α-BaCaAlF₇

α-Barium calcium aluminium heptafluoride is a member of the isotypic complex fluorides of general formula BaCaMF₇ (M = Al, Ga, Cr). The crystal structure is made up of triple layers which extend parallel to the (001) plane, and are stacked along [001], with a stacking unit of c/2. The central unit of one triple layer consists of edge-sharing _∞²[CaF_8/2] polyhedra, forming a distorted fluorite-type arrangement. On both sides of this layer, isolated [AlF₆] octahedra are attached by edge-sharing with the central unit. The triple layers are held together by 11-coordinate Ba atoms that are located between the layers.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Al-F) = 0.002 Å
• R factor = 0.019
• wR factor = 0.043
• Data-to-parameter ratio = 17.2

checkCIF results

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ADDSYM reports no extra symmetry

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S1. Comment

Complex fluorides are interesting host lattices for doping with rare-earth or transition metal ions to obtain luminescent materials with slightly different characteristics as compared to the widespread used oxides (Rubio, 1991; Kubel et al., 1997; Joubert et al., 2001).

Numerous phases in the pseudobinary system MF₂—AlF₃ with M = Ca, Sr, Ba have been reported and the corresponding structures solved and refined either from single-crystal or powder data [M = Ca: CaAlF₅ (Hemon & Courbion, 1991) and Ca₂AlF₇ (Domesle & Hoppe, 1980); M = Sr: two polymorphs of SrAlF₅ (von der Muehll et al., 1971; Kubel, 1998; Weil et al., 2001) and Sr₅Al₂F₁₆ (Weil, 2001); M = Ba: four polymorphic forms of BaAlF₅ (Domesle & Hoppe, 1982a; Le Bail et al., 1990; Weil et al., 2001), three polymorphic forms of Ba₃AlF₉ (Renaudin et al., 1990; Renaudin et al., 1991; Le Bail, 1993) and Ba₃Al₂F₁₂ (Domesle & Hoppe, 1982b)], whereas the number of structurally well characterized compounds in the pseudo-ternary system MF₂–M'F₂–AlF₃ (M, M' = Ca, Sr, Ba) is restricted to only three representatives, with composition Ba_0.43Sr_0.57AlF₅ (Kubel, 1998), Sr_0.92Ca_0.08AlF₅ and Sr_0.67Ca_0.33AlF₅ (Weil et al., 2001) (all single-crystal data). Additionally, two more phases with composition SrCaAlF₇ and BaCaAlF₇ are reported, each with three polymorphic forms denoted as α, β and γ (Hoffman, 1972). For four modifications (all SrCaAlF₇ polymorphs and α-BaCaAlF₇), non-indexed powder data are given and their Eu^II activation was measured. The latter phase is a very interesting candidate as a luminescent material since it shows a high Eu^II emission line. Fig. 1 shows that the phase previously described as α-BaCaAlF₇ is identical to the title compound. Except for an impurity line at 2θ = 27.4°, the entry in the powder diffraction file (PDF # 27–0090; ICDD, 2001) is in agreement with the powder pattern calculated on the basis of the single-crystal structure refinement.

α-BaCaAlF₇ is isotypic with BaCaGaF₇ and BaCaCrF₇ (Holler & Babel, 1985). According to the slightly smaller ionic radius of Al^III (0.54 Å) with respect to Ga^III and Cr^III (both 0.62 Å, values from Shannon, 1976), the unit-cell volume of the title compound is slightly smaller (Al: 544 ų; Ga: 553 ų; Cr: 556 ų). The structure is built up from triple layers which extend parallel to (001) and consist of a central unit and two side units (Fig. 2). The central unit is composed of edge-sharing ²_∞[CaF_8/2] polyhedra with distorted square antiprisms as the corresponding coordination figures. Although the intrapolyhedral geometry differs significantly from that of a cube found in the ideal fluorite structure, this arrangement can be considered as part of a distorted CaF₂ structure. This assumption is confirmed by comparable distances d(Ca1—Ca2) of 3.703 (1) and 3.902 (1) Å, as well as the average Ca—F distance of 2.372 Å and the value of the a and b axis with respect to the distances and the lattice parameter in CaF₂ itself [d(Ca—Ca) = 3.869 Å, d(Ca—F) = 2.369 Å and a = 5.471 Å; values from Zhurova et al. (1996)]. The side units are made up from isolated [AlF₆] octahedra which share common edges with the central fluorite-type ²_∞[CaF_8/2] layer. The average Al—F distance of 1.808 Å is in the typical range for [AlF₆] octahedra found in other complex aluminium fluorides. The largest deviation from the ideal octahedral angle is 6.6°. Two triple layers are stacked along [001] with a stacking unit of c/2 and are held together by 11-fold coordinated Ba atoms. The resulting [BaF₁₁] polyhedron might be described as a distorted tricapped rectangular prism (Fig. 3), with an average Ba—F distance of 2.850 Å. The F atoms exhibit either a coordination number (CN) of 3, resulting in a distorted trigonal planar environment (F1, F2 and F4), or CN = 4, with a distorted tetrahedral coordination geometry (F3, F5, F6 and F7).

S2. Experimental

A mixture of 457 mg BaF₂ (Riedel-de Haën, 97%), 41 mg CaF₂ (Merck, Suprapur) and 44 mg AlF₃ (CERAC, 99.9%), corresponding to a molar ratio of 5:1:1, was added to 1 g ZnCl₂ (Aldrich, 98%) and placed in a platinum crucible. The reaction mixture was then heated under atmospheric conditions to 873 K over a period of 2 h. The flux was kept at that temperature for 2 h and then cooled to 473 K over a period of 12 h. After cooling to room temperature, the solidified melt was leached with demineralized water. Colourless plates of the title compound were isolated from the residue. Additionally, tetragonal plates of matlockite-type BaFCl (Sauvage, 1974) were found in the crystal mixture.

Computing details

Data collection: SMART (Siemens, 1996); cell refinement: SAINT (Siemens, 1996); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS for Windows (Dowty, 2000); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Powder patterns of α-BaCaAlF7. Top: calculated from single-crystal data; bottom: PDF entry # 27–0090 (ICDD, 2001); 2θ values are for Cu Kα1 radiation.
	Fig. 2. Projection of the crystal structure along [010]; the unit cell and the [AlF6] octahedra are outlined.
	Fig. 3. Displacement ellipsoid plot of the coordination environment of the Ba atom with ellipsoids drawn at the 75% probability level.

Barium calcium aluminium heptafluoride

Crystal data

BaCaAlF7	F(000) = 608
Mr = 337.40	Dx = 4.123 Mg m^−3
Monoclinic, P2/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -p 2yac	Cell parameters from 3828 reflections
a = 5.3664 (5) Å	θ = 3.3–30.5°
b = 5.3846 (6) Å	µ = 8.49 mm^−1
c = 18.8262 (19) Å	T = 293 K
β = 92.319 (2)°	Plate, colourless
V = 543.55 (10) Å^3	0.12 × 0.08 × 0.06 mm
Z = 4

Data collection

Area detector diffractometer	1603 independent reflections
Radiation source: fine-focus sealed tube	1477 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
ω scans	θmax = 30.5°, θmin = 2.2°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −7→7
Tmin = 0.429, Tmax = 0.630	k = −7→7
5719 measured reflections	l = −26→26

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.019	w = 1/[σ^2(Fo^2) + (0.0157P)^2 + 1.3785P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.044	(Δ/σ)max < 0.001
S = 1.11	Δρmax = 0.76 e Å^−3
1603 reflections	Δρmin = −0.72 e Å^−3
93 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0038 (3)

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
Ba	0.24906 (3)	0.24041 (3)	−0.067757 (8)	0.01162 (7)
Al	0.20069 (15)	0.22470 (14)	0.38092 (4)	0.00837 (15)
Ca1	0.7500	0.20769 (13)	0.7500	0.00786 (13)
Ca2	0.2500	0.68150 (13)	0.7500	0.00703 (13)
F1	0.5997 (3)	0.5949 (3)	0.56404 (9)	0.0165 (3)
F2	0.5649 (3)	0.8622 (3)	0.68045 (9)	0.0141 (3)
F3	0.0901 (3)	0.4812 (3)	0.32626 (9)	0.0133 (3)
F4	0.4141 (3)	0.3614 (3)	0.69206 (8)	0.0133 (3)
F5	0.0530 (3)	0.7185 (3)	0.56042 (9)	0.0159 (3)
F6	0.0154 (3)	−0.0288 (3)	0.67494 (8)	0.0122 (3)
F7	0.2750 (3)	−0.0454 (3)	0.43446 (9)	0.0167 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba	0.01257 (9)	0.01309 (10)	0.00916 (9)	0.00207 (6)	−0.00020 (5)	0.00030 (5)
Al	0.0091 (3)	0.0089 (4)	0.0071 (3)	0.0002 (3)	0.0000 (3)	0.0002 (3)
Ca1	0.0084 (3)	0.0073 (3)	0.0078 (3)	0.000	−0.0005 (2)	0.000
Ca2	0.0064 (3)	0.0077 (3)	0.0070 (3)	0.000	0.0004 (2)	0.000
F1	0.0170 (8)	0.0201 (8)	0.0122 (8)	−0.0069 (7)	−0.0017 (6)	−0.0016 (6)
F2	0.0139 (8)	0.0135 (8)	0.0153 (8)	−0.0004 (6)	0.0054 (6)	−0.0031 (6)
F3	0.0129 (7)	0.0121 (8)	0.0146 (8)	−0.0013 (6)	−0.0023 (6)	0.0047 (6)
F4	0.0136 (8)	0.0140 (8)	0.0121 (7)	0.0027 (6)	−0.0017 (6)	−0.0031 (6)
F5	0.0132 (8)	0.0200 (9)	0.0151 (8)	0.0027 (6)	0.0053 (6)	−0.0009 (6)
F6	0.0107 (7)	0.0138 (8)	0.0120 (7)	−0.0022 (6)	−0.0002 (6)	−0.0012 (6)
F7	0.0190 (8)	0.0144 (8)	0.0167 (8)	0.0033 (6)	0.0005 (7)	0.0049 (6)

Geometric parameters (Å, º)

Ba—F4^i	2.5498 (16)	Ca1—F4^xiii	2.2282 (17)
Ba—F1^i	2.6763 (17)	Ca1—F4	2.2282 (17)
Ba—F7^ii	2.7542 (17)	Ca1—F3^vii	2.3880 (16)
Ba—F1^iii	2.7831 (17)	Ca1—F3^xiv	2.3880 (16)
Ba—F5^i	2.7860 (17)	Ca1—F6^xv	2.4102 (16)
Ba—F6^i	2.8244 (16)	Ca1—F6^xvi	2.4102 (16)
Ba—F5^iv	2.8660 (18)	Ca1—F2^xvii	2.4609 (18)
Ba—F7^i	2.9510 (17)	Ca1—F2^xviii	2.4609 (18)
Ba—F5^v	3.0058 (18)	Ca1—Al^x	3.4086 (9)
Ba—F7^vi	3.0102 (17)	Ca1—Al^xix	3.4086 (9)
Ba—F3^iv	3.1421 (17)	Ca1—Ca2^xvi	3.7025 (7)
Ba—Al^i	3.5178 (9)	Ca1—Ca2	3.7025 (7)
Al—F1^vii	1.7535 (19)	Ca2—F4^xv	2.2392 (17)
Al—F2^vii	1.8038 (17)	Ca2—F4	2.2392 (17)
Al—F7	1.8047 (18)	Ca2—F2	2.3868 (16)
Al—F3	1.8083 (18)	Ca2—F2^xv	2.3868 (16)
Al—F5^viii	1.8133 (18)	Ca2—F6^xx	2.4231 (17)
Al—F6^ix	1.8614 (18)	Ca2—F6^xxi	2.4231 (17)
Al—Ca1^x	3.4086 (9)	Ca2—F3^viii	2.4389 (17)
Al—Ca2^viii	3.4201 (9)	Ca2—F3^xiv	2.4389 (17)
Al—Ba^i	3.5178 (9)	Ca2—Al^viii	3.4201 (9)
Al—Ba^xi	3.6427 (8)	Ca2—Al^xiv	3.4201 (9)
Al—Ba^xii	3.9106 (8)	Ca2—Ca1^xxii	3.7025 (7)
F4^i—Ba—F1^i	68.11 (5)	F7—Al—F5^viii	87.15 (8)
F4^i—Ba—F7^ii	80.11 (5)	F3—Al—F5^viii	88.88 (8)
F1^i—Ba—F7^ii	67.93 (5)	F1^vii—Al—F6^ix	178.18 (9)
F4^i—Ba—F1^iii	129.51 (5)	F2^vii—Al—F6^ix	85.73 (8)
F1^i—Ba—F1^iii	61.48 (6)	F7—Al—F6^ix	88.70 (8)
F7^ii—Ba—F1^iii	78.96 (5)	F3—Al—F6^ix	85.92 (8)
F4^i—Ba—F5^i	85.73 (5)	F5^viii—Al—F6^ix	88.51 (8)
F1^i—Ba—F5^i	66.76 (5)	F4^xiii—Ca1—F4	136.39 (9)
F7^ii—Ba—F5^i	134.60 (5)	F4^xiii—Ca1—F3^vii	74.37 (6)
F1^iii—Ba—F5^i	77.55 (5)	F4—Ca1—F3^vii	75.42 (6)
F4^i—Ba—F6^i	67.82 (5)	F4^xiii—Ca1—F3^xiv	75.42 (6)
F1^i—Ba—F6^i	135.93 (5)	F4—Ca1—F3^xiv	74.37 (6)
F7^ii—Ba—F6^i	104.80 (5)	F3^vii—Ca1—F3^xiv	90.92 (8)
F1^iii—Ba—F6^i	162.52 (5)	F4^xiii—Ca1—F6^xv	113.25 (6)
F5^i—Ba—F6^i	109.20 (5)	F4—Ca1—F6^xv	89.87 (6)
F4^i—Ba—F5^iv	156.29 (5)	F3^vii—Ca1—F6^xv	163.38 (6)
F1^i—Ba—F5^iv	107.31 (5)	F3^xiv—Ca1—F6^xv	77.46 (6)
F7^ii—Ba—F5^iv	120.75 (5)	F4^xiii—Ca1—F6^xvi	89.87 (6)
F1^iii—Ba—F5^iv	53.08 (5)	F4—Ca1—F6^xvi	113.25 (6)
F5^i—Ba—F5^iv	71.58 (6)	F3^vii—Ca1—F6^xvi	77.46 (6)
F6^i—Ba—F5^iv	112.66 (5)	F3^xiv—Ca1—F6^xvi	163.38 (6)
F4^i—Ba—F7^i	152.69 (5)	F6^xv—Ca1—F6^xvi	116.22 (8)
F1^i—Ba—F7^i	107.15 (5)	F4^xiii—Ca1—F2^xvii	147.39 (6)
F7^ii—Ba—F7^i	73.51 (6)	F4—Ca1—F2^xvii	73.69 (6)
F1^iii—Ba—F7^i	51.97 (5)	F3^vii—Ca1—F2^xvii	110.94 (6)
F5^i—Ba—F7^i	117.99 (5)	F3^xiv—Ca1—F2^xvii	134.67 (5)
F6^i—Ba—F7^i	112.10 (5)	F6^xv—Ca1—F2^xvii	71.17 (6)
F5^iv—Ba—F7^i	50.75 (5)	F6^xvi—Ca1—F2^xvii	61.58 (5)
F4^i—Ba—F5^v	112.88 (5)	F4^xiii—Ca1—F2^xviii	73.69 (6)
F1^i—Ba—F5^v	155.78 (5)	F4—Ca1—F2^xviii	147.39 (7)
F7^ii—Ba—F5^v	88.15 (5)	F3^vii—Ca1—F2^xviii	134.67 (5)
F1^iii—Ba—F5^v	111.77 (5)	F3^xiv—Ca1—F2^xviii	110.94 (6)
F5^i—Ba—F5^v	136.74 (6)	F6^xv—Ca1—F2^xviii	61.58 (5)
F6^i—Ba—F5^v	52.08 (5)	F6^xvi—Ca1—F2^xviii	71.17 (6)
F5^iv—Ba—F5^v	81.12 (5)	F2^xvii—Ca1—F2^xviii	81.79 (8)
F7^i—Ba—F5^v	60.06 (5)	F4^xv—Ca2—F4	79.34 (9)
F4^i—Ba—F7^vi	112.88 (5)	F4^xv—Ca2—F2	151.92 (7)
F1^i—Ba—F7^vi	154.80 (5)	F4—Ca2—F2	75.25 (6)
F7^ii—Ba—F7^vi	137.13 (7)	F4^xv—Ca2—F2^xv	75.25 (6)
F1^iii—Ba—F7^vi	113.73 (5)	F4—Ca2—F2^xv	151.92 (7)
F5^i—Ba—F7^vi	88.07 (5)	F2—Ca2—F2^xv	131.87 (9)
F6^i—Ba—F7^vi	52.00 (4)	F4^xv—Ca2—F6^xx	114.63 (6)
F5^iv—Ba—F7^vi	60.92 (5)	F4—Ca2—F6^xx	125.05 (6)
F7^i—Ba—F7^vi	83.08 (5)	F2—Ca2—F6^xx	72.22 (6)
F5^v—Ba—F7^vi	48.98 (5)	F2^xv—Ca2—F6^xx	77.31 (6)
F4^i—Ba—F3^iv	58.84 (5)	F4^xv—Ca2—F6^xxi	125.05 (6)
F1^i—Ba—F3^iv	95.87 (5)	F4—Ca2—F6^xxi	114.63 (6)
F7^ii—Ba—F3^iv	138.91 (5)	F2—Ca2—F6^xxi	77.31 (6)
F1^iii—Ba—F3^iv	127.61 (5)	F2^xv—Ca2—F6^xxi	72.22 (6)
F5^i—Ba—F3^iv	50.21 (4)	F6^xx—Ca2—F6^xxi	99.85 (8)
F6^i—Ba—F3^iv	60.14 (4)	F4^xv—Ca2—F3^viii	73.17 (6)
F5^iv—Ba—F3^iv	99.88 (4)	F4—Ca2—F3^viii	74.72 (6)
F7^i—Ba—F3^iv	146.66 (4)	F2—Ca2—F3^viii	110.68 (6)
F5^v—Ba—F3^iv	105.15 (4)	F2^xv—Ca2—F3^viii	86.55 (6)
F7^vi—Ba—F3^iv	66.75 (4)	F6^xx—Ca2—F3^viii	158.97 (6)
F1^vii—Al—F2^vii	95.58 (9)	F6^xxi—Ca2—F3^viii	61.92 (6)
F1^vii—Al—F7	90.02 (9)	F4^xv—Ca2—F3^xiv	74.72 (6)
F2^vii—Al—F7	90.26 (8)	F4—Ca2—F3^xiv	73.17 (6)
F1^vii—Al—F3	95.26 (9)	F2—Ca2—F3^xiv	86.55 (6)
F2^vii—Al—F3	93.15 (8)	F2^xv—Ca2—F3^xiv	110.68 (6)
F7—Al—F3	173.40 (9)	F6^xx—Ca2—F3^xiv	61.92 (6)
F1^vii—Al—F5^viii	90.13 (9)	F6^xxi—Ca2—F3^xiv	158.97 (6)
F2^vii—Al—F5^viii	173.74 (9)	F3^viii—Ca2—F3^xiv	137.88 (8)

Symmetry codes: (i) −x+1/2, y, −z+1/2; (ii) x−1/2, −y, z−1/2; (iii) x−1/2, −y+1, z−1/2; (iv) x+1/2, −y+1, z−1/2; (v) −x+1/2, y−1, −z+1/2; (vi) x+1/2, −y, z−1/2; (vii) −x+1, −y+1, −z+1; (viii) −x, −y+1, −z+1; (ix) −x, −y, −z+1; (x) −x+1, −y, −z+1; (xi) x−1/2, −y, z+1/2; (xii) x−1/2, −y+1, z+1/2; (xiii) −x+3/2, y, −z+3/2; (xiv) x+1/2, −y+1, z+1/2; (xv) −x+1/2, y, −z+3/2; (xvi) x+1, y, z; (xvii) x, y−1, z; (xviii) −x+3/2, y−1, −z+3/2; (xix) x+1/2, −y, z+1/2; (xx) −x+1/2, y+1, −z+3/2; (xxi) x, y+1, z; (xxii) x−1, y, z.