Lead zinc borate, PbZn₂(BO₃)₂

PbZn₂(BO₃)₂ crystallizes in the space group Pccn, with the Pb cation at a site with imposed twofold symmetry. The compound represents a new structure type in which ZnBO₃ layers are bridged by Pb²⁺ cations, giving rise to a three-dimensional framework. Channels parallel to the [010] direction accommodate the stereochemically active lone pairs of the Pb²⁺ cations.

S1. Comment

Borates have attracted much attention because they have important practical applications in second harmonic generation (SHG). For example, β-BaB₂O₄, LiB₃O₅ and YCa₄(BO₃)₃O are all well known nonlinear optical (NLO) crystals (Becker, 1998). So far, many investigations have been carried out on the alkali and alkali-earth metal borates, while borates incorporating other main group elements together with transition metal elements are relatively less explored. The title compound, PbZn₂(BO₃)₂, was previously reported by Petzoldt (1966) who presented its powder X-ray diffraction pattern. However, its crystal structure remains as yet undetermined. In the course of our investigation of novel borate NLO materials, we have unexpectedly obtained single crystals of PbZn₂(BO₃)₂. Our X-ray structural analysis has established that PbZn₂(BO₃)₂ crystallizes in a new unique structure type (Pearson symbol oP44), never observed for any of the known borates. We report here its crystal structure.

PbZn₂(BO₃)₂ is characterized by a complex three-dimensional network consisting of Pb²⁺ cations, tetratahedral Zn²⁺ centers and BO₃^3- anions. We have chosen a description emphasizing the substructures to help visualize the crystal structure. In this sense, the structure can be considered as being built upon two-dimensional sheets in the following way (Fig. 1): within the (100) plane, the fundamental building units of BO₃ triangles and ZnO₄ tetrahedra are linked together by sharing common vertices to generate a two-dimensional ZnBO₃ layer. Applying the c-glide symmetry operation to this layer produces the neighboring equivalent layers along the [100] direction. These layers are further bridged by the Pb²⁺ ions, resulting in the formation of the final three-dimensional framework. This framework contains V-shaped open channels running parallel to the [010] direction that are filled by the Pb²⁺ 6s² lone pairs.

All atoms occupy general positions, except Pb, which lies on a twofold axis, giving two sets of short bond lengths [Pb—O2 = 2.273 (5) Å and Pb—O1 = 2.515 (5) Å; Table 1]. The Pb atom is also weakly bonded to four more O atoms at distances of 3.301 (6)–3.652 (8) Å. Bond valence sum (BVS) calculations (Brown & Altermatt, 1985) produced a value of 1.97 for the four shorter Pb—O bonds, which indicates that the longer distances need not be included in the coordination environment of Pb. All the short Pb—O distances fall within the same hemisphere around lead, leaving room for the 6s² lone pair of Pb²⁺ in the opposite direction (Fig. 2). A similar highly distorted Pb²⁺ coordination geometry has also been observed in Pb₆B₁₀O₂₁, with four short Pb—O distances of 2.30 (2)–2.48 (2) Å (Krogh-Moe & Wold-Hansen, 1973). In contrast, the ZnO₄ and BO₃ groups are only slightly distorted. The B—O distances lie within a narrow range of 1.371 (9)–1.384 (10) Å, with an average of 1.376 Å, which is consistent with the value reported in Li₃In(BO₃)₂ [1.371 (1) Å; Penin et al., 2001]. The Zn—O distances of 1.941 (5)–1.958 (5) Å (average 1.950 Å), are also very reasonable when compared with the ranges 1.926 (6)–1.977 (6) Å [average 1.95 (2) Å] in BaZn₂(BO₃)₂ and 1.95 (3)–2.03 (4) Å [average 1.98 (3) Å] in Ba₂Zn(BO₃)₂, all featuring tetrahedrally coordinated Zn²⁺ (Smith & Keszler, 1992; Smith & Koliha, 1994). Bond valence analysis gave values of 2.06 for Zn and 2.96 for B, in good agreement with their expected formal valences.

Both Zn₃(BO₃)₂ (Chen et al., 2005) and BaZn₂(BO₃)₂ (Smith & Keszler, 1992) are closely related to PbZn₂(BO₃)₂ in stoichiometry but differ in structures. In Zn₃(BO₃)₂, the BO₃ triangles share common vertices with irregular ZnO₄ tetrahedra to form a three-dimensional framework. The framework contains small unoccupied six-edge and four-edge channels running along the b axis. The structure of BaZn₂(BO₃)₂ also consists of a three-dimensional framework of corner-sharing ZnO₄ tetrahedra and BO₃ triangles. However, the channels within this framework host seven-coordinate Ba²⁺ cations. It is the variation in the coordination environments around the metal cations that is mainly responsible for the structural differences between Zn₃(BO₃)₂, BaZn₂(BO₃)₂ and PbZn₂(BO₃)₂.

It is clear from Fig. 1 that two rows of Pb²⁺ cations have their stereochemically active non-bonded electron pairs pointing in the opposite direction, which yields a structure without polarity. To confirm this, SHG measurements were performed on crushed crystals of PbZn₂(BO₃)₂ using a modified Kurtz NLO system with a 1064 nm light source (Kurtz & Perry, 1968). No second-harmonic signal at 532 nm was observed, which further supports the description of this new structure type in the centrosymmetric Pccn space group.

S2. Experimental

A powder mixture of PbO (4.520 g, 20.250 mmol), ZnO (1.099 g, 13.500 mmol), Bi₂O₃ (3.146 g, 6.751 mmol) and H₃BO₃ (2.504 g, 40.500 mmol) was transferred to a Pt crucible. The sample was melted at 1023 K for one day, then cooled to 823 K at a rate of 2 K h^-1, followed by cooling to room temperature at a rate of 20 K h^-1. Colourless plate-like crystals of PbZn₂(BO₃)₂ with dimensions up to 0.6 × 0.6 × 1.0 mm were recovered. They were isolated mechanically from the reaction product for further characterization by single-crystal X-ray diffraction. The powder X-ray diffraction pattern of the ground crystals is in good agreement with that calculated from the single-crystal data. The IR spectrum of PbZn₂(BO₃)₂ exhibits three sets of bands characteristic of the planar triangular BO₃ group. They are the out-of-plane bending modes (ν2) occurring at 761.8 and 719.3 cm^-1, the antisymmetric stretch (ν3) at 1223.6 cm^-1, and the in-plane mode (ν4) at 625.5 cm^-1. These values correspond well to those reported in the literature (Thompson et al., 1991).

S3. Refinement

Direct phase determination yielded the positions of Pb and Zn atoms. The B and O atoms were located from the subsequent difference Fourier synthesis. All atoms were refined anisotropically. The highest residual electronic density peaks were located 1.54 Å from the Pb atoms.

Computing details

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku Corporation, 1994); cell refinement: Rigaku/AFC Diffractometer Control Software; data reduction: Rigaku/AFC Diffractometer Control Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The crystal structure of PbZn2(BO3)2 projected along the [010] direction, where black circles, doubly shaded circles, single shaded circles and open circles are Pb, Zn, B, and O atoms, respectively. The Pb—O contacts [2.273 (5) and 2.515 (5) Å] are drawn with thin solid lines.
	Fig. 2. The local coordination geometries of Pb, Zn and B in PbZn2(BO3)2, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) 3/2 - x, 1/2 - y, z; (ii) 1 - x, 1/2 + y, 1/2 - z; (iii) 1 - x, 1 - y, 1 - z; (iv) x, 1 + y, z; (v) 3/2 - x, 3/2 - y, z.]

Lead zinc borate

Crystal data

PbZn2(BO3)2	F(000) = 800
Mr = 455.55	Dx = 5.145 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 25 reflections
a = 11.1709 (8) Å	θ = 20.2–22.5°
b = 4.9674 (7) Å	µ = 36.61 mm^−1
c = 10.5987 (8) Å	T = 290 K
V = 588.13 (10) Å^3	Plate, colorless
Z = 4	0.1 × 0.06 × 0.02 mm

Data collection

Rigaku AFC-7R diffractometer	972 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.031
Graphite monochromator	θmax = 34.9°, θmin = 3.7°
2θ–ω scans	h = 0→18
Absorption correction: ψ scan (Kopfmann & Huber, 1968)	k = 0→8
Tmin = 0.085, Tmax = 0.473	l = 0→16
1522 measured reflections	3 standard reflections every 150 reflections
1289 independent reflections	intensity decay: 1.6%

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)] = 0.045	Secondary atom site location: difference Fourier map
wR(F^2) = 0.145	w = 1/[σ^2(Fo^2) + (0.0998P)^2 + 0.0518P] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max < 0.001
1289 reflections	Δρmax = 3.69 e Å^−3
51 parameters	Δρmin = −3.06 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
Pb	0.7500	0.7500	0.56467 (3)	0.01330 (16)
Zn	0.50171 (8)	0.67690 (16)	0.35005 (6)	0.01124 (19)
B	0.6023 (8)	0.1894 (18)	0.4163 (7)	0.0128 (13)
O1	0.6227 (5)	0.3426 (11)	0.5216 (4)	0.0127 (9)
O2	0.6277 (5)	−0.0832 (11)	0.4122 (5)	0.0137 (8)
O3	0.5539 (7)	0.3107 (13)	0.3117 (4)	0.0163 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb	0.0090 (2)	0.0182 (2)	0.0127 (2)	−0.00054 (12)	0.000	0.000
Zn	0.0117 (3)	0.0104 (4)	0.0116 (3)	−0.0012 (3)	−0.0002 (3)	−0.0004 (2)
B	0.012 (3)	0.016 (3)	0.010 (2)	−0.001 (3)	−0.004 (3)	0.000 (3)
O1	0.008 (2)	0.016 (2)	0.0150 (19)	−0.0024 (19)	0.0005 (16)	−0.0028 (15)
O2	0.011 (2)	0.011 (2)	0.0190 (17)	−0.0012 (19)	−0.0030 (18)	0.0001 (17)
O3	0.023 (3)	0.012 (2)	0.014 (2)	0.007 (3)	−0.0084 (18)	−0.0012 (15)

Geometric parameters (Å, º)

Pb—O2^i	2.273 (5)	Zn—O3	1.953 (6)
Pb—O2^ii	2.273 (5)	Zn—O2^ii	1.958 (5)
Pb—O1	2.515 (5)	B—O1	1.371 (9)
Pb—O1^iii	2.515 (5)	B—O3	1.372 (9)
Zn—O3^iv	1.941 (5)	B—O2	1.384 (10)
Zn—O1^v	1.947 (5)
O2^i—Pb—O2^ii	89.3 (3)	O1—B—O3	118.7 (7)
O2^i—Pb—O1	85.28 (17)	O1—B—O2	122.3 (7)
O2^ii—Pb—O1	79.89 (19)	O3—B—O2	119.0 (7)
O2^i—Pb—O1^iii	79.89 (19)	B—O1—Zn^v	115.0 (5)
O2^ii—Pb—O1^iii	85.28 (17)	B—O1—Pb	133.5 (5)
O1—Pb—O1^iii	159.1 (2)	Zn^v—O1—Pb	108.5 (2)
O3^iv—Zn—O1^v	113.9 (3)	B—O2—Zn^vi	117.3 (5)
O3^iv—Zn—O3	103.34 (19)	B—O2—Pb^vi	117.2 (4)
O1^v—Zn—O3	108.2 (3)	Zn^vi—O2—Pb^vi	116.7 (2)
O3^iv—Zn—O2^ii	108.6 (2)	B—O3—Zn^vii	133.6 (5)
O1^v—Zn—O2^ii	108.0 (2)	B—O3—Zn	111.0 (5)
O3—Zn—O2^ii	115.0 (3)	Zn^vii—O3—Zn	114.0 (3)

Symmetry codes: (i) −x+3/2, −y+1/2, z; (ii) x, y+1, z; (iii) −x+3/2, −y+3/2, z; (iv) −x+1, y+1/2, −z+1/2; (v) −x+1, −y+1, −z+1; (vi) x, y−1, z; (vii) −x+1, y−1/2, −z+1/2.