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Crystals of the title compound were extracted from the bulk of grown SrAlF5 crystals as unexpected inclusions that were identified as the long sought after aluminium oxyfluoride. The structure of AlOF is built up from tetra­hedral and octa­hedral polyhedra. Each tetra­hedron is bisected by a mirror plane, with the Al atom and two vertex anions in the plane. All tetra­hedral vertices are positions of competing oxide and fluoride ions and are shared with octa­hedra. These shared vertices belong to two octa­hedral edges which join the octa­hedra to form infinite zigzag chains. The chains are strung along twofold screw axes that run parallel to the unit-cell b axis. The remaining two octa­hedral vertices are occupied only by fluoride ions. A small deficiency in the occupation of the octa­hedral Al position was suggested by the refinement. However, the stoichiometry of the compound is AlOF within experimental uncertainty. The Al-F(O) distances are separated into three groups with average values of 1.652 (3) (tetra­hedra), 1.800 (2) (octa­hedra) and 1.894 (2) Å (octa­hedra). This structure differs widely from the reported tetra­gonal phase Al1-xO1-3xF1+3x (x = 0.0886) [Kutoglu (1992). Z. Kristallogr. 199, 197-201], which consists solely of octa­hedral structural units.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109010671/sq3190sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270109010671/sq3190Isup2.hkl
Contains datablock I

Comment top

Aluminium oxyhalides, especially the bromides and iodides, are frequently present in industrial processes, often as so-called unwanted by-products. An example of their occurrence is the deterioration of discharge lamps, where polycrystalline alumina is preferred over quartz as the material for the envelope of the lamp. During operation there is some corrosion of the alumina wall. This is believed to be due to the formation of AlX3 and AlOX, where X is a halogen (Swihart & Catoire, 2000). Another occurrence of aluminium oxyhalides is the cathode surface in the aluminium electrolysis cell (Sharapova et al., 2005). Petrographic analysis of cell materials has revealed oxyfluoride glasses with variable compositions of AlOF1-x and with index of refraction n = 1.33–1.362. Although the crystalline aluminium oxyhalides AlOCl, AlOBr and AlOI are known (Rouxel, 1962; Hagenmuller, Rouxel & le Neindre, 1961; Hagenmuller, Rouxel et al., 1961; Schafer et al., 1958), the corresponding compound AlOF is absent from the FIZ/NIST Inorganic Crystal Structure Database (Release 2008; ICSD, 2008). Some authors have tried to prepare AlOF using the procedure commonly used to prepare the other AlOX compounds, namely the reaction of AlF3 with As2O3 in a sealed evacuated glass ampoule at elevated temperatures. However, the stoichiometric compound was not formed at temperatures up to 750 K (Siegel & Johnson, 1964). The crystal structure of tetragonal Al1-xO1-3xF1+3x (x = 0.0886) with the rutile structure has been reported (Kutoglu, 1992).

During the examination of SrAlF5 single crystals synthesized from the melt using the Bridgman technique, some crystalline inclusions into the basal crystalline body were detected. Optical polarization investigations revealed considerable variation within the grown crystals, with a rough extinction due to blockings, systematic distortions and crystal inclusions. The inclusions have the form of small faceted crystals, distributed randomly over the bulk of the matrix substance (Fig. 1). The birefringence of the inclusions is greater by an order of magnitude than that of the matrix SrAlF5 crystal, indicating that the inclusions may be grown crystals with another composition. Such inclusions are believed to be the consequence of a small excess of AlF3 included in the reaction mixture, as they were not present if a rigorously stoichiometric loading of the components was implemented for the synthesis.

It has been observed that during repeated thermal cycling to 800 K, the matrix substance becomes more and more cracked near the inclusions. We were therefore readily able to pull some inclusions out of the matrix SrAlF5 crystal for subsequent experiments. They were found to be without damage, transparent, and with ideal orthorhombic faceting and straight extinctions. The value of the birefringence (Δnb = na- nc) in a pinacoid with a thickness of 40 mkm [µm?] is 0.0113. The temperature dependence of the birefringence shows that the optical anisotropy of the included crystals varies only slightly up to 800 K with no appreciable anomalies.

The structure of the included AlOF crystals consists of AlF(O)6 octahedra and AlF(O)4 tetrahedra (Fig. 2). Connecting via their edges, the octahedra line up in a zigzag chain along the unit cell b axis (Fig. 3a) and common edges are formed from the competitive F/O sites only. Each chain is connected to four adjacent ones via the F1 vertices (Fig. 3b). In addition, every F/O ion is the vertex of a tetrahedron which joins three adjacent octahedral chains in the same manner (Fig. 4). As a result, the tetrahedra have common vertices with the octahedra and do not connect to one another.

The interionic distances are strictly separated into three groups. The Al—F(O) distances are 2 × 1.650 (2), 1.652 (3) and 1.658 (3) Å in the tetrahedron but 1.885 (2), 2 × 1.896 (2) and 1.900 (2) Å in the octahedron, and the Al—F distances in the octahedron are 1.792 (2) and 1.808 (2) Å. Tetrahedral distortions are minimal, with tetrahedral angles in the range 107.44 (13)–110.23 (9)°. The interior octahedral angles differ from 90° by as much as 10.5°. By contrast, in the reported tetragonal phase Al1-xO1-3xF1+3x (x = 0.0886; Kutoglu, 1992), the basic structural elements are octahedra which are joined via edges and form infinite columns along the c direction of the unit cell. Each column is connected via all its vertices to four adjacent columns. The Al—F(O) distances are 1.980 (1) and 1.982 (1) Å.

We attempted to refine the site occupancy factors (s.o.f.) for all ions in the structure. The refinement revealed that ions Al1 and F1 occupy their positions completely, while the Al2 position appears to possess some deficiency. Separate refinements of s.o.f.(Al2) and s.o.f.(O and F) with charge-balance retention have shown that, to within two standard uncertainties, s.o.f.(O) = 72% and s.o.f.(F) = 26% in all positions except F1, when s.o.f.(Al2) = 0.977 (4) with the R factor lowered slightly to 0.0297. Based on this refinement, we may assign to the crystal under investigation the composition Al0.987O0.96F1.04, with about 2% deficiency of Al2 in the octahedral position. Nevertheless, in the range of three s.u. the composition of the compound is AlOF. Consequently, in the final refinement the Al2 site was treated as fully occupied and the mixed F/O sites were fixed at 25% F and 75% O. We cannot, however, rule out a slight non-stoichiometry in this compound.

Experimental top

AlF3 (99.99%) and SrF2 (99.99%) were used as starting reagents. Due to the high volatility of AlF3, it was added in a 2–5% excess to the starting reaction mixture. The crystallization temperature was 1140 K. The excess of AlF3 appears to be necessary for the formation of the AlOF inclusions; when exact stoichiometric amounts of the reagents were used, no inclusions formed in the product SrAlF5 crystals. The optical quality of the grown transparent single crystals of SrAlF5 was studied using an Axioskop 40 Pol polarizing microscope with high permission [Permissivity?] and contrast objective `Plan-Neofluar' (Zeiss).

Refinement top

A transparent colourless crystal inclusion was chosen for structure studies. Neither Patterson function nor direct methods produced a correct structure solution. Repeated electron-density syntheses with a cull of `atoms' which acquired heightened displacement parameters, new maxima substitution and variation of atom types allowed us to isolate octahedral and tetrahedral configurations among the maxima disposition and then to define the elemental composition. The analysis indicated that Al is the only metal in the structure. Subsequent refinement of the structural model required the introduction of oxide ions which occupy positions in competition with the fluoride ions. The model refined well under the assumption that the Al, fluoride and oxide ions take part in the compound in equal amounts. The F and O ions compete everywhere in an F:O ratio of 1:3, except the position of F1 where the O does not admix.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 2001); data reduction: SAINT-Plus (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. Photograph of the AlOF inclusion into the SrAlF5 crystal body.
[Figure 2] Fig. 2. The asymmetric unit of the AlOF structure, with the atom-labelling scheme. Symmetry-equivalent anions are shown to complete the Al coordination polyhedra. [Symmetry codes: (i) 1/2 - x, 1 - y, 1/2 + z; (ii) x - 1/2, 1/2 - y, 3/2 - z; (iii) 1 - x, 1 - y, 1 - z; (iv) x - 1/2, y, 1/2 - z; (v) x - 1/2, 1/2 - y, 1/2 - z.]
[Figure 3] Fig. 3. The octahedral ordering in AlOF crystals. (a) The octahedral chain in the structure; (101) plane projection. (b) The unit cell completed with the octahedral chains.
[Figure 4] Fig. 4. The connections of the tetrahedron (centre, shaded darker) in the structure.
Aluminium oxyfluoride top
Crystal data top
AlOFF(000) = 360
Mr = 61.98Dx = 3.565 Mg m3
Orthorhombic, PnmaMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2nCell parameters from 411 reflections
a = 8.825 (2) Åθ = 4.6–26.7°
b = 8.408 (2) ŵ = 1.08 mm1
c = 4.669 (1) ÅT = 299 K
V = 346.44 (14) Å3Prism, colourless
Z = 120.15 × 0.08 × 0.04 mm
Data collection top
Bruker SMART 4K CCD area-detector
diffractometer
441 independent reflections
Radiation source: fine-focus sealed tube337 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.055
ϕ and ω scansθmax = 28.0°, θmin = 4.6°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
h = 1111
Tmin = 0.818, Tmax = 0.958k = 1111
2858 measured reflectionsl = 66
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.031Secondary atom site location: difference Fourier map
wR(F2) = 0.078 w = 1/[σ2(Fo2) + (0.0435P)2]
where P = (Fo2 + 2Fc2)/3
S = 1.02(Δ/σ)max = 0.002
441 reflectionsΔρmax = 0.46 e Å3
49 parametersΔρmin = 0.55 e Å3
Crystal data top
AlOFV = 346.44 (14) Å3
Mr = 61.98Z = 12
Orthorhombic, PnmaMo Kα radiation
a = 8.825 (2) ŵ = 1.08 mm1
b = 8.408 (2) ÅT = 299 K
c = 4.669 (1) Å0.15 × 0.08 × 0.04 mm
Data collection top
Bruker SMART 4K CCD area-detector
diffractometer
441 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
337 reflections with I > 2σ(I)
Tmin = 0.818, Tmax = 0.958Rint = 0.055
2858 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03149 parameters
wR(F2) = 0.0780 restraints
S = 1.02Δρmax = 0.46 e Å3
441 reflectionsΔρmin = 0.55 e Å3
Special details top

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. 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 (Å2) top
xyzUiso*/UeqOcc. (<1)
Al10.05993 (12)0.25000.3977 (2)0.0033 (3)
Al20.36924 (9)0.41728 (9)0.59558 (17)0.0073 (3)
F10.25174 (17)0.55600 (17)0.4000 (3)0.0081 (4)
O20.4677 (3)0.25000.7945 (5)0.0088 (6)0.75
F20.4677 (3)0.25000.7945 (5)0.0088 (6)0.25
O30.5112 (2)0.4081 (2)0.2900 (3)0.0089 (4)0.75
F30.5112 (2)0.4081 (2)0.2900 (3)0.0089 (4)0.25
O40.2452 (3)0.25000.4570 (5)0.0097 (6)0.75
F40.2452 (3)0.25000.4570 (5)0.0097 (6)0.25
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Al10.0031 (5)0.0034 (5)0.0033 (5)0.0000.0005 (4)0.000
Al20.0081 (4)0.0066 (4)0.0074 (5)0.0004 (3)0.0007 (3)0.0002 (3)
F10.0088 (7)0.0079 (7)0.0075 (8)0.0014 (6)0.0020 (6)0.0003 (6)
O20.0112 (13)0.0040 (12)0.0112 (12)0.0000.0010 (10)0.000
F20.0112 (13)0.0040 (12)0.0112 (12)0.0000.0010 (10)0.000
O30.0072 (9)0.0105 (9)0.0090 (9)0.0030 (7)0.0003 (7)0.0012 (7)
F30.0072 (9)0.0105 (9)0.0090 (9)0.0030 (7)0.0003 (7)0.0012 (7)
O40.0135 (13)0.0037 (12)0.0117 (13)0.0000.0009 (11)0.000
F40.0135 (13)0.0037 (12)0.0117 (13)0.0000.0009 (11)0.000
Geometric parameters (Å, º) top
Al1—F3i1.6495 (18)Al2—O41.8962 (19)
Al1—O3i1.6495 (18)Al2—F3iv1.8850 (19)
Al1—F3ii1.6495 (18)Al2—O3iv1.8850 (19)
Al1—O3ii1.6495 (18)Al2—Al1v3.1802 (13)
Al1—F2iii1.652 (3)Al2—Al2vi2.8130 (16)
Al1—O2iii1.652 (3)Al2—Al2iv2.8385 (16)
Al1—O41.658 (3)O2—Al1vii1.652 (3)
Al1—Al2i3.1802 (13)O3—Al1v1.6495 (18)
Al1—Al2ii3.1802 (13)O3—Al2iv1.8850 (19)
Al2—O31.9002 (19)O4—Al2vi1.8962 (19)
Al2—F11.8080 (16)
F3i—Al1—O3i0.00 (14)Al2i—Al1—Al2ii52.50 (3)
F3i—Al1—F3ii107.44 (13)O3—Al2—F191.43 (8)
O3i—Al1—F3ii107.44 (13)O3—Al2—O495.41 (10)
F3i—Al1—O3ii107.44 (13)F1—Al2—O488.57 (8)
O3i—Al1—O3ii107.44 (13)O3—Al2—F3iv82.84 (8)
F3ii—Al1—O3ii0.00 (15)F1—Al2—F3iv87.81 (8)
F3i—Al1—F2iii109.49 (8)O4—Al2—F3iv175.94 (9)
O3i—Al1—F2iii109.49 (8)O3—Al2—O3iv82.84 (8)
F3ii—Al1—F2iii109.49 (8)F1—Al2—O3iv87.81 (8)
O3ii—Al1—F2iii109.49 (8)O4—Al2—O3iv175.94 (9)
F3i—Al1—O2iii109.49 (8)F3iv—Al2—O3iv0.00 (11)
O3i—Al1—O2iii109.49 (8)O3—Al2—Al1v24.43 (5)
F3ii—Al1—O2iii109.49 (8)F1—Al2—Al1v102.89 (6)
O3ii—Al1—O2iii109.49 (8)O4—Al2—Al1v74.35 (8)
F2iii—Al1—O2iii0.00 (15)F3iv—Al2—Al1v104.68 (6)
F3i—Al1—O4110.23 (9)O3iv—Al2—Al1v104.68 (6)
O3i—Al1—O4110.23 (9)O3—Al2—Al2vi87.68 (5)
F3ii—Al1—O4110.23 (9)F1—Al2—Al2vi130.17 (5)
O3ii—Al1—O4110.23 (9)O4—Al2—Al2vi42.12 (6)
F2iii—Al1—O4109.91 (14)F3iv—Al2—Al2vi141.14 (6)
O2iii—Al1—O4109.91 (14)O3iv—Al2—Al2vi141.14 (6)
F3i—Al1—Al2i28.46 (6)Al1v—Al2—Al2vi63.751 (18)
O3i—Al1—Al2i28.46 (6)O3—Al2—Al2iv41.22 (6)
F3ii—Al1—Al2i80.44 (7)F1—Al2—Al2iv89.51 (6)
O3ii—Al1—Al2i80.44 (7)O4—Al2—Al2iv136.52 (9)
F2iii—Al1—Al2i111.68 (9)F3iv—Al2—Al2iv41.62 (5)
O2iii—Al1—Al2i111.68 (9)O3iv—Al2—Al2iv41.62 (5)
O4—Al1—Al2i129.99 (9)Al1v—Al2—Al2iv63.82 (3)
F3i—Al1—Al2ii80.44 (7)Al2vi—Al2—Al2iv119.34 (3)
O3i—Al1—Al2ii80.44 (7)Al2—O3—Al1v127.11 (10)
F3ii—Al1—Al2ii28.46 (6)Al2—O3—Al2iv97.16 (8)
O3ii—Al1—Al2ii28.46 (6)Al1v—O3—Al2iv129.22 (11)
F2iii—Al1—Al2ii111.68 (9)Al1—O4—Al2128.77 (8)
O2iii—Al1—Al2ii111.68 (9)Al1—O4—Al2vi128.77 (8)
O4—Al1—Al2ii129.99 (9)Al2—O4—Al2vi95.76 (12)
Symmetry codes: (i) x1/2, y+1/2, z+1/2; (ii) x1/2, y, z+1/2; (iii) x1/2, y, z+3/2; (iv) x+1, y+1, z+1; (v) x+1/2, y, z+1/2; (vi) x, y+1/2, z; (vii) x+1/2, y, z+3/2.

Experimental details

Crystal data
Chemical formulaAlOF
Mr61.98
Crystal system, space groupOrthorhombic, Pnma
Temperature (K)299
a, b, c (Å)8.825 (2), 8.408 (2), 4.669 (1)
V3)346.44 (14)
Z12
Radiation typeMo Kα
µ (mm1)1.08
Crystal size (mm)0.15 × 0.08 × 0.04
Data collection
DiffractometerBruker SMART 4K CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2004)
Tmin, Tmax0.818, 0.958
No. of measured, independent and
observed [I > 2σ(I)] reflections
2858, 441, 337
Rint0.055
(sin θ/λ)max1)0.660
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.031, 0.078, 1.02
No. of reflections441
No. of parameters49
Δρmax, Δρmin (e Å3)0.46, 0.55

Computer programs: SMART (Bruker, 2001), SAINT-Plus (Bruker, 2001), SHELXTL (Sheldrick, 2008).

 

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