the LiNbO 3 or Whitlockite type [Whitlockite = MgCa 9 (PO 4 ) 6 (HPO 4 )]. Compounds containing lithium in this space group numbered 179, of which 148 belong again to LiNbO 3 type. The isomorphic molybdate (ICSD col 65006, col 420160) represents a structure type of its own. The  Hydrogen bonds from water molecules to the S1 and S2 sulfate groups.

Figure 12
Larger part of the hydrogen-bond network projected on (a) the ab plane and (b) the ac plane.

Synthesis, crystallization and characterization
Single crystals were grown from about 120 mL of an aqueous solution containing Li 2 SO 4 and Na 2 SO 4 in a molar ratio of approx. 1:1 and an absolute concentration well below the solubility line (Fig. 13). The solution was kept in an desiccator with 50% H 2 SO 4 solution as drying agent. Over two weeks, a number of crystals with sizes of 1-7 mm were formed that showed the typical trigonal-pyramidal form. Small pieces were cut for XRD measurements. The density of 1.995 g cm À3 calculated from the parameters at 293 K (Table 1) is in excellent agreement with the experimental value of 2.009 g cm À3 as cited in Groth (1908). Attempts were made to record powder XRD patterns from quickly ground crystals. Large crystals appear stable at least for some minutes on a filter paper. However, when grinding to achieve a crystal powder, dehydration took place. In cases of less intensive grinding, the texture effects were too large for a representative powder XRD pattern. Thus, particularly for powder XRD measurements, a suspension of fine crystals was prepared: To a 2 molar solution of Na 2 SO 4 , an equivalent amount of anhydrous Li 2 SO 4 was added. The suspension was stirred two days at 298 K. The supernatant solution was decanted and subsequently some slurry was transferred into the expanded, upper part of a Hilgenberg glass capillary. By means of a centrifuge (30 minutes at 4000 r.p.m.), the crystals were pressed into the capillary. This way the available capillary volume was effectively filled with crystals ( Fig. 14). A PXRD pattern obtained under rotation is shown in Fig. 15 in comparison with the one calculated from the crystal structure.
The powder pattern was measured at room temperature on a Bruker D8 Discover diffractometer in Bragg-Brentano geometry with Cu K radiation ( = 1.5406 Å ) and a linear detector Vå ntec-1 (geometry angle 1 ). The measurements were made with a Gö bel mirror as monochromator with a 1.0 mm slit and a 2.5 primary soller. The generator was set to 40 kV/40 mA. The program TOPAS 5.0 (Bruker, 2009) was used to refine the lattice parameters ( Fig. 1). The solved structure from single crystal XRD at 293K was used as starting point of the refinement.
Thermal analyses ( Fig. 16) were performed from roughly crushed, large single crystals. Water is released in one step below 353 K. The mass loss of 29.2% is near the theoretical value of 28.7%. In a second experiment, the measured value was 29.1%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Structure solution using direct methods and a refinement of the atomic positions with respect to the isotropic displacement parameters led to the positions of the Na, Li, S and O atoms. The positions of the H atoms could be located from residual electron-density maxima after further refinement. H atoms were refined isotropically.

Figure 14
Image of a Hilgenberg capillary (diameter 0.5 mm) filled with crystals of LiNa 3 (SO 4 ) 2 Á6H 2 O by means of centrifugal compaction.

Figure 15
Powder XRD pattern of LiNa 3 (SO 4 ) 2 Á6H 2 O recorded from a rotating capillary. Scan rate: 20 sec, steps 0.023 . For comparison, the calculated powder pattern from structural data at 293 K is also shown.

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.

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.

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )

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.