PbGaBO₄, an orthoborate with a new structure-type

The crystal structure of a new ternary borate, lead gallium boron tetraoxide, PbGaBO₄, has been determined by X-ray diffraction using a single-crystal grown from a PbO flux. The structure consists of infinite [010] chains of edge-sharing GaO₆ octahedra bridged by BO₃ triangles. The Pb²⁺ cations and their stereoactive lone pair occupy the apex of PbO₄ square pyramids. The structure represents a new structure-type for an­hydrous orthoborates. Bond-valence analysis reveals the presence of strain within the octahedral chains.

Key indicators

• Single-crystal X-ray study
• T = 299 K
• Mean [sigma](Ga-O) = 0.002 Å
• R factor = 0.028
• wR factor = 0.066
• Data-to-parameter ratio = 36.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level A:



ABSTM_02  Alert A Crystal and compound unsuitable for non-numerical
          corrections. Product of mu and tmid > 3.0
           Value of mu given =    57.220 tmid =     0.070


 Alert Level C:



DIFMN_02  Alert C The minimum difference density is < -0.1*ZMAX*0.75
          _refine_diff_density_min given =     -6.702
          Test value =     -6.150

DIFMN_03  Alert C The minimum difference density is < -0.1*ZMAX*0.75
          The relevant atom site should be identified.


 1 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 2 Alert Level C = Please check

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S1. Comment

Inorganic borates continue to be an active area of research with the aim of finding new compounds with interesting optical properties. Previous studies in our laboratory have resulted in the successful structure determinations of two new gallium borate compounds, MGa₂B₂O₇ (M = Sr, Ba) (Park & Barbier, 2000). Our current investigation is focused on the PbO–X₂O₃–B₂O₃ systems (X = Al, Ga), which have not yet been explored. The structure of PbGaBO₄ represents a new structure-type for the family of anhydrous orthoborates. It is based on a distorted octahedral coordination of Ga, a regular triangular coordination of B and the expected irregular fourfold coordination of divalent Pb with a stereoactive lone pair (Fig. 1). The structure is built of infinite chains of edge-sharing GaO₆ octahedra parallel to the b axis and linked by BO₃ triangles (Figs. 2 and 3). The short B—O bonds (1.37 Å) in the BO₃ groups bridging adjacent octahedra impose a strong angular distortion along the octahedral chains; the dihedral angle between adjacent octahedra deviate from the ideal angle of 60° and range from 49.8 to 77.7° (Fig. 2). Bond-valence analysis (Brese & O'Keefe, 1991) indicates the presence of structural strain as a result of distortions in the GaO₆ octahedra; the bond-valence sum around O1 is high [σ(s) = 2.23] due to two short Ga—O1 bonds (1.889 Å, s = 0.65), whereas the bond-valence sum around O3 is low [σ(s)= 1.79] due to two long Ga—O3 bonds (2.076 Å, s = 0.40).

S2. Experimental

Single crystals of PbGaBO₄ were grown using a PbO flux. A stoichiometric mixture of PbO, Ga₂O₃ and H₃BO₃ powders with 50 mole% excess PbO (total weight 12.00 g) was melted at 1173 K in a covered Pt crucible and cooled to 773 K (at 3 K h^-1). A large quantity of colourless prismatic crystals were recovered after dissolving the PbO flux in dilute aqueous HNO₃.

S3. Refinement

The locations of maximum and minimum peaks in the residual electron-density map: highest peak 5.73 e Å^-3 at 0.0592, 0.1547, 0.3570 (0.56 Å from Pb) and deepest hole -6.70 e Å^-3 at 0.0553, 1/4, 0.4608 (0.84 Å from Pb).

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XTALDRAW (Bartelmeh & Downs, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Part of the PbGaBO4 structure. The displacement ellipsoids are drawn at the 90% probability level.
	Fig. 2. View of the PbGaBO4 structure approximately along the [100] direction. Strong angular distortions are associated with the bridging borate groups.
	Fig. 3. View of the PbGaBO4 structure along the [010] direction. The GaO6 octahedral chains are linked by the BO3 triangles.

lead gallium boron tetraoxide

Crystal data

PbGaBO4	Dx = 6.871 Mg m^−3
Mr = 351.72	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 7743 reflections
a = 6.9944 (10) Å	θ = 3.8–45.4°
b = 5.8925 (8) Å	µ = 57.22 mm^−1
c = 8.2495 (11) Å	T = 299 K
V = 340.00 (8) Å^3	Prism, colourless
Z = 4	0.10 × 0.07 × 0.04 mm
F(000) = 600

Data collection

CCD area detector diffractometer	1493 independent reflections
Radiation source: rotating anode	1336 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.042
ϕ and ω scans	θmax = 45.4°, θmin = 3.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −14→11
Tmin = 0.016, Tmax = 0.101	k = −7→11
7743 measured reflections	l = −16→15

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)] = 0.028	w = 1/[σ^2(Fo^2) + (0.0348P)^2 + 0.5503P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.066	(Δ/σ)max = 0.001
S = 1.14	Δρmax = 5.73 e Å^−3
1493 reflections	Δρmin = −6.70 e Å^−3
41 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0065 (5)

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	0.06200 (2)	0.2500	0.358777 (19)	0.00640 (5)
Ga	0.0000	0.0000	0.0000	0.00398 (8)
O3	−0.1783 (5)	−0.2500	0.0921 (4)	0.0069 (5)
O1	0.1013 (5)	−0.2500	−0.1146 (4)	0.0041 (4)
O2	0.1717 (4)	−0.0466 (4)	0.1922 (3)	0.0073 (3)
B	−0.2237 (7)	0.2500	−0.2639 (6)	0.0036 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb	0.00665 (7)	0.00891 (7)	0.00364 (7)	0.000	0.00060 (4)	0.000
Ga	0.00501 (16)	0.00327 (14)	0.00366 (16)	0.00093 (12)	−0.00066 (13)	−0.00089 (12)
O3	0.0085 (12)	0.0073 (10)	0.0048 (12)	0.000	0.0040 (10)	0.000
O1	0.0025 (9)	0.0038 (9)	0.0059 (11)	0.000	0.0005 (8)	0.000
O2	0.0103 (9)	0.0052 (7)	0.0064 (9)	0.0016 (7)	−0.0051 (7)	−0.0001 (6)
B	0.0034 (14)	0.0045 (13)	0.0030 (15)	0.000	−0.0012 (11)	0.000

Geometric parameters (Å, º)

Pb—O1^i	2.316 (4)	Ga—Ga^v	2.9463 (4)
Pb—O2	2.352 (2)	Ga—Ga^iv	2.9463 (4)
Pb—O2^ii	2.352 (2)	Ga—Pb^i	3.3344 (4)
Pb—O1^iii	2.365 (3)	O3—B^vi	1.372 (6)
Pb—Ga	3.3344 (4)	O3—Ga^v	2.074 (2)
Pb—Ga^iv	3.3344 (4)	O1—Ga^v	1.888 (2)
Ga—O1	1.888 (2)	O1—Pb^i	2.316 (4)
Ga—O1^i	1.888 (2)	O1—Pb^vii	2.365 (3)
Ga—O2	2.008 (2)	O2—B^i	1.385 (3)
Ga—O2^i	2.008 (2)	B—O3^viii	1.372 (6)
Ga—O3	2.074 (2)	B—O2^iv	1.385 (3)
Ga—O3^i	2.074 (2)	B—O2^i	1.385 (3)
O1^i—Pb—O2	69.67 (8)	O2^i—Ga—Ga^iv	82.13 (7)
O1^i—Pb—O2^ii	69.67 (8)	O3—Ga—Ga^iv	135.25 (7)
O2—Pb—O2^ii	96.00 (12)	Ga^v—Ga—Ga^iv	180.0
O1^i—Pb—O1^iii	124.88 (9)	O1—Ga—Pb	137.75 (10)
O2—Pb—O1^iii	74.31 (8)	O2^i—Ga—Pb	135.91 (7)
O2^ii—Pb—O1^iii	74.31 (8)	O3—Ga—Pb	93.84 (9)
O2^ii—Pb—Ga	81.49 (6)	O3^i—Ga—Pb	86.16 (9)
O1^iii—Pb—Ga	102.24 (8)	Ga^v—Ga—Pb	116.219 (4)
O2—Pb—Ga^iv	81.49 (6)	Ga^iv—Ga—Pb	63.781 (4)
O1^iii—Pb—Ga^iv	102.24 (8)	O1^i—Ga—Pb^i	137.75 (10)
Ga—Pb—Ga^iv	52.437 (9)	O2—Ga—Pb^i	135.91 (7)
O1—Ga—O1^i	180.0 (3)	O3—Ga—Pb^i	86.16 (9)
O1—Ga—O2	93.67 (13)	O3^i—Ga—Pb^i	93.84 (9)
O1^i—Ga—O2	86.33 (13)	Ga^v—Ga—Pb^i	63.781 (4)
O1—Ga—O2^i	86.33 (13)	Ga^iv—Ga—Pb^i	116.219 (4)
O1^i—Ga—O2^i	93.67 (13)	Pb—Ga—Pb^i	180.000 (6)
O2—Ga—O2^i	180.00 (9)	B^vi—O3—Ga	128.14 (15)
O1—Ga—O3	81.65 (11)	B^vi—O3—Ga^v	128.14 (15)
O1^i—Ga—O3	98.35 (11)	Ga—O3—Ga^v	90.51 (14)
O2—Ga—O3	88.47 (13)	Ga—O1—Ga^v	102.55 (16)
O2^i—Ga—O3	91.53 (13)	Ga—O1—Pb^i	104.50 (12)
O1—Ga—O3^i	98.35 (11)	Ga^v—O1—Pb^i	104.50 (12)
O1^i—Ga—O3^i	81.65 (11)	Ga—O1—Pb^vii	114.84 (11)
O2—Ga—O3^i	91.53 (13)	Ga^v—O1—Pb^vii	114.84 (11)
O2^i—Ga—O3^i	88.47 (13)	Pb^i—O1—Pb^vii	114.22 (14)
O3—Ga—O3^i	180.0	B^i—O2—Ga	127.8 (2)
O1^i—Ga—Ga^v	141.28 (8)	B^i—O2—Pb	118.6 (2)
O2—Ga—Ga^v	82.13 (7)	Ga—O2—Pb	99.48 (10)
O2^i—Ga—Ga^v	97.87 (7)	O3^viii—B—O2^iv	120.08 (19)
O3^i—Ga—Ga^v	135.25 (7)	O3^viii—B—O2^i	120.08 (19)
O1—Ga—Ga^iv	141.28 (8)	O2^iv—B—O2^i	119.8 (4)
O2—Ga—Ga^iv	97.87 (7)

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