Crystal structure of the solid solution Ba8.35Pb0.65(B3O6)6.

Single crystals of lead barium borate, Ba8.35Pb0.65(B3O6)6, octabarium lead(II) hexa-kis-(triborate), have been obtained by spontaneous nucleation from a high-temperature melt. Its three-dimensional structure is constructed on the basis of a BaO9 polyhedron, a (Pb/Ba)O6 octa-hedron (occupancy ratio Pb:Ba = 0.216:0.784) and a condensed B3O6 ring anion. In the crystal, the planar B3O6 anions are stacked in an alternating fashion with Ba and (Pb/Ba) atoms along [001]. A comparison is made with the structures of related solid solutions in the system Ba/Pb/B/O.

Chemical context

The study of inorganic borates is motivated by their possible non-linear optical properties, transparency in a wide range of wavelengths, high laser-damage tolerance, piezoelectricity and luminescent and other useful properties for technical applications of the respective compounds. For example, β-BaB2O4 (Chen et al., 1985 ▸), LiB3O5 (Chen et al., 1989 ▸), CsB3O5 (Sasaki et al., 2000 ▸), Sr2Be2B2O7 (Chen et al., 1995 ▸), K5Ba10(BO3)8F (Liu et al., 2016 ▸), PbB4O7 (Bartwal et al., 2001 ▸), Pb2B5O9 X (X = Cl, Br, I) (Huang et al., 2010 ▸) or Ba3Sr4(BO3)3F5 (Zhang et al., 2009 ▸) have been studied because of their second-order non-linear optical behavior. Among inorganic borates synthesized and characterized over the past decades, some lead(II) borates show comprehensive applications. These features are associated with the highly asymmetric stereochemistry typical for a lead(II) atom due to the stereoactivity of the 6s 2 lone pair (Zhang et al., 2016 ▸; Mutailipu et al., 2016 ▸). Accordingly, numerous studies have been devoted to this family of compounds. Some lead borates are particularly attractive because of their high second-harmonic generation (SHG) response (Wu et al., 2012 ▸; Dong et al., 2015 ▸; Jing et al., 2015 ▸) or large birefringence (Liu et al., 2015 ▸).

In this communication, we report on the synthesis and crystal structure of the solid solution Ba8.35Pb0.65(B3O6)6.

Structural commentary

The crystal structure of Ba8.35Pb0.65(B3O6)6 is based on a Ba2O9 polyhedron, a (Pb/Ba1)O6 polyhedron and a condensed B3O6 anion, as shown in Fig. 1 ▸. The planar B3O6 anions (point group symmetry 3.) are isolated from each other and distributed layer upon layer perpendicular to [001]. The occupationally disordered (Pb/Ba)1 site (occupancy ratio Pb:Ba = 0.216:0.784) and the Ba2 site are located alternately between the B3O6 sheets in (Pb/Ba)1 and Ba2 layers, as shown in Fig. 2 ▸ a. The B atom is bound to one O1 atom and two O2 atoms to from a BO3 triangle. Three BO3 triangles are condensed through vertex-sharing to build a planar and cyclic B3O6 unit. The B—O bond lengths vary from 1.318 (5) to 1.406 (5) Å (Table 1 ▸), and the O—B—O angles are between 116.8 (4) and 122.6 (4)°.

Figure 1

The principal building units in the crystal structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x, −y, −z; (ii) −y, x − y, z; (iii) y, −x + y, −z; (iv) x − y, x, −z; (v) −x + y, −x, z; (vi) −y + 1, x − y, z; (vii) −x + y + 1, −x + 1, z; (viii) y + , −x + y + , −z + ; (ix) x − y + , x + , −z + ; (x) −x + , −y + , −z + ; (xiii) −y, x − y − 1, z; (xiv) −x + y + 1, −x, z.]

Figure 2

The crystal structures of related solid solutions in the system Ba/Pb/B/O viewed down [010]: (a) Ba8.35Pb0.65(B3O6)6; (b) Ba7.87Pb1.13(B3O6)6 (Wu et al., 2012 ▸); (c) Ba2Pb(B3O6)2 (Li et al., 2014 ▸); (d) Ba2Pb(B3O6)2 (Tang et al., 2015 ▸). The numbers indicate the bond lengths (Å) of the PbO6 or (Ba/Pb)O6 octa­hedra.

Table 1

Selected geometric parameters (Å, °)

(Pb/Ba)1—O1	2.537 (3)	B—O1	1.318 (5)
Ba2—O1	2.766 (3)	B—O2	1.397 (5)
Ba2—O1i	2.810 (3)	B—O2ii	1.406 (5)
Ba2—O2	3.030 (3)
O1—B—O2	120.6 (4)	O2—B—O2ii	116.8 (4)
O1—B—O2ii	122.6 (4)

Symmetry codes: (i) ; (ii) .

The Ba2 atom (site symmetry 3.) is coordinated by nine O atoms. The Ba—O bond lengths of the Ba2O9 polyhedron range from 2.766 (3) to 3.030 (3) Å, with a mean distance of 2.869 Å (Table 1 ▸). A similar environment for Ba is observed in the crystal structures of Na3Ba2(B3O6)2F (Zhang et al., 2015 ▸), PbBa2(B3O6)2 (Li et al., 2014 ▸) and α-BBO (Wu et al., 2002 ▸). Each of the Ba2O9 polyhedra shares edges with adjacent Ba2O9 polyhedra to form six-membered rings that are arranged in corrugated layers extending parallel to (001) (Fig. 3 ▸). The (Pb/Ba)1 site (site symmetry .) is surrounded by six O atoms; the corresponding (Pb/Ba)1O6 octa­hedra are isolated from each other. The six (Pb/Ba1)—O bonds have an identical length of 2.537 (3) Å (Table 1 ▸, Fig. 2 ▸ a). In comparison with the M2 site, the M1 site has a more narrow coordin­ation environment which seems to be the reason why Pb atoms exclusively substitute Ba atoms at the latter position due to their smaller ionic radius.

Figure 3

The formation of a corrugated layer of Ba2O9 polyhedra in the crystal structure of Ba8.35Pb0.65(B3O6)6 viewed down [001].

Comparison with the structures of related solid solutions

It is inter­esting to compare the structure of Ba8.35Pb0.65(B3O6)6 with those of the related solid solutions Ba7.87Pb1.13(B3O6)6 (Wu et al., 2012 ▸) and Ba2Pb(B3O6)2 (Li et al., 2014 ▸; Tang et al., 2015 ▸). Whereas the title compound Ba8.35Pb0.65(B3O6)6 crystallizes in space group R , Ba7.87Pb1.13(B3O6)6 was solved and refined in space group R32 on the basis of single crystal X-ray diffraction data (Wu et al., 2012 ▸); the lattice parameters of both compounds are very similar. Ba2Pb(B3O6)2 on the other hand was reported to crystallize either in space group R with lattice parameters in the same range as the previous two structures (single crystal X-ray diffraction data; Li et al., 2014 ▸) or in space group R c with a doubled c axis in comparison with the other structures (powder X-ray diffraction data using the Rietveld method; Tang et al., 2015 ▸). All four crystal structures are characterized by an alternating stacking of cationic and anionic (001) layers along [001], as shown in Fig. 2 ▸. In each case, the Ba site is coordinated by nine O atoms to form BaO9 polyhedra, and the Pb or the (Pb/Ba) site is coordinated by six O atoms to form distorted PbO6 octa­hedra [in Ba2Pb(B3O6)2] or (Pb/Ba)O6 octa­hedra [in Ba8.35Pb0.65(B3O6)6 and Ba7.87Pb1.13(B3O6)6].

The arrangement of the planar B3O6 rings in the crystal structures is a determining factor in whether a non-centrosymmetric or a centrosymmetric structure is obtained. In Ba7.87Pb1.13(B3O6)6 (Wu et al., 2012 ▸), the rings are aligned in a chiral arrangement (Fig. 4 ▸ b), responsible for the SHG effect. In Ba2Pb(B3O6)2 (Li et al., 2014 ▸), the B3O6 rings are parallel to each other, distributed layer upon layer along [001], and the B3O6 rings in neighbouring layers point in exactly opposite directions (Fig. 4 ▸ c), just like in the title compound (Fig. 4 ▸ a). In the Ba2Pb(B3O6)2 structure with doubled volume (Tang et al., 2015 ▸), all of the B3O6 rings are parallel to (001), and the B3O6 rings in two neighbouring layers are rotated slightly relative to each other (Fig. 4 ▸ d).

Figure 4

The arrangement of B3O6 groups along the [001] direction in the different solid solutions: (a) Ba8.35Pb0.65(B3O6)6; (b) Ba7.87Pb1.13(B3O6)6 (Wu et al., 2012 ▸); (c) Ba2Pb(B3O6)2 (Li et al., 2014 ▸); (d) Ba2Pb(B3O6)2 (Tang et al., 2015 ▸).

Synthesis and crystallization

Suitable crystals of the solid solution Ba8.35Pb0.65(B3O6)6 were obtained by spontaneous nucleation from a high-temperature melt mixture originating from PbO, H3BO3 and BaF2 in molar ratios of 4:5:1. The starting materials were weighed and melted in a platinum crucible in several batches. The crucible position was fixed at the centre of a resistance-heated furnace. The temperature of the furnace was controlled within 0.1–1 K by an Al-708P controller and a Pt/Pt–Rh thermocouple. The temperature was raised by about 50 K h−1 to 50 K above the melting point and held for 15 h to ensure a homogenous mixture of the solution. After cooling down the furnace to 1073 K, a slow cooling rate of 5 K d−1, was applied, followed by cooling to room temperature at 20 K h−1. Colorless crystals in the millimetre range were obtained.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. From the two large cation positions in the structure (Wyckoff positions 3a and 6c), only those of M1 at 3a are occupationally disordered by Ba and Pb atoms. Refinement of the occupancy of Ba:Pb at this site under consideration of EXYZ and EADP commands (Sheldrick, 2008 ▸) resulted in a 21.6 (7)% occupancy of Pb. The highest peak and the deepest hole are located 0.98 and 2.06 Å from the Ba2 and B atoms, respectively.

Table 2

Experimental details

Crystal data
Chemical formula	Ba8.35Pb0.65(B3O6)6
M r	2051.83
Crystal system, space group	Trigonal, R
Temperature (K)	296
a, c (Å)	7.206 (2), 18.653 (11)
V (Å3)	838.7 (6)
Z	1
Radiation type	Mo Kα
μ (mm−1)	13.00
Crystal size (mm)	0.16 × 0.08 × 0.02
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Numerical (face-indexed using SADABS; Bruker, 2000 ▸)
T min, T max	0.141, 0.547
No. of measured, independent and observed [I > 2σ(I)] reflections	1745, 441, 430
R int	0.024
(sin θ/λ)max (Å−1)	0.652
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.019, 0.048, 1.23
No. of reflections	441
No. of parameters	35
Δρmax, Δρmin (e Å−3)	0.64, −0.74

Computer programs: APEX2 and SAINT (Bruker, 2000 ▸), SHELXS97, SHELXL97 and SHELXTL (Sheldrick, 2008 ▸) and DIAMOND (Brandenburg, 2006 ▸).

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989017001864/wm5348sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989017001864/wm5348Isup2.hkl

Crystal structure. DOI: 10.1107/S2056989017001864/wm5348sup3.txt

CCDC reference: 1530747

Additional supporting information: crystallographic information; 3D view; checkCIF report

Ba8.35Pb0.65(B3O6)6	Dx = 4.062 Mg m−3
Mr = 2051.83	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3	Cell parameters from 1161 reflections
Hall symbol: -R 3	θ = 3.3–27.6°
a = 7.206 (2) Å	µ = 13.00 mm−1
c = 18.653 (11) Å	T = 296 K
V = 838.7 (6) Å3	Plate, colourless
Z = 1	0.16 × 0.08 × 0.02 mm
F(000) = 899

Bruker APEXII CCD diffractometer	441 independent reflections
Radiation source: fine-focus sealed tube	430 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.024
phi and ω scans	θmax = 27.6°, θmin = 3.3°
Absorption correction: numerical (face-indexed using SADABS; Bruker, 2000)	h = −8→9
Tmin = 0.141, Tmax = 0.547	k = −9→9
1745 measured reflections	l = −14→23

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.019	w = 1/[σ2(Fo2) + (0.0178P)2 + 5.852P] where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.048	(Δ/σ)max < 0.001
S = 1.23	Δρmax = 0.64 e Å−3
441 reflections	Δρmin = −0.74 e Å−3
35 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraints	Extinction coefficient: 0.00125 (17)

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 > 2sigma(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.

	x	y	z	Uiso*/Ueq	Occ. (<1)
Pb1	0.0000	0.0000	0.0000	0.0147 (2)	0.216 (7)
Ba1	0.0000	0.0000	0.0000	0.0147 (2)	0.784 (7)
Ba2	0.6667	0.3333	0.12947 (2)	0.01537 (18)
B	0.2953 (7)	−0.1579 (7)	0.0864 (3)	0.0175 (9)
O1	0.2633 (5)	0.0063 (4)	0.09165 (18)	0.0218 (6)
O2	0.5029 (5)	−0.1261 (5)	0.08345 (18)	0.0234 (7)

	U11	U22	U33	U12	U13	U23
Pb1	0.0136 (2)	0.0136 (2)	0.0169 (3)	0.00680 (12)	0.000	0.000
Ba1	0.0136 (2)	0.0136 (2)	0.0169 (3)	0.00680 (12)	0.000	0.000
Ba2	0.01176 (19)	0.01176 (19)	0.0226 (3)	0.00588 (9)	0.000	0.000
B	0.019 (2)	0.017 (2)	0.018 (2)	0.0103 (18)	−0.0009 (18)	−0.0002 (17)
O1	0.0185 (14)	0.0159 (13)	0.0320 (17)	0.0093 (12)	−0.0011 (12)	−0.0022 (12)
O2	0.0162 (14)	0.0142 (13)	0.0384 (18)	0.0066 (11)	0.0012 (13)	0.0017 (13)

(Pb/Ba)1—O1i	2.537 (3)	Ba2—O1vii	2.810 (3)
(Pb/Ba)1—O1ii	2.537 (3)	Ba2—O2	3.030 (3)
(Pb/Ba)1—O1	2.537 (3)	Ba2—O2viii	3.030 (3)
(Pb/Ba)1—O1iii	2.537 (3)	Ba2—O2ix	3.030 (3)
(Pb/Ba)1—O1iv	2.537 (3)	Ba2—Bviii	3.296 (5)
(Pb/Ba)1—O1v	2.537 (3)	Ba2—Bix	3.296 (5)
Pb1—Ba2vi	3.803 (2)	Ba2—Pb1xii	3.803 (2)
Pb1—Ba2vii	3.803 (2)	B—O1	1.318 (5)
Ba2—O1viii	2.766 (3)	B—O2	1.397 (5)
Ba2—O1	2.766 (3)	B—O2xiii	1.406 (5)
Ba2—O1ix	2.766 (3)	O1—Ba2vii	2.810 (3)
Ba2—O1x	2.810 (3)	O2—Bxiv	1.406 (5)
Ba2—O1xi	2.810 (3)
O1i—Pb1—O1ii	100.43 (11)	O1xi—Ba2—O2viii	67.48 (9)
O1i—Pb1—O1	180.00 (17)	O1vii—Ba2—O2viii	135.55 (8)
O1ii—Pb1—O1	79.57 (11)	O2—Ba2—O2viii	112.31 (6)
O1i—Pb1—O1iii	79.57 (11)	O1viii—Ba2—O2ix	147.14 (9)
O1ii—Pb1—O1iii	180.00 (11)	O1—Ba2—O2ix	67.39 (8)
O1—Pb1—O1iii	100.43 (11)	O1ix—Ba2—O2ix	47.75 (8)
O1i—Pb1—O1iv	79.57 (11)	O1x—Ba2—O2ix	135.55 (8)
O1ii—Pb1—O1iv	100.43 (11)	O1xi—Ba2—O2ix	107.59 (8)
O1—Pb1—O1iv	100.43 (11)	O1vii—Ba2—O2ix	67.48 (9)
O1iii—Pb1—O1iv	79.57 (11)	O2—Ba2—O2ix	112.31 (6)
O1i—Pb1—O1v	100.43 (11)	O2viii—Ba2—O2ix	112.31 (6)
O1ii—Pb1—O1v	79.57 (11)	O1viii—Ba2—Bviii	23.05 (10)
O1—Pb1—O1v	79.57 (11)	O1—Ba2—Bviii	134.00 (10)
O1iii—Pb1—O1v	100.43 (11)	O1ix—Ba2—Bviii	92.24 (10)
O1iv—Pb1—O1v	180.00 (17)	O1x—Ba2—Bviii	89.14 (10)
O1i—Pb1—Ba2vi	47.64 (7)	O1xi—Ba2—Bviii	75.27 (10)
O1ii—Pb1—Ba2vi	132.36 (7)	O1vii—Ba2—Bviii	144.45 (11)
O1—Pb1—Ba2vi	132.36 (7)	O2—Ba2—Bviii	89.88 (9)
O1iii—Pb1—Ba2vi	47.64 (7)	O2viii—Ba2—Bviii	25.06 (9)
O1iv—Pb1—Ba2vi	47.64 (7)	O2ix—Ba2—Bviii	134.51 (10)
O1v—Pb1—Ba2vi	132.36 (7)	O1viii—Ba2—Bix	134.00 (10)
O1i—Pb1—Ba2vii	132.36 (7)	O1—Ba2—Bix	92.24 (10)
O1ii—Pb1—Ba2vii	47.64 (7)	O1ix—Ba2—Bix	23.05 (10)
O1—Pb1—Ba2vii	47.64 (7)	O1x—Ba2—Bix	144.45 (11)
O1iii—Pb1—Ba2vii	132.36 (7)	O1xi—Ba2—Bix	89.14 (10)
O1iv—Pb1—Ba2vii	132.36 (7)	O1vii—Ba2—Bix	75.27 (10)
O1v—Pb1—Ba2vii	47.64 (7)	O2—Ba2—Bix	134.51 (10)
Ba2vi—Pb1—Ba2vii	180.0	O2viii—Ba2—Bix	89.88 (10)
O1viii—Ba2—O1	113.73 (6)	O2ix—Ba2—Bix	25.06 (9)
O1viii—Ba2—O1ix	113.73 (6)	Bviii—Ba2—Bix	114.27 (7)
O1—Ba2—O1ix	113.73 (6)	O1viii—Ba2—Pb1xii	104.78 (7)
O1viii—Ba2—O1x	76.27 (10)	O1—Ba2—Pb1xii	104.78 (7)
O1—Ba2—O1x	89.15 (12)	O1ix—Ba2—Pb1xii	104.78 (7)
O1ix—Ba2—O1x	145.30 (7)	O1x—Ba2—Pb1xii	41.85 (7)
O1viii—Ba2—O1xi	89.15 (12)	O1xi—Ba2—Pb1xii	41.85 (7)
O1—Ba2—O1xi	145.30 (7)	O1vii—Ba2—Pb1xii	41.85 (7)
O1ix—Ba2—O1xi	76.27 (10)	O2—Ba2—Pb1xii	106.46 (7)
O1x—Ba2—O1xi	70.59 (10)	O2viii—Ba2—Pb1xii	106.46 (7)
O1viii—Ba2—O1vii	145.31 (7)	O2ix—Ba2—Pb1xii	106.46 (7)
O1—Ba2—O1vii	76.27 (10)	Bviii—Ba2—Pb1xii	104.10 (8)
O1ix—Ba2—O1vii	89.15 (12)	Bix—Ba2—Pb1xii	104.10 (8)
O1x—Ba2—O1vii	70.59 (10)	O1—B—O2	120.6 (4)
O1xi—Ba2—O1vii	70.59 (10)	O1—B—O2xiii	122.6 (4)
O1viii—Ba2—O2	67.39 (8)	O2—B—O2xiii	116.8 (4)
O1—Ba2—O2	47.75 (8)	B—O1—Pb1	113.5 (3)
O1ix—Ba2—O2	147.14 (9)	B—O1—Ba2	101.7 (3)
O1x—Ba2—O2	67.48 (9)	Pb1—O1—Ba2	130.17 (12)
O1xi—Ba2—O2	135.55 (8)	B—O1—Ba2vii	117.4 (3)
O1vii—Ba2—O2	107.59 (8)	Pb1—O1—Ba2vii	90.51 (10)
O1viii—Ba2—O2viii	47.75 (8)	Ba2—O1—Ba2vii	103.73 (10)
O1—Ba2—O2viii	147.14 (9)	B—O2—Bxiv	122.9 (4)
O1ix—Ba2—O2viii	67.39 (8)	B—O2—Ba2	88.2 (2)
O1x—Ba2—O2viii	107.59 (8)	Bxiv—O2—Ba2	143.4 (3)