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
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.
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.
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??).
lithium disodium aluminium hexafluoride
top
Crystal data top
LiNa2AlF6 | Z = 2 |
Mr = 193.89 | Melting point: decomp. 1053 K K |
Monoclinic, P21/n | Cu Kα radiation, λ = 1.540562, 1.544390 Å |
a = 5.2863 (4) Å | T = 293 K |
b = 5.3603 (4) Å | Particle morphology: thin powder |
c = 7.5025 (6) Å | white |
β = 90.005 (2)° | circular flat plate, 20 × 20 mm |
V = 212.59 Å3 | Specimen 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-28 | Scan method: step |
Graphite monochromator | 2θmin = 15°, 2θmax = 118°, 2θstep = 0.02° |
Specimen mounting: packed powder pellet | |
Refinement top
Refinement on F2 | Excluded region(s): none |
Least-squares matrix: full | Profile function: Pearson VII |
Rp = 0.075 | 40 parameters |
Rwp = 0.108 | 0 restraints |
Rexp = 0.059 | 0 constraints |
RBragg = 0.044 | Weighting scheme based on measured s.u.'s |
R(F2) = 0.030 | |
χ2 = 3.423 | Preferred orientation correction: March-Dollase correction |
5150 data points | |
Crystal data top
LiNa2AlF6 | β = 90.005 (2)° |
Mr = 193.89 | V = 212.59 Å3 |
Monoclinic, P21/n | Z = 2 |
a = 5.2863 (4) Å | Cu Kα radiation, λ = 1.540562, 1.544390 Å |
b = 5.3603 (4) Å | T = 293 K |
c = 7.5025 (6) Å | circular flat plate, 20 × 20 mm |
Data collection top
DRON-4 powder diffractometer | Scan method: step |
Specimen mounting: packed powder pellet | 2θmin = 15°, 2θmax = 118°, 2θstep = 0.02° |
Data collection mode: reflection | |
Refinement top
Rp = 0.075 | χ2 = 3.423 |
Rwp = 0.108 | 5150 data points |
Rexp = 0.059 | 40 parameters |
RBragg = 0.044 | 0 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 | x | y | z | Uiso*/Ueq | |
Al | 0.0000 | 0.0000 | 0.0000 | 0.0087 (6)* | |
Li | 0.0000 | 0.0000 | 0.5000 | 0.014 (2)* | |
Na | −0.0078 (7) | 0.4658 (3) | 0.2506 (6) | 0.022 (6)* | |
F1 | 0.0733 (6) | 0.0191 (4) | 0.2328 (9) | 0.017 (1)* | |
F2 | 0.2232 (7) | 0.3092 (7) | 0.5341 (9) | 0.015 (1)* | |
F3 | 0.1941 (6) | 0.2730 (6) | 0.9632 (7) | 0.014 (1)* | |
Geometric parameters (Å, º) top
Li—F3i | 2.042 (3) | Li—F2vi | 2.051 (4) |
Na—F3ii | 2.346 (6) | Na—F1vii | 2.318 (5) |
Al—F2iii | 1.803 (4) | Na—F2 | 2.592 (7) |
Naiv—F3 | 2.618 (6) | Na—F1 | 2.436 (3) |
Al—F3v | 1.808 (3) | Naii—F2 | 2.315 (7) |
Li—F1 | 2.044 (7) | Na—F2viii | 2.614 (6) |
Al—F1 | 1.792 (7) | Naix—F3 | 2.582 (6) |
| | | |
F1—Al—F2iii | 89.7 (2) | F2viii—Al—F3v | 91.3 (2) |
F1—Al—F2viii | 90.3 (2) | F1—Al—F3vi | 91.15 (19) |
F1—Al—F3v | 88.85 (19) | F2viii—Al—F3vi | 88.7 (2) |
Symmetry codes: (i) −x+1/2, y−1/2, −z+3/2; (ii) −x, −y+1, −z+1; (iii) −x+1/2, y−1/2, −z+1/2; (iv) x, y, z+1; (v) x, y, z−1; (vi) −x, −y, −z+1; (vii) −x+1/2, y+1/2, −z+1/2; (viii) x−1/2, −y+1/2, z−1/2; (ix) x+1/2, −y+1/2, z+1/2. |
Experimental details
Crystal data |
Chemical formula | LiNa2AlF6 |
Mr | 193.89 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 293 |
a, b, c (Å) | 5.2863 (4), 5.3603 (4), 7.5025 (6) |
β (°) | 90.005 (2) |
V (Å3) | 212.59 |
Z | 2 |
Radiation type | Cu Kα, λ = 1.540562, 1.544390 Å |
Specimen shape, size (mm) | Circular flat plate, 20 × 20 |
|
Data collection |
Diffractometer | DRON-4 powder diffractometer |
Specimen mounting | Packed powder pellet |
Data collection mode | Reflection |
Scan method | Step |
2θ values (°) | 2θmin = 15 2θmax = 118 2θstep = 0.02 |
|
Refinement |
R factors and goodness of fit | Rp = 0.075, Rwp = 0.108, Rexp = 0.059, RBragg = 0.044, R(F2) = 0.030, χ2 = 3.423 |
No. of data points | 5150 |
No. of parameters | 40 |
Selected geometric parameters (Å, º) topLi—F3i | 2.042 (3) | Li—F2vi | 2.051 (4) |
Na—F3ii | 2.346 (6) | Na—F1vii | 2.318 (5) |
Al—F2iii | 1.803 (4) | Na—F2 | 2.592 (7) |
Naiv—F3 | 2.618 (6) | Na—F1 | 2.436 (3) |
Al—F3v | 1.808 (3) | Naii—F2 | 2.315 (7) |
Li—F1 | 2.044 (7) | Na—F2viii | 2.614 (6) |
Al—F1 | 1.792 (7) | Naix—F3 | 2.582 (6) |
| | | |
F1—Al—F2iii | 89.7 (2) | F2viii—Al—F3v | 91.3 (2) |
F1—Al—F2viii | 90.3 (2) | F1—Al—F3vi | 91.15 (19) |
F1—Al—F3v | 88.85 (19) | F2viii—Al—F3vi | 88.7 (2) |
Symmetry codes: (i) −x+1/2, y−1/2, −z+3/2; (ii) −x, −y+1, −z+1; (iii) −x+1/2, y−1/2, −z+1/2; (iv) x, y, z+1; (v) x, y, z−1; (vi) −x, −y, −z+1; (vii) −x+1/2, y+1/2, −z+1/2; (viii) x−1/2, −y+1/2, z−1/2; (ix) x+1/2, −y+1/2, z+1/2. |
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.