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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.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270105040709/bc1089sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270105040709/bc1089Isup2.hkl
Contains datablock I

Comment top

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 BO33- 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.

Experimental top

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).

Refinement top

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.

Structure description top

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 BO33- 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.

Computing details top

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 top
[Figure 1] 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.
[Figure 2] 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 top
Crystal data top
PbZn2(BO3)2F(000) = 800
Mr = 455.55Dx = 5.145 Mg m3
Orthorhombic, PccnMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2acCell parameters from 25 reflections
a = 11.1709 (8) Åθ = 20.2–22.5°
b = 4.9674 (7) ŵ = 36.61 mm1
c = 10.5987 (8) ÅT = 290 K
V = 588.13 (10) Å3Plate, colorless
Z = 40.1 × 0.06 × 0.02 mm
Data collection top
Rigaku AFC-7R
diffractometer
972 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.031
Graphite monochromatorθmax = 34.9°, θmin = 3.7°
2θω scansh = 018
Absorption correction: ψ scan
(Kopfmann & Huber, 1968)
k = 08
Tmin = 0.085, Tmax = 0.473l = 016
1522 measured reflections3 standard reflections every 150 reflections
1289 independent reflections intensity decay: 1.6%
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.045Secondary atom site location: difference Fourier map
wR(F2) = 0.145 w = 1/[σ2(Fo2) + (0.0998P)2 + 0.0518P]
where P = (Fo2 + 2Fc2)/3
S = 1.11(Δ/σ)max < 0.001
1289 reflectionsΔρmax = 3.69 e Å3
51 parametersΔρmin = 3.06 e Å3
Crystal data top
PbZn2(BO3)2V = 588.13 (10) Å3
Mr = 455.55Z = 4
Orthorhombic, PccnMo Kα radiation
a = 11.1709 (8) ŵ = 36.61 mm1
b = 4.9674 (7) ÅT = 290 K
c = 10.5987 (8) Å0.1 × 0.06 × 0.02 mm
Data collection top
Rigaku AFC-7R
diffractometer
972 reflections with I > 2σ(I)
Absorption correction: ψ scan
(Kopfmann & Huber, 1968)
Rint = 0.031
Tmin = 0.085, Tmax = 0.4733 standard reflections every 150 reflections
1522 measured reflections intensity decay: 1.6%
1289 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04551 parameters
wR(F2) = 0.1450 restraints
S = 1.11Δρmax = 3.69 e Å3
1289 reflectionsΔρmin = 3.06 e Å3
Special details top

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 (Å2) top
xyzUiso*/Ueq
Pb0.75000.75000.56467 (3)0.01330 (16)
Zn0.50171 (8)0.67690 (16)0.35005 (6)0.01124 (19)
B0.6023 (8)0.1894 (18)0.4163 (7)0.0128 (13)
O10.6227 (5)0.3426 (11)0.5216 (4)0.0127 (9)
O20.6277 (5)0.0832 (11)0.4122 (5)0.0137 (8)
O30.5539 (7)0.3107 (13)0.3117 (4)0.0163 (11)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Pb0.0090 (2)0.0182 (2)0.0127 (2)0.00054 (12)0.0000.000
Zn0.0117 (3)0.0104 (4)0.0116 (3)0.0012 (3)0.0002 (3)0.0004 (2)
B0.012 (3)0.016 (3)0.010 (2)0.001 (3)0.004 (3)0.000 (3)
O10.008 (2)0.016 (2)0.0150 (19)0.0024 (19)0.0005 (16)0.0028 (15)
O20.011 (2)0.011 (2)0.0190 (17)0.0012 (19)0.0030 (18)0.0001 (17)
O30.023 (3)0.012 (2)0.014 (2)0.007 (3)0.0084 (18)0.0012 (15)
Geometric parameters (Å, º) top
Pb—O2i2.273 (5)Zn—O31.953 (6)
Pb—O2ii2.273 (5)Zn—O2ii1.958 (5)
Pb—O12.515 (5)B—O11.371 (9)
Pb—O1iii2.515 (5)B—O31.372 (9)
Zn—O3iv1.941 (5)B—O21.384 (10)
Zn—O1v1.947 (5)
O2i—Pb—O2ii89.3 (3)O1—B—O3118.7 (7)
O2i—Pb—O185.28 (17)O1—B—O2122.3 (7)
O2ii—Pb—O179.89 (19)O3—B—O2119.0 (7)
O2i—Pb—O1iii79.89 (19)B—O1—Znv115.0 (5)
O2ii—Pb—O1iii85.28 (17)B—O1—Pb133.5 (5)
O1—Pb—O1iii159.1 (2)Znv—O1—Pb108.5 (2)
O3iv—Zn—O1v113.9 (3)B—O2—Znvi117.3 (5)
O3iv—Zn—O3103.34 (19)B—O2—Pbvi117.2 (4)
O1v—Zn—O3108.2 (3)Znvi—O2—Pbvi116.7 (2)
O3iv—Zn—O2ii108.6 (2)B—O3—Znvii133.6 (5)
O1v—Zn—O2ii108.0 (2)B—O3—Zn111.0 (5)
O3—Zn—O2ii115.0 (3)Znvii—O3—Zn114.0 (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, y1, z; (vii) x+1, y1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaPbZn2(BO3)2
Mr455.55
Crystal system, space groupOrthorhombic, Pccn
Temperature (K)290
a, b, c (Å)11.1709 (8), 4.9674 (7), 10.5987 (8)
V3)588.13 (10)
Z4
Radiation typeMo Kα
µ (mm1)36.61
Crystal size (mm)0.1 × 0.06 × 0.02
Data collection
DiffractometerRigaku AFC-7R
Absorption correctionψ scan
(Kopfmann & Huber, 1968)
Tmin, Tmax0.085, 0.473
No. of measured, independent and
observed [I > 2σ(I)] reflections
1522, 1289, 972
Rint0.031
(sin θ/λ)max1)0.806
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.045, 0.145, 1.11
No. of reflections1289
No. of parameters51
Δρmax, Δρmin (e Å3)3.69, 3.06

Computer programs: Rigaku/AFC Diffractometer Control Software (Rigaku Corporation, 1994), Rigaku/AFC Diffractometer Control Software, SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), ATOMS (Dowty, 1999), SHELXL97.

Selected geometric parameters (Å, º) top
Pb—O2i2.273 (5)Zn—O2iv1.958 (5)
Pb—O12.515 (5)B—O11.371 (9)
Zn—O3ii1.941 (5)B—O31.372 (9)
Zn—O1iii1.947 (5)B—O21.384 (10)
Zn—O31.953 (6)
O3ii—Zn—O1iii113.9 (3)O3—Zn—O2iv115.0 (3)
O3ii—Zn—O3103.34 (19)O1—B—O3118.7 (7)
O1iii—Zn—O3108.2 (3)O1—B—O2122.3 (7)
O3ii—Zn—O2iv108.6 (2)O3—B—O2119.0 (7)
O1iii—Zn—O2iv108.0 (2)
Symmetry codes: (i) x+3/2, y+1/2, z; (ii) x+1, y+1/2, z+1/2; (iii) x+1, y+1, z+1; (iv) x, y+1, z.
 

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