Lithium carnallite, LiCl·MgCl₂·7H₂O

The title compound, lithium magnesium chloride hepta­hydrate, LiCl·MgCl₂·7H₂O, was analyzed in 1988 by powder X-ray diffraction [Emons, Brand, Pohl & Köhnke (1988). Z. Anorg. Allg. Chem. 563, 180–184] and a monoclinic crystal lattice was determined. In the present work, the structure was solved from single-crystal diffraction data. A trigonal structure was found, exhibiting a network structure of Mg(H₂O)₆ octa­hedra and Li(H₂O)Cl₃ tetra­hedra connected by H...Cl hydrogen bonds. The [Li(H₂O)]⁺ unit is coordinated by distorted edge-connected Cl⁻ octa­hedra.

S1. Comment

LiCl.MgCl₂.7H₂O belongs to the group of double salts MX.MgX₂.6H₂O with M equal to Li(H₂O)⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺ and H₃O⁺, and X equal to Cl^-, Br^- and I^-. The most important member of this group is the mineral carnallite, KCl.MgCl₂.6H₂O. Carnallite is formed during evaporation of potassium and magnesium rich waters. It belongs to the natural salts and represents a source for potash fertilizer production and the recovery of magnesium chloride.

The structure analysis of LiCl.MgCl₂.7H₂O is motivated by a growing interest in natural lithium resources for battery materials. Lithium carnallite is formed during evaporation of brines from salt lakes in South America.

In order to study the crystal chemistry of MX.MgX₂.6H₂O compounds, Emons et al. (1988) performed powder diffraction experiments on all of the above-mentioned carnallites, except (H₃O)X.MgX₂.6H₂O. Crystal structures were determined for KCl.MgCl₂.6H₂O (Fischer, 1973; Schlemper et al., 1985), NH₄Cl.MgCl₂.6H₂O (Nakayasu, 1983; Solans et al., 1983; Marsh, 1992b), RbCl.MgCl₂.6H₂O (Waizumi, Masuda, Ohtaki, Burkov & Scripkin, 1991; Marsh, 1992a), CsCl.MgCl₂.6H₂O (Waizumi, Masuda & Ohtaki, 1991), and RbBr.MgBr₂.6H₂O and CsBr.MgBr₂.6H₂O (Dinnebier et al., 2008).

The structure of LiCl.MgCl₂.7H₂O consists, as shown in Fig. 1, basically of Mg(H₂O)₆ octahedra and Li(H₂O)Cl₃ pseudo-tetrahedra. The dumpbell-shaped Li(H₂O)⁺ unit is oriented with the bonding axis parallel to the threefold symmetry axis. Consequently, the Li(H₂O)Cl₃ tetrahedron consists of three symmetry equivalent Cl^- and H⁺ positions, where the two H atoms of the water molecule occupy statistically three energetically equivalent positions. Typical for MX.MgX₂.6H₂O structures, but not necessarily expected for LiCl.MgCl₂.7H₂O, is the octahedral coordination of M by Cl^-. Considering the Li(H₂O)⁺ unit as coordination center, a distorted Li(H₂O)Cl₆ octahedra results, as presented in Fig. 2. A combination of three Li···Cl bonds of 2.3806 (10) Å and three H···Cl hydrogen bonds of 2.52 (5) Å length leads to an edge-connected three-dimensional network of Li(H₂O)Cl₆ octahedra, which is stabilized by hydrogen-bonded Mg(H₂O)₆ octahedra. A main reason for the trigonal lattice, as shown for the unit cell in Fig. 3, can be seen in the trigonal symmetry of the Li(H₂O) dumbbell (Fig. 2). Despite its trigonal structure, LiCl.MgCl₂.7H₂O is structurally related to a cubic lattice. In the rhombohedral setting of the unit cell, the lattice parameters are a = 6.675 Å and α = 87.51°. This nearly cubic atomic arrangement is obvious from Fig. 4.

Related literature

For related literature, see: Dinnebier et al. (2008); Emons et al. (1988); Fischer (1973); Marsh (1992, 1992); Nakayasu (1983); Schlemper et al. (1985); Solans et al. (1983); Waizumi, Masuda & Ohtaki (1991); Waizumi, Masuda, Ohtaki, Burkov & Scripkin (1991).

S2. Experimental

LiCl.MgCl₂.7H₂O was prepared by cooling a solution of MgCl₂.6H₂O (23.4 g) and LiCl (14.9 g) in deionized water (17.6 g) from 345 to 303 K within a period of 3 d. This was carried out in closed test tubes with constant rotation of the bubbler in a climatic chamber (Vötsch VC4043). To prevent contact of the air humidity with the crystals they were covered by n-hexane. A crystal of 0.3 × 0.3 × 0.3 mm was selected and embedded in a two-compound adhesive-based epoxy resin (UHU plus sofortfest) before being mounted on the single-crystal diffractometer.

S3. Refinement

A 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 Mg, O and Cl atoms. The positions of the H and Li atoms could be located from residual electron-density maxima after further refinement. The site occupancy of the H atom belonging to O3 (Fig. 1) refined to a value of 2/3, indicating that two H atoms share three equivalent positions.

Computing details

Data collection: APEX2 (Bruker, 2005); cell refinement: APEX2 (Bruker, 2005); data reduction: APEX2 (Bruker, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2009).

Figures

	Fig. 1. The asymmetric unit and symmetry-related atoms of LiCl.MgCl2.7H2O. Displacement ellipsoids are drawn at the 50% probability level. H atoms are not labeled. [Symmetry codes: (i) -y, x - y, z; (ii) -x + y, -x, z; (iii) -y + 1, x - y + 1, z; (iv) -x + y, -x + 1, z.]
	Fig. 2. The octahedral coordination of the Li(H2O)^+unit by Cl^-.
	Fig. 3. A projection along the [001] direction of a trigonal unit cell of LiCl.MgCl2.7H2O.
	Fig. 4. A projection showing the rhombohedral and nearly cubic framework of the lithium carnallite structure.

Lithium magnesium chloride heptahydrate

Crystal data

LiCl·MgCl2·7H2O	Dx = 1.476 Mg m^−3
Mr = 263.71	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 432 reflections
Hall symbol: R 3	θ = 3.1–30.0°
a = 9.2322 (3) Å	µ = 0.82 mm^−1
c = 12.0541 (5) Å	T = 293 K
V = 889.77 (6) Å^3	Rhombohedral, colourless
Z = 3	0.30 × 0.30 × 0.30 mm
F(000) = 408

Data collection

Bruker X8 kappa diffractometer	1134 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.033
Graphite monochromator	θmax = 30.0°, θmin = 3.1°
ϕ scans, and ω scans	h = −12→12
4767 measured reflections	k = −12→12
1150 independent reflections	l = −16→16

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.016	w = 1/[σ^2(Fo^2) + (0.0256P)^2 + 0.0237P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.038	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 0.23 e Å^−3
1150 reflections	Δρmin = −0.20 e Å^−3
58 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.0105 (10)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 572 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: 0.01 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cl1	0.28090 (3)	0.14521 (3)	0.50017 (2)	0.02998 (7)
Li1	0.0000	0.0000	0.4348 (2)	0.0320 (6)
Mg1	0.3333	0.6667	0.51437 (4)	0.02086 (11)
O1	0.33936 (12)	0.48624 (11)	0.61057 (6)	0.03604 (17)
O2	0.51991 (10)	0.67841 (12)	0.41485 (6)	0.03345 (16)
O3	0.0000	0.0000	0.27800 (13)	0.0559 (4)
H1	0.387 (3)	0.493 (3)	0.664 (2)	0.061 (6)*
H2	0.309 (3)	0.395 (3)	0.5826 (16)	0.049 (4)*
H3	0.597 (2)	0.679 (2)	0.4376 (16)	0.046 (4)*
H4	0.543 (2)	0.712 (2)	0.3523 (16)	0.046 (4)*
H5	0.052 (7)	0.086 (5)	0.234 (3)	0.103 (15)*	0.67

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.03165 (11)	0.02948 (11)	0.02796 (10)	0.01464 (10)	0.00257 (8)	0.00054 (7)
Li1	0.0339 (9)	0.0339 (9)	0.0281 (12)	0.0170 (4)	0.000	0.000
Mg1	0.02171 (16)	0.02171 (16)	0.01916 (19)	0.01085 (8)	0.000	0.000
O1	0.0527 (5)	0.0289 (4)	0.0296 (4)	0.0227 (3)	−0.0118 (3)	−0.0005 (3)
O2	0.0304 (3)	0.0503 (5)	0.0259 (3)	0.0248 (3)	0.0044 (3)	0.0025 (3)
O3	0.0684 (7)	0.0684 (7)	0.0307 (7)	0.0342 (4)	0.000	0.000

Geometric parameters (Å, º)

Cl1—Li1	2.3806 (10)	Mg1—O2^iii	2.0569 (7)
Li1—O3	1.890 (3)	Mg1—O2	2.0569 (7)
Li1—Cl1^i	2.3806 (10)	O1—H1	0.77 (2)
Li1—Cl1^ii	2.3806 (10)	O1—H2	0.81 (2)
Mg1—O1^iii	2.0531 (8)	O2—H3	0.76 (2)
Mg1—O1^iv	2.0531 (8)	O2—H4	0.803 (19)
Mg1—O1	2.0531 (8)	O3—H5	0.87 (4)
Mg1—O2^iv	2.0569 (7)
O3—Li1—Cl1	109.34 (7)	O1—Mg1—O2^iii	88.75 (3)
O3—Li1—Cl1^i	109.34 (7)	O2^iv—Mg1—O2^iii	89.42 (3)
Cl1—Li1—Cl1^i	109.60 (7)	O1^iii—Mg1—O2	178.17 (4)
O3—Li1—Cl1^ii	109.34 (7)	O1^iv—Mg1—O2	88.75 (3)
Cl1—Li1—Cl1^ii	109.60 (7)	O1—Mg1—O2	90.60 (4)
Cl1^i—Li1—Cl1^ii	109.60 (7)	O2^iv—Mg1—O2	89.42 (3)
O1^iii—Mg1—O1^iv	91.23 (4)	O2^iii—Mg1—O2	89.42 (3)
O1^iii—Mg1—O1	91.23 (4)	Mg1—O1—H1	131.0 (17)
O1^iv—Mg1—O1	91.23 (4)	Mg1—O1—H2	118.1 (13)
O1^iii—Mg1—O2^iv	88.75 (3)	H1—O1—H2	109 (2)
O1^iv—Mg1—O2^iv	90.60 (4)	Mg1—O2—H3	123.0 (14)
O1—Mg1—O2^iv	178.17 (4)	Mg1—O2—H4	128.1 (12)
O1^iii—Mg1—O2^iii	90.60 (4)	H3—O2—H4	106.1 (19)
O1^iv—Mg1—O2^iii	178.17 (4)	Li1—O3—H5	127 (2)

Symmetry codes: (i) −y, x−y, z; (ii) −x+y, −x, z; (iii) −x+y, −x+1, z; (iv) −y+1, x−y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···Cl1^v	0.77 (3)	2.46 (3)	3.2156 (10)	171 (2)
O1—H2···Cl1	0.82 (2)	2.40 (2)	3.2056 (9)	168 (3)
O2—H3···Cl1^vi	0.76 (2)	2.43 (2)	3.1803 (11)	172.2 (17)
O2—H4···Cl1^vii	0.803 (19)	2.382 (19)	3.1845 (8)	179 (2)
O3—H5···Cl1^viii	0.87 (4)	2.53 (4)	3.3631 (7)	160 (4)

Symmetry codes: (v) −y+2/3, x−y+1/3, z+1/3; (vi) −x+y+1, −x+1, z; (vii) x+1/3, y+2/3, z−1/3; (viii) −x+y+1/3, −x+2/3, z−1/3.