PbCa₂[Al₈O₁₅] with a novel three-dimensional aluminate anion

The asymmetric unit of the title compound, lead(II) dicalcium octa­aluminate, contains one Pb, one Ca, four Al and eight O atoms, with the Pb atom and one O atom situated on mirror planes. Three Al atoms exhibit slightly distorted tetra­hedral coordinations with a mean Al-O bond length of 1.76 Å. The fourth Al atom is in a considerably distorted trigonal-bipyramidal coordination with a mean Al-O bond length of 1.89 Å. One AlO₄ tetra­hedron forms infinite chains parallel to [100] via corner-sharing. These chains are linked by parallel chains of edge-sharing AlO₅ trigonal bipyramids into layers A of six-membered double rings extending parallel to (010). The second layer B is made up of the remaining two AlO₄ tetra­hedra. These tetra­hedra share corners, resulting in likewise six-membered double rings. Finally, the parallel layers A and B are linked into a three-dimensional framework by common corners. Charge compensation is achieved by the Pb²⁺ and Ca²⁺ cations, which are situated in the cavities of the anionic framework, and which are surrounded by seven and six O atoms, respectively, both within highly irregular coordination polyhedra.

S1. Comment

Compared with the high number of known silicates, the number of structurally characterized aluminates is significantly smaller. However, as well as the tetrahedral XO₄ group as the main structural motif and the less frequently found octahedral XO₆ group observed in the multifarious structural chemistry of silicates (Liebau, 1985), trigonal–bipyramidal XO₅ groups are also known in the crystal chemistry of aluminates (Santamaría-Pérez & Vegas, 2003), albeit with only a few representatives, e.g. in the structure of the mineral andalusite, Al₂SiO₅ (Burnham & Buerger, 1961). Such a trigonal–bipyramidal AlO₅ group is also present in the crystal structure of the title aluminate, PbCa₂[Al₈O₁₅], which was obtained serendipitously from a lead oxide flux.

The crystal structure of PbCa₂[Al₈O₁₅] contains the novel three-dimensional [Al₈O₁₅]^6- aluminate anion. Although the phases BaCa₂[Al₈O₁₅] (Brisi & Montorsi, 1962) and CsCa₂[Al₈O₁₅] (van Hoek et al., 1989) have also been reported with the same composition for the anion, their crystal structures remained undetermined. The anion in the title compound is composed of three different AlO₄ tetrahedra (Al1–Al3) and one trigonal–bipyramidal AlO₅ group (Al4). The Al1O₄ tetrahedra are linked into infinite zweier single chains (Liebau, 1985) parallel to [100] via corner-sharing. These chains are corner-linked above and below their propagation by parallel chains of edge-sharing Al4O₅ trigonal bipyramids into layers A of six-membered double rings extending parallel to (010) (Fig. 1). The second layer B consists of the remaining Al2O₄ and Al3O₄ tetrahedra. Each of them is likewise assembled into infinite zweier single chains that are further linked via corner-sharing to build up six-membered double rings (Fig. 2). Since atom O2 that links two single ring layers is situated on a special position with site symmetry m, layer B is bisected by a mirror plane. Finally, layers A and B are linked by common corners along [010] into a three-dimensional anionic framework structure. The Ca²⁺ and Pb²⁺ cations are situated in the cavities of the A and B layers, respectively (Fig. 3).

The three AlO₄ tetrahedra are distorted, with Al—O distances in the range 1.7214 (13)–1.7758 (13) Å (average 1.759 Å) and O—Al—O angles between 101.27 (6)–119.09 (8)° (average 109.4°). The AlO₅ group likewise shows a distortion, with three short Al—O distances in the equatorial plane (average 1.761 Å) and two longer but different Al—O distances (Table 1) to the apices of the polyhedron, with an overall mean of 1.885 Å. A displacement of the Al atom by 0.230 (1) Å from the equatorial plane towards the more tightly bonded of the two apical O atoms is observed. The spread of interatomic distances and angles within the aluminate framework are in normal ranges [Standard reference?] and match other structures where different AlO_x coordination polyhedra are present. For example, AlO₄, AlO₅ and AlO₆ polyhedra coexist in Ca₄Al₆O₁₃ that has been prepared under high-pressure conditions (Kahlenberg et al., 2000). In this structure, mean values of 1.777 Å for the Al—O distances in two AlO₄ tetrahedra, 1.856 Å in two AlO₅ trigonal bipyramids and 1.930 Å in three AlO₆ octahedra are observed. The ideal values for these polyhedra, calculated using the corresponding ionic radii for Al³⁺ and O^2- (Shannon, 1976), are 1.75, 1.84 and 1.90 Å, respectively.

Another structural aspect that has been used to classify aluminates is the distribution of Al···Al distances, which show two distinct maxima (Isea et al., 1998). One sharp maximum is observed around 2.86 Å, which represents the Al···Al distance in elementary aluminium (face-centred cubic) and corresponds to the separation between two Al atoms which occupy edge-sharing AlO₆ octahedra. The second, broader, maximum is around 3.3 Å and coincides with the distribution of distances between two tetra-coordinated Al atoms bridged by one O atom. In PbCa₂[Al₈O₁₅], the Al4···Al4 distance of 2.8644 (8) Å between the two edge-sharing AlO₅ groups is in agreement with this concept, and likewise the Al1···Al1 and Al2···Al3 separations between two corner-sharing AlO₄ tetrahedra of 3.0980 (8) and 3.1323 (8) Å, respectively. However, the very short Al3···Al3 distance of 2.9040 (8) Å between two AlO₄ tetrahedra is too short with respect to this simple concept.

The Ca²⁺ cation exhibits a coordination number of 6 within a very irregular coordination polyhedron. The Ca—O distances vary between 2.2925 (13) and 2.7121 (13) Å (mean 2.43 Å), in good agreement with numerous other CaO₆ polyhedra and with the overall mean of the average Ca—O distances for CaO_x polyhedra of 2.46 Å (Chiari, 1990). The Pb²⁺ cation is situated on a mirror plane in the B layer of the anionic framework and is bonded to seven O atoms, with Pb—O bond lengths ranging from 2.2925 (12) to 3.2809 (14) Å. The resulting PbO₇ polyhedron is heavily distorted and shows the typical influence of a stereochemically active electron lone pair (Galy et al., 1975). Most probably, the electron lone pair points towards the layer voids, which are arranged parallel to the [301] direction.

Bond-valence sum (BVS) analysis (Brown, 2002) using the parameters of Brese & O'Keeffe (1991) revealed slight deviations from the expected values of 2 valence units (v.u.) for Ca, Pb and O atoms and of 3 v.u. for Al atoms. The deviations reflect the distortions in the PbO₇, CaO₆ and AlO_x polyhedra: Pb1 (coordination number = 7; BVS = 1.88 v.u.), Ca1 (6; 1.86), Al1 (4; 2.80), Al2 (4; 2.71), Al3 (4; 2.74), Al4 (5, 2.75), O1 (3; 1.84), O2 (3; 1.78), O3 (3; 1.86), O4 (4; 1.71), O5 (3, 1.69), O6 (3; 1.93), O7 (4; 1.99), O8 (4, 1.89).

Related literature

For related literature, see: Brese & O'Keeffe (1991); Brisi & Montorsi (1962); Brown (2002); Burnham & Buerger (1961); Chiari (1990); Galy et al. (1975); Hoek et al. (1989); Isea et al. (1998); Kahlenberg et al. (2000); Liebau (1985); Santamaría-Pérez & Vegas (2003); Shannon (1976).

S2. Experimental

Single crystals of the title compound grew accidentally from an attacked corundum crucible during attempts to synthesize phases with new compositions in the system CaO–PbO–TeO₃ using a lead oxide flux. A mixture of 2PbCO₃.Pb(OH)₂ (1.16 g), CaCO₃ (0.075 g) and TeO₂ (0.117 g) was thoroughly ground in an agate mortar and placed in a corundum crucible. The reaction mixture was heated over a period of 18 h to 1233 K, kept at this temperature for 6 h and cooled over a period of 18 h to room temperature. A few colourless crystals of the title compound grew on top of a brick-red microcrystalline matrix that mainly consisted of aluminium. The crystals were manually separated and cleaned under a polarizing microscope.

S3. Refinement

The largest positive and negative residual electron densities are located 0.55 and 0.38 Å, respectively, from atom Pb1. Reflection 020 was affected by the beam stop and was omitted from the refinement.

Computing details

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Layer A, composed of chains of corner-sharing Al1O4 tetrahedra (light grey) and chains of edge-sharing Al4O5 trigonal bipyramids (dark grey). Displacement ellipsoids are drawn at the 90% probability level.
	Fig. 2. Layer B, composed of corner-sharing Al2O4 and Al3O4 tetrahedra. Displacement ellipsoids are drawn at the 90% probability level.
	Fig. 3. The structure of PbCa2[Al8O15] in a projection along [100].

lead(II) dicalcium octaaluminate

Crystal data

PbCa2[Al8O15]	F(000) = 1384
Mr = 743.19	Dx = 3.698 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 14887 reflections
a = 5.3702 (1) Å	θ = 2.9–45.0°
b = 27.9903 (4) Å	µ = 14.02 mm^−1
c = 8.8811 (1) Å	T = 296 K
V = 1334.95 (3) Å^3	Block, colourless
Z = 4	0.12 × 0.12 × 0.10 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	5602 independent reflections
Radiation source: fine-focus sealed tube	5068 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
ω and ϕ scans	θmax = 45.1°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −10→5
Tmin = 0.187, Tmax = 0.246	k = −55→55
39453 measured reflections	l = −17→17

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.024	Secondary atom site location: difference Fourier map
wR(F^2) = 0.060	w = 1/[σ^2(Fo^2) + (0.0289P)^2 + 2.432P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.003
5602 reflections	Δρmax = 3.91 e Å^−3
121 parameters	Δρmin = −3.21 e Å^−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
Pb1	0.428096 (16)	0.2500	1.044280 (10)	0.00950 (2)
Ca1	0.06048 (5)	0.110812 (11)	0.56641 (3)	0.00579 (4)
Al1	0.12604 (10)	0.038859 (18)	0.83700 (6)	0.00556 (7)
Al2	0.01849 (10)	0.189427 (18)	0.84718 (6)	0.00551 (7)
Al3	−0.02687 (10)	0.161644 (19)	1.18773 (6)	0.00613 (7)
Al4	0.19008 (11)	0.058665 (19)	1.19384 (6)	0.00718 (8)
O1	0.3975 (2)	0.02335 (4)	1.30357 (15)	0.00844 (17)
O2	0.0424 (4)	0.2500	0.8979 (2)	0.0099 (3)
O3	−0.1119 (2)	0.15916 (5)	1.00045 (14)	0.00934 (18)
O4	−0.1677 (2)	0.17341 (5)	0.69052 (14)	0.00905 (17)
O5	0.0309 (3)	0.10693 (5)	1.28228 (14)	0.01041 (19)
O6	0.2544 (2)	0.05825 (5)	1.00378 (13)	0.00903 (17)
O7	−0.2424 (2)	0.19359 (5)	1.29953 (15)	0.00955 (18)
O8	0.3353 (2)	0.06358 (4)	0.70435 (14)	0.00792 (16)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.00860 (3)	0.00848 (3)	0.01142 (3)	0.000	0.00116 (2)	0.000
Ca1	0.00480 (9)	0.00603 (9)	0.00653 (9)	−0.00098 (7)	−0.00031 (7)	0.00050 (7)
Al1	0.00483 (16)	0.00669 (17)	0.00517 (16)	−0.00032 (14)	−0.00011 (13)	−0.00047 (13)
Al2	0.00503 (16)	0.00585 (17)	0.00565 (17)	−0.00049 (14)	−0.00012 (14)	0.00014 (13)
Al3	0.00624 (17)	0.00637 (18)	0.00576 (17)	0.00052 (14)	−0.00010 (14)	0.00041 (13)
Al4	0.00831 (19)	0.00789 (18)	0.00534 (17)	0.00240 (15)	0.00083 (15)	0.00068 (14)
O1	0.0078 (4)	0.0069 (4)	0.0106 (4)	−0.0006 (3)	−0.0019 (3)	0.0017 (3)
O2	0.0122 (7)	0.0059 (5)	0.0117 (6)	0.000	−0.0014 (5)	0.000
O3	0.0108 (4)	0.0112 (4)	0.0060 (4)	−0.0045 (4)	−0.0001 (3)	0.0011 (3)
O4	0.0063 (4)	0.0120 (4)	0.0089 (4)	0.0015 (3)	−0.0022 (3)	−0.0023 (3)
O5	0.0148 (5)	0.0085 (4)	0.0080 (4)	0.0038 (4)	0.0013 (4)	0.0008 (3)
O6	0.0089 (4)	0.0131 (4)	0.0051 (4)	−0.0032 (4)	0.0001 (3)	−0.0011 (3)
O7	0.0072 (4)	0.0093 (4)	0.0122 (4)	−0.0015 (3)	0.0021 (3)	−0.0038 (3)
O8	0.0056 (4)	0.0092 (4)	0.0089 (4)	0.0008 (3)	0.0018 (3)	0.0020 (3)

Geometric parameters (Å, º)

Pb1—O7^i	2.2925 (12)	Al1—O1^ix	1.7708 (13)
Pb1—O7^ii	2.2925 (12)	Al2—O3	1.7496 (13)
Pb1—O2	2.4457 (19)	Al2—O2	1.7589 (7)
Pb1—O4^iii	3.0345 (13)	Al2—O4	1.7709 (13)
Pb1—O4^iv	3.0345 (13)	Al2—O4^iii	1.7758 (13)
Pb1—O7^v	3.2809 (14)	Al3—O3	1.7262 (14)
Pb1—O7^vi	3.2809 (14)	Al3—O7	1.7676 (13)
Ca1—O6^vii	2.2925 (13)	Al3—O7^ii	1.7738 (14)
Ca1—O3^iii	2.2976 (13)	Al3—O5	1.7740 (14)
Ca1—O8	2.3294 (12)	Al3—O5^x	2.8381 (16)
Ca1—O4	2.4055 (13)	Al4—O6	1.7230 (13)
Ca1—O5^viii	2.5306 (13)	Al4—O1	1.7797 (13)
Ca1—O8^vii	2.7121 (13)	Al4—O5	1.7811 (14)
Al1—O6	1.7214 (13)	Al4—O1^x	1.8567 (14)
Al1—O8^vii	1.7470 (13)	Al4—O5^ii	2.2849 (16)
Al1—O8	1.7690 (13)
O7^i—Pb1—O7^ii	87.07 (7)	O7—Al3—O5^x	70.88 (5)
O7^i—Pb1—O2	89.06 (5)	O7^ii—Al3—O5^x	170.62 (6)
O7^ii—Pb1—O2	89.06 (5)	O5—Al3—O5^x	68.67 (5)
O7^i—Pb1—O4^iii	146.59 (4)	O6—Al4—O1	114.03 (7)
O7^ii—Pb1—O4^iii	82.04 (4)	O6—Al4—O5	122.23 (7)
O2—Pb1—O4^iii	59.41 (4)	O1—Al4—O5	118.69 (7)
O7^i—Pb1—O4^iv	82.04 (4)	O6—Al4—O1^x	100.26 (6)
O7^ii—Pb1—O4^iv	146.59 (4)	O1—Al4—O1^x	103.12 (6)
O2—Pb1—O4^iv	59.41 (4)	O5—Al4—O1^x	89.56 (7)
O4^iii—Pb1—O4^iv	89.89 (5)	O6—Al4—O5^ii	86.24 (6)
O7^i—Pb1—O7^v	57.71 (2)	O1—Al4—O5^ii	77.07 (5)
O7^ii—Pb1—O7^v	97.40 (3)	O5—Al4—O5^ii	83.97 (6)
O2—Pb1—O7^v	145.51 (3)	O1^x—Al4—O5^ii	172.62 (6)
O4^iii—Pb1—O7^v	154.99 (3)	Al4—O1—Al4^ii	103.92 (7)
O4^iv—Pb1—O7^v	103.07 (3)	Al1^xi—O1—Al1^xii	66.70 (4)
O7^i—Pb1—O7^vi	97.40 (3)	Al4—O1—Al1^xii	124.39 (6)
O7^ii—Pb1—O7^vi	57.71 (2)	Al4^ii—O1—Al1^xii	68.50 (4)
O2—Pb1—O7^vi	145.51 (3)	Al1^xi—O1—Al1^ii	89.65 (5)
O4^iii—Pb1—O7^vi	103.07 (3)	Al4—O1—Al1^ii	131.29 (6)
O4^iv—Pb1—O7^vi	154.99 (3)	Al2—O2—Al2^xiii	149.14 (12)
O7^v—Pb1—O7^vi	57.54 (4)	Al2—O2—Pb1	101.41 (6)
O6^vii—Ca1—O3^iii	148.96 (4)	Al2^xiii—O2—Pb1	101.41 (6)
O6^vii—Ca1—O8	103.47 (5)	Al2—O2—Pb1^xiv	91.49 (7)
O3^iii—Ca1—O8	89.15 (5)	Al2^xiii—O2—Pb1^xiv	91.49 (7)
O6^vii—Ca1—O4	103.11 (5)	Pb1—O2—Pb1^xiv	126.37 (7)
O3^iii—Ca1—O4	94.55 (5)	Al3—O3—Al2	128.62 (8)
O8—Ca1—O4	119.67 (4)	Al3—O3—Ca1^vii	118.38 (6)
O6^vii—Ca1—O5^viii	69.90 (4)	Al2—O3—Ca1^vii	113.00 (6)
O3^iii—Ca1—O5^viii	79.37 (5)	Al2—O4—Al2^vii	128.32 (8)
O8—Ca1—O5^viii	122.66 (4)	Al2—O4—Ca1	104.88 (6)
O4—Ca1—O5^viii	117.15 (4)	Al2^vii—O4—Ca1	125.51 (6)
O6^vii—Ca1—O8^vii	64.63 (4)	Al2—O4—Pb1^vii	105.39 (6)
O3^iii—Ca1—O8^vii	145.34 (4)	Al2^vii—O4—Pb1^vii	81.54 (5)
O8—Ca1—O8^vii	67.11 (2)	Ca1—O4—Pb1^vii	96.51 (4)
O4—Ca1—O8^vii	77.52 (4)	Al3—O5—Al4	122.01 (8)
O5^viii—Ca1—O8^vii	134.40 (4)	Al3—O5—Al4^x	114.46 (7)
O6—Al1—O8^vii	114.44 (7)	Al4—O5—Al4^x	88.67 (6)
O6—Al1—O8	101.27 (6)	Al3—O5—Ca1^xv	116.48 (6)
O8^vii—Al1—O8	105.83 (5)	Al4—O5—Ca1^xv	116.25 (6)
O6—Al1—O1^ix	118.87 (7)	Al4^x—O5—Ca1^xv	89.03 (5)
O8^vii—Al1—O1^ix	106.89 (6)	Al1—O6—Al4	139.90 (8)
O8—Al1—O1^ix	108.56 (6)	Al1—O6—Ca1^iii	104.80 (6)
O3—Al2—O2	107.27 (8)	Al4—O6—Ca1^iii	113.94 (6)
O3—Al2—O4	105.23 (7)	Al1—O6—Al4^ii	155.89 (7)
O2—Al2—O4	119.09 (8)	Al3—O7—Al3^x	110.17 (7)
O3—Al2—O4^iii	113.85 (7)	Al3—O7—Pb1^x	115.27 (6)
O2—Al2—O4^iii	102.85 (8)	Al3^x—O7—Pb1^x	130.73 (7)
O4—Al2—O4^iii	108.89 (5)	Al3—O7—Pb1^xiv	102.03 (6)
O3—Al3—O7	112.85 (7)	Al1^iii—O8—Al1	123.56 (7)
O3—Al3—O7^ii	108.04 (7)	Al1^iii—O8—Ca1	132.88 (7)
O7—Al3—O7^ii	105.85 (7)	Al1—O8—Ca1	99.78 (6)
O3—Al3—O5	117.88 (7)	Al1^iii—O8—Ca1^iii	87.26 (5)
O7—Al3—O5	106.58 (7)	Al1—O8—Ca1^iii	88.51 (5)
O7^ii—Al3—O5	104.72 (7)	Ca1—O8—Ca1^iii	113.62 (5)
O3—Al3—O5^x	81.22 (5)

Symmetry codes: (i) x+1/2, −y+1/2, −z+5/2; (ii) x+1/2, y, −z+5/2; (iii) x+1/2, y, −z+3/2; (iv) x+1/2, −y+1/2, −z+3/2; (v) x+1, −y+1/2, z; (vi) x+1, y, z; (vii) x−1/2, y, −z+3/2; (viii) x, y, z−1; (ix) −x+1/2, −y, z−1/2; (x) x−1/2, y, −z+5/2; (xi) −x+1/2, −y, z+1/2; (xii) −x+1, −y, −z+2; (xiii) x, −y+1/2, z; (xiv) x−1, y, z; (xv) x, y, z+1.