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Lithium sodium aluminium fluoride was obtained as a white powder by melting a stoichiometric mixture of AlF3, NaF and LiF at 1223 K, and then cooling to 923 K and sintering at this temperature for 4 h. The ab initio crystal structure determination was carried out using X-ray powder diffraction techniques. The monoclinic structure of LiNa2AlF6 can be related to cubic elpasolite. The Li and Al atoms lie on inversion centres. The main octahedral AlF6 structural elements are not deformed, but are rotated slightly with respect to the unit-cell axes. The Li atoms have octahedral coordinations, whereas the Na atoms have cubo-octahedral coordinations. The Na coordination polyhedron is distorted in comparison with that of elpasolite.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102018462/iz1026sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108270102018462/iz1026Isup2.rtv
Contains datablock I

Comment top

Several producers of raw aluminium use an electrolyte with up to 2.5% of a LiF additive to improve the processing characteristics. Maintaining the ideal LiF and other constituent concentrations is an important technological task. Fast monitoring of electrolyte chemical compositions can be achieved using X-ray diffraction to quantify the phases from a cooled electrolyte sample. The procedure needs reliable X-ray diffraction reference data for the crystallized phases. The present study of LiNa2AlF6 was prompted particularly by the task of lithium regulation in electrolytes, and consequently the relevant lithium-bearing phases were under consideration.

The Na3AlF6—Li3AlF6 phase diagram has been examined several times (Holm & Holm, 1970). However, it is not yet completely clear what kind of diffraction data can be applied for phase identification and what are the exact phase compositions. According to the phase diagram of Holm & Holm (1970), there are three lithium-bearing phases, viz. Li3AlF6, Li3Na3Al2F12 and LiNa2AlF6. The first and second phases have been structurally characterized (Burns et al., 1968; Geller, 1971). Concerning the third, Holm & Holm reported that they had found an orthorhombic cell. However, they were inclined to consider it as monoclinic because this improved the understanding of the phase transformation. It was also reported that the system demonstrates several phase transitions below the solidus temperature and has extensive fields of solid solutions. The current investigation was focused on LiNa2AlF6, since this is the most closely related phase to that which formed during a sample-taking procedure in the course of electrolyte monitoring. X-ray powder diffraction techniques were used because the phase was a product of subsolidus transformations and a single-crystal was not accessible.

An almost pure substance was obtained and an ab initio crystal structure determination was carried out. X-ray powder indexing without reference to the systematically absent reflections and the crystal structure actually gives an orthorhombic cell, because the deviation of β from 90° is rather small (0.06°). However, the more accurate analysis of overlapped groups of reflections, and especially the total powder diffraction profile-fitting procedure (Le Bail et al., 1988), positively identifies a monoclinic cell. The final structure refinement confirmed this choice completely.

The crystal structure of LiNa2AlF6 is presented in Fig. 1. It is built up from AlF6 octahedra arranged according to a body-centred cell. The geometry of AlF6 is almost perfectly regular and the variation in Al—F bond lengths is no more than 0.01 Å. These lengths correspond well with those in Na3AlF6 (Hawthorne & Ferguson, 1975). The angles deviate from 90° by slightly more than 1°. This seems acceptable because the average octahedral angle deviation in Na3AlF6 (Hawthorne & Ferguson, 1975) is about 0.7°, and that in Li3AlF6 is even more than 1° (Burns et al., 1968). The Li atoms are surrounded by distorted fluorine octahedra, with average bond lengths of about 2.048 Å. The Na atom is in a more spacious position, with distorted cubo-octahedral geometry.

According to the geometrical features, the structure may be considered as a cryolite type, which is often referred to the elpasolite family, NaK2AlF6 (Morss, 1974). More accurate referring can be achieved by comparing with α- and β-Na3AlF6. The β-cryolite is characterized by higher symmetry, space group Immm (Yang et al., 1993), with greater differentiation between the cationic positions, whereas α-cryolite is monoclinic with a smaller variation in Na—F distance (Hawthorne & Ferguson, 1975). It seems that a monoclinic distortion compensates for cationic inequality.

It is also relevant to note the remarkable structural differences between LiNa2AlF6 and Li3Na3Al2F12, with a similar composition. In Li3Na3Al2F12, the Li has a tetrahedral coordination and the Na has an eightfold coordination, whereas in LiNa2AlF6, the Li has an octahedral coordination and the Na has a 12-fold coordination. This is the basis for a supposition that LiNa2AlF6 should evolve preferentially towards the crylolyte Should this be cryolite? structure by loss of Li upon heating.

Thus, the structure of LiNa2AlF6 has commonality with both α- and β-cryolites. It is similar to the α-phase in symmetry, atom arrangement and AlF6 octahedral orientation, and to the β-phase in the specific differentiation between alkali metal positions.

Experimental top

LiNa2AlF6 was obtained as a white powder by melting a stoichiometric mixture of AlF3, NaF and LiF at 1223 K, and then cooling down to 923 K and sintering at this temperature for 4 h. The initial high purity AlF3, NaF and LiF ingredients were used as received (REACHIM). An alternative synthesis route consists of heating a stoichiometric mixture of Na5AlF14, NaF and LiF at 973 K.

Refinement top

Experimental data were collected on an automatic diffractometer with Bragg-Brentano geometry under ambient conditions. The sample was prepared using a top-loading standard quartz sample holder. Corundum was used as the external standard. Cell parameters were obtained using the programs described by Kirik et al. (1979) and Visser (1969). Analysis of the systematic absences gave space group P21/n. The errors given in the tables primarily report the precision of the measurements rather than their accuracy.

Computing details top

Data collection: DRON-4 data collection software; cell refinement: modified Rietveld program DBWM; data reduction: XDIG (local program); program(s) used to solve structure: Patterson and Fourier synthesis (local program); program(s) used to refine structure: modified Rietveld program DBWM; molecular graphics: XP (Siemens, 19??).

Figures top
[Figure 1] Fig. 1. The crystal structure of LiNa2AlF6.
[Figure 2] Fig. 2. Rietveld plot for LiNa2AlF6; the difference profile is shown underneath.
lithium disodium aluminium hexafluoride top
Crystal data top
LiNa2AlF6Z = 2
Mr = 193.89Melting point: decomp. 1053 K K
Monoclinic, P21/nCu Kα radiation, λ = 1.540562, 1.544390 Å
a = 5.2863 (4) ÅT = 293 K
b = 5.3603 (4) ÅParticle morphology: thin powder
c = 7.5025 (6) Åwhite
β = 90.005 (2)°circular flat plate, 20 × 20 mm
V = 212.59 Å3Specimen preparation: Prepared at 973 K and 1000 kPa
Data collection top
DRON-4 powder
diffractometer
Data collection mode: reflection
Radiation source: conventional sealed X-ray tube, BSV-28Scan method: step
Graphite monochromator2θmin = 15°, 2θmax = 118°, 2θstep = 0.02°
Specimen mounting: packed powder pellet
Refinement top
Refinement on F2Excluded region(s): none
Least-squares matrix: fullProfile function: Pearson VII
Rp = 0.07540 parameters
Rwp = 0.1080 restraints
Rexp = 0.0590 constraints
RBragg = 0.044Weighting scheme based on measured s.u.'s
R(F2) = 0.030
χ2 = 3.423Preferred orientation correction: March-Dollase correction
5150 data points
Crystal data top
LiNa2AlF6β = 90.005 (2)°
Mr = 193.89V = 212.59 Å3
Monoclinic, P21/nZ = 2
a = 5.2863 (4) ÅCu Kα radiation, λ = 1.540562, 1.544390 Å
b = 5.3603 (4) ÅT = 293 K
c = 7.5025 (6) Åcircular flat plate, 20 × 20 mm
Data collection top
DRON-4 powder
diffractometer
Scan method: step
Specimen mounting: packed powder pellet2θmin = 15°, 2θmax = 118°, 2θstep = 0.02°
Data collection mode: reflection
Refinement top
Rp = 0.075χ2 = 3.423
Rwp = 0.1085150 data points
Rexp = 0.05940 parameters
RBragg = 0.0440 restraints
R(F2) = 0.030
Special details top

Refinement. R_prof-backgr = 0.076

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Al0.00000.00000.00000.0087 (6)*
Li0.00000.00000.50000.014 (2)*
Na0.0078 (7)0.4658 (3)0.2506 (6)0.022 (6)*
F10.0733 (6)0.0191 (4)0.2328 (9)0.017 (1)*
F20.2232 (7)0.3092 (7)0.5341 (9)0.015 (1)*
F30.1941 (6)0.2730 (6)0.9632 (7)0.014 (1)*
Geometric parameters (Å, º) top
Li—F3i2.042 (3)Li—F2vi2.051 (4)
Na—F3ii2.346 (6)Na—F1vii2.318 (5)
Al—F2iii1.803 (4)Na—F22.592 (7)
Naiv—F32.618 (6)Na—F12.436 (3)
Al—F3v1.808 (3)Naii—F22.315 (7)
Li—F12.044 (7)Na—F2viii2.614 (6)
Al—F11.792 (7)Naix—F32.582 (6)
F1—Al—F2iii89.7 (2)F2viii—Al—F3v91.3 (2)
F1—Al—F2viii90.3 (2)F1—Al—F3vi91.15 (19)
F1—Al—F3v88.85 (19)F2viii—Al—F3vi88.7 (2)
Symmetry codes: (i) x+1/2, y1/2, z+3/2; (ii) x, y+1, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x, y, z+1; (v) x, y, z1; (vi) x, y, z+1; (vii) x+1/2, y+1/2, z+1/2; (viii) x1/2, y+1/2, z1/2; (ix) x+1/2, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaLiNa2AlF6
Mr193.89
Crystal system, space groupMonoclinic, P21/n
Temperature (K)293
a, b, c (Å)5.2863 (4), 5.3603 (4), 7.5025 (6)
β (°) 90.005 (2)
V3)212.59
Z2
Radiation typeCu Kα, λ = 1.540562, 1.544390 Å
Specimen shape, size (mm)Circular flat plate, 20 × 20
Data collection
DiffractometerDRON-4 powder
diffractometer
Specimen mountingPacked powder pellet
Data collection modeReflection
Scan methodStep
2θ values (°)2θmin = 15 2θmax = 118 2θstep = 0.02
Refinement
R factors and goodness of fitRp = 0.075, Rwp = 0.108, Rexp = 0.059, RBragg = 0.044, R(F2) = 0.030, χ2 = 3.423
No. of data points5150
No. of parameters40

Computer programs: DRON-4 data collection software, modified Rietveld program DBWM, XDIG (local program), Patterson and Fourier synthesis (local program), XP (Siemens, 19??).

Selected geometric parameters (Å, º) top
Li—F3i2.042 (3)Li—F2vi2.051 (4)
Na—F3ii2.346 (6)Na—F1vii2.318 (5)
Al—F2iii1.803 (4)Na—F22.592 (7)
Naiv—F32.618 (6)Na—F12.436 (3)
Al—F3v1.808 (3)Naii—F22.315 (7)
Li—F12.044 (7)Na—F2viii2.614 (6)
Al—F11.792 (7)Naix—F32.582 (6)
F1—Al—F2iii89.7 (2)F2viii—Al—F3v91.3 (2)
F1—Al—F2viii90.3 (2)F1—Al—F3vi91.15 (19)
F1—Al—F3v88.85 (19)F2viii—Al—F3vi88.7 (2)
Symmetry codes: (i) x+1/2, y1/2, z+3/2; (ii) x, y+1, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x, y, z+1; (v) x, y, z1; (vi) x, y, z+1; (vii) x+1/2, y+1/2, z+1/2; (viii) x1/2, y+1/2, z1/2; (ix) x+1/2, y+1/2, z+1/2.
 

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