Dilead mercury chromate(VI), Pb₂HgCrO₆

The structure of Pb₂HgCrO₆ (space group P) can be described as consisting of isolated [CrO₄]^2- tetrahedra and nearly linear [HgO₂]^2- dumb-bells, which form layers of composition [HgCrO₆]^4-. These are intercalated with corrugated pseudo-hexagonal Pb²⁺ layers. The Pb²⁺ cation is stereochemically active and has coordination 3+5.

S1. Comment

Pb₂HgCrO₆ was first obtained as a by-product of different syntheses aiming to produce ternary silver oxides in steel autoclaves with perchloric acid as an accelerator (Curda et al., 2001). Subsequently, single crystals of Pb₂HgCrO₆ have been prepared by solid state reaction of a mixture of HgO, PbO₂ and elemental Cr (molar ratio 1:2:1) under an elevated oxygen pressure.

The structure of Pb₂HgCrO₆ can be described as consisting of three basic building units: isolated [CrO₄]^2- tetrahedra and nearly linear [HgO₂]^2- dumbbells form layers of composition [HgCrO₆]^4- which are intercalated with corrugated pseudohexagonal Pb²⁺ layers perpendicular to the [110] direction (Fig. 1).

The Pb²⁺ is stereochemically active. Every Pb²⁺ has a first coordination sphere formed by three O atoms at distances ranging from 2.24 to 2.45 Å. The second coordination sphere is formed by five further O atoms at distances of 2.64–3.66 Å (Table 1; Fig. 2).

Bond valence sums for the cations calculated according to Brese & O'Keeffe (1991) are 6.16 for Cr, 1.98 for Hg, and 2.14 and 2.21 for Pb. While for the O atoms O1, O5 and O6 the bond valence sums are within the normal range (2.09, 2.11 and 1.96, respectively), for O2 (2.22), O3 (1.79) and O4 (2.30), higher deviations from the ideal value are observed.

S2. Experimental

To synthesize Pb₂HgCrO₆, a mixture of HgO, PbO₂ and elemental Cr (molar ratio 1:2:1), under an elevated oxygen pressure of 11 MPa, was annealed for 120 h at 773 K in silver crucibles placed in stainless steel autoclaves (Linke & Jansen, 1997) using H₂O (2 ml) as an accelerator. Single crystals of Pb₂HgCrO₆ were formed.

S3. Refinement

Refinement was also carried out in space group P1, but did not lead to better agreement factors and was therefore discarded.

Computing details

Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-RED (Stoe & Cie, 2001); program(s) used to solve structure: SIR97 (Altomare et al., 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1998-2000).

Figures

	Fig. 1. A projection onto ab of the structure of Pb2HgCrO6. The [CrO4]^2- tetrahedra are shown.
	Fig. 2. Views of the two environments of the Pb^2+ ions in Pb2HgCrO6.

Dilead mercury chromate(VI)

Crystal data

Pb2HgCrO6	Z = 2
Mr = 762.98	F(000) = 632
Triclinic, P1	Dx = 7.651 Mg m^−3
a = 6.505 (2) Å	Mo Kα radiation, λ = 0.71069 Å
b = 7.201 (3) Å	Cell parameters from 8070 reflections
c = 7.605 (3) Å	θ = 2.6–24.5°
α = 91.82 (3)°	µ = 75.35 mm^−1
β = 92.17 (3)°	T = 293 K
γ = 111.33 (3)°	Prismatic, red
V = 331.2 (2) Å^3	0.20 × 0.05 × 0.02 mm

Data collection

Stoe IPDS II diffractometer	1838 independent reflections
Radiation source: fine-focus sealed tube	1583 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.070
image plate scans	θmax = 29.5°, θmin = 2.7°
Absorption correction: numerical (Coppens, 1970)	h = −9→7
Tmin = 0.011, Tmax = 0.081	k = −9→9
3475 measured reflections	l = −10→10

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)] = 0.049	w = 1/[σ^2(Fo^2) + (0.0857P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.127	(Δ/σ)max < 0.001
S = 0.97	Δρmax = 3.68 e Å^−3
1838 reflections	Δρmin = −3.30 e Å^−3
92 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0073 (7)

Special details

Experimental. measured with Stoe IPDS II; 1. run: phi=0; omega=0–180; delta omega=1; time=1 min 2. run: phi=90; omega=0–180; delta omega=1; time=1 min detector distance= 100 mm

Integration: Coefficient A=12.0 Coefficient B=2.0 EMS=0.010

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
Hg1	0.68673 (8)	0.70067 (9)	0.63551 (7)	0.03091 (19)
Pb1	−0.21924 (9)	0.25922 (8)	0.13343 (8)	0.0320 (2)
Pb2	0.17992 (9)	0.74491 (8)	0.39612 (7)	0.0317 (2)
Cr1	0.7086 (4)	0.7665 (4)	0.1477 (3)	0.0301 (5)
O1	0.463 (2)	0.6478 (18)	0.2086 (19)	0.049 (3)
O2	0.0538 (16)	0.3898 (15)	0.3672 (14)	0.031 (2)
O3	0.844 (2)	0.9460 (17)	0.2982 (15)	0.039 (2)
O4	0.4134 (16)	0.7651 (17)	0.6257 (14)	0.035 (2)
O5	−0.1528 (19)	0.6160 (18)	0.1271 (16)	0.042 (3)
O6	0.686 (2)	0.8686 (18)	−0.0350 (15)	0.042 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg1	0.0240 (3)	0.0354 (3)	0.0366 (3)	0.0149 (2)	0.0000 (2)	0.0017 (2)
Pb1	0.0300 (3)	0.0332 (3)	0.0344 (3)	0.0134 (2)	0.0011 (2)	0.0004 (2)
Pb2	0.0265 (3)	0.0341 (3)	0.0364 (3)	0.0136 (2)	−0.0018 (2)	0.0036 (2)
Cr1	0.0287 (11)	0.0302 (10)	0.0339 (12)	0.0138 (8)	−0.0007 (9)	0.0015 (9)
O1	0.037 (6)	0.036 (5)	0.068 (8)	0.007 (5)	0.009 (6)	0.010 (5)
O2	0.021 (4)	0.034 (5)	0.043 (5)	0.018 (4)	−0.005 (4)	−0.002 (4)
O3	0.044 (6)	0.035 (5)	0.043 (6)	0.020 (5)	0.000 (5)	0.000 (4)
O4	0.021 (5)	0.054 (6)	0.040 (5)	0.026 (4)	−0.004 (4)	0.002 (5)
O5	0.028 (5)	0.043 (6)	0.049 (7)	0.008 (4)	−0.009 (5)	−0.007 (5)
O6	0.052 (7)	0.043 (6)	0.036 (6)	0.024 (5)	0.002 (5)	0.007 (5)

Geometric parameters (Å, º)

Hg1—O4	1.993 (9)	Pb2—O2	2.384 (10)
Hg1—O2^i	2.019 (9)	Pb2—O2^iv	2.395 (11)
Hg1—O1^i	2.673 (9)	Pb2—O1	2.642 (14)
Hg1—O6^ii	2.749 (9)	Pb2—O5	2.795 (14)
Hg1—O3	3.150 (9)	Pb2—O3^vii	3.109 (14)
Hg1—O3^iii	3.193 (9)	Pb2—O3^viii	3.215 (14)
Hg1—O1	3.459 (9)	Pb2—O4^viii	3.571 (14)
Pb1—O4^iv	2.240 (11)	Pb2—Pb1^iv	3.5791 (16)
Pb1—O2	2.375 (10)	Cr1—O1	1.609 (12)
Pb1—O5	2.448 (12)	Cr1—O6	1.619 (11)
Pb1—O3^v	2.765 (12)	Cr1—O5^ix	1.650 (13)
Pb1—O6^vi	2.893 (12)	Cr1—O3	1.661 (12)
Pb1—O6^v	2.895 (12)	O2—Hg1^i	2.019 (9)
Pb1—O5^vi	3.080 (12)	O2—Pb2^iv	2.395 (11)
Pb1—O1^vi	3.209 (12)	O4—Pb1^iv	2.240 (11)
Pb1—Pb2^iv	3.5791 (16)	O5—Cr1^vii	1.650 (13)
Pb2—O4	2.236 (9)
O4—Hg1—O2^i	174.5 (4)	O6—Cr1—O3	108.5 (6)
O4^iv—Pb1—O2	75.7 (4)	O5^ix—Cr1—O3	108.0 (5)
O4^iv—Pb1—O5	90.6 (4)	Cr1—O1—Pb2	134.6 (7)
O2—Pb1—O5	78.9 (3)	Hg1^i—O2—Pb1	116.2 (5)
O4—Pb2—O2	93.6 (4)	Hg1^i—O2—Pb2	110.3 (4)
O4—Pb2—O2^iv	75.4 (4)	Pb1—O2—Pb2	111.8 (4)
O2—Pb2—O2^iv	71.5 (4)	Hg1^i—O2—Pb2^iv	112.1 (4)
O4—Pb2—O1	85.2 (4)	Pb1—O2—Pb2^iv	97.2 (4)
O2—Pb2—O1	72.0 (4)	Pb2—O2—Pb2^iv	108.5 (4)
O2^iv—Pb2—O1	137.1 (3)	Hg1—O4—Pb2	129.4 (5)
O1—Cr1—O6	107.5 (7)	Hg1—O4—Pb1^iv	119.8 (5)
O1—Cr1—O5^ix	111.0 (6)	Pb2—O4—Pb1^iv	106.2 (4)
O6—Cr1—O5^ix	112.2 (6)	Cr1^vii—O5—Pb1	138.9 (6)
O1—Cr1—O3	109.5 (7)

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