β-K₂Cr₂O₇

The monoclinic modification of dipotassium dichromate, β-K₂Cr₂O₇, has been synthesized in the K₂Cr₂O₇–H₂O system. The structure consists of K⁺ cations and Cr₂O₇²⁻ dimers. In contrast with triclinic α-K₂Cr₂O₇ [Kuz'min, Ilyukhin, Kharitonov & Belov (1969). Krist. Tech. 4, 441–461], the Cr₂O₇²⁻ groups in β-K₂Cr₂O₇ have twofold crystallographic symmetry and are parallel to each other.

Computing details

Data collection: SMART-NT (Bruker, 1998); cell refinement: SAINT-NT (Bruker, 1998); data reduction: SAINT-NT; program(s) used to solve structure: SHELXTL (Bruker, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: CrystalMaker (Palmer, 1999); software used to prepare material for publication: SHELXTL.

dipotassium dichromate

Crystal data

K2Cr2O7	? # Insert any comments here.
Mr = 294.20	Dx = 2.724 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 13.0339 (17) Å	Cell parameters from 523 reflections
b = 7.375 (1) Å	θ = 3–23°
c = 7.4672 (10) Å	µ = 4.18 mm^−1
β = 91.923 (2)°	T = 293 K
V = 717.38 (17) Å^3	Plate, orange
Z = 4	0.46 × 0.24 × 0.09 mm
F(000) = 568

Data collection

Bruker PLATFORM with a SMART CCD area detector diffractometer	798 independent reflections
Radiation source: fine-focus sealed tube	718 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.045
ω scans	θmax = 28.3°, θmin = 3.1°
Absorption correction: ψ-scan empirical (using intensity measurements) (SAINT-NT; Bruker, 1998)	h = −16→17
Tmin = 0.317, Tmax = 0.686	k = −9→9
1779 measured reflections	l = −6→9

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)] = 0.029	w = 1/[σ^2(Fo^2) + (0.0513P)^2 + 0.5383P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.084	(Δ/σ)max < 0.001
S = 1.11	Δρmax = 0.50 e Å^−3
798 reflections	Δρmin = −0.52 e Å^−3
52 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0074 (11)

Special details

Experimental. ? #Insert any special details here.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cr	0.39714 (3)	0.17429 (5)	0.13497 (5)	0.0213 (2)
K	0.14473 (5)	0.15065 (9)	0.35562 (8)	0.0328 (2)
O1	0.44153 (18)	0.3071 (3)	−0.0176 (3)	0.0337 (5)
O2	0.33753 (18)	0.2974 (3)	0.2780 (3)	0.0341 (5)
O3	1/2	0.0595 (4)	1/4	0.0283 (6)
O4	0.32025 (19)	0.0254 (3)	0.0494 (3)	0.0408 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr	0.0206 (3)	0.0226 (3)	0.0207 (3)	−0.00247 (14)	0.00292 (18)	−0.00125 (14)
K	0.0336 (4)	0.0382 (4)	0.0266 (4)	−0.0056 (3)	0.0002 (3)	−0.0043 (2)
O1	0.0334 (12)	0.0398 (11)	0.0281 (10)	−0.0049 (9)	0.0069 (9)	0.0076 (8)
O2	0.0379 (12)	0.0373 (10)	0.0277 (10)	0.0088 (9)	0.0082 (9)	−0.0012 (8)
O3	0.0256 (13)	0.0251 (12)	0.0343 (15)	0	0.0020 (11)	0
O4	0.0371 (12)	0.0368 (11)	0.0479 (13)	−0.0112 (9)	−0.0072 (11)	−0.0054 (10)

Geometric parameters (Å, º)

Cr—O1	1.624 (2)	K—O1^iii	3.039 (2)
Cr—O2	1.620 (2)	K—O2^iv	2.764 (2)
Cr—O3	1.7819 (13)	K—O2^iii	2.802 (2)
Cr—O4	1.605 (2)	K—O2	2.814 (3)
K—O1^i	2.746 (2)	K—O4^v	2.886 (2)
K—O1^ii	2.859 (2)	K—O4^vi	2.965 (2)
O4—Cr—O2	109.82 (13)	O1^i—K—O4^vi	141.59 (7)
O4—Cr—O1	111.46 (12)	O2^iv—K—O4^vi	62.63 (7)
O2—Cr—O1	108.33 (12)	O2^iii—K—O4^vi	72.24 (7)
O4—Cr—O3	108.45 (11)	O2—K—O4^vi	65.75 (7)
O2—Cr—O3	108.56 (9)	O1^ii—K—O4^vi	126.46 (7)
O1—Cr—O3	110.17 (9)	O4^v—K—O4^vi	100.75 (6)
Cr^vii—O3—Cr	123.25 (15)	O1^i—K—O1^iii	108.35 (7)
O1^i—K—O2^iv	155.77 (7)	O2^iv—K—O1^iii	75.01 (6)
O1^i—K—O2^iii	79.30 (6)	O2^iii—K—O1^iii	53.33 (6)
O2^iv—K—O2^iii	118.43 (6)	O2—K—O1^iii	138.36 (7)
O1^i—K—O2	96.20 (7)	O1^ii—K—O1^iii	66.42 (7)
O2^iv—K—O2	96.02 (6)	O4^v—K—O1^iii	139.30 (7)
O2^iii—K—O2	101.49 (5)	O4^vi—K—O1^iii	74.36 (7)
O1^i—K—O1^ii	86.37 (7)	O1^i—K—O4	71.22 (6)
O2^iv—K—O1^ii	72.84 (7)	O2^iv—K—O4	131.63 (6)
O2^iii—K—O1^ii	107.91 (7)	O2^iii—K—O4	56.30 (6)
O2—K—O1^ii	150.43 (6)	O2—K—O4	48.95 (6)
O1^i—K—O4^v	100.01 (6)	O1^ii—K—O4	154.12 (6)
O2^iv—K—O4^v	67.47 (7)	O4^v—K—O4	108.63 (6)
O2^iii—K—O4^v	164.51 (7)	O4^vi—K—O4	71.65 (5)
O2—K—O4^v	63.09 (7)	O1^iii—K—O4	107.83 (6)
O1^ii—K—O4^v	87.42 (7)

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