Spinel-type HgCr₂O₄ from single-crystal data

The crystal structure of mercury(II) chromium(III) tetroxide has been redetermined from a single crystal grown from an HgO-CrO₃ mixture at 743 K in an evacuated silica ampoule. The present investigation confirms the previous study, which was based on powder data [Wessels, Czekalla & Jeitschko (1998). Mater. Res. Bull. 33, 95-191], but with higher precision and with all displacement parameters refined anisotropically. HgCr₂O₄ adopts the normal 2-3-spinel structure, with mercury in one-eighth of the tetra­hedral and chromium in one-half of the octa­hedral voids of the cubic closed-packed O atoms.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](r-O) = 0.001 Å
• R factor = 0.011
• wR factor = 0.023
• Data-to-parameter ratio = 15.3

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       0.25 Ratio
PLAT127_ALERT_1_C Implicit Hall Symbol  Inconsistent with Explicit    -F 4vw 2vw

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           39.95
           From the CIF: _reflns_number_total                122
           Count of symmetry unique reflns            0
           Completeness (_total/calc)      Infinity    %
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured       122
           Fraction of Friedel pairs measured Infinity
           Are heavy atom types Z>Si present        yes
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported _diffrn_ambient_temperature .        293 K

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   3 ALERT level C = Check and explain
   3 ALERT level G = General alerts; check

   5 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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Computing details

Data collection: CAD-4 Software (Enraf–Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA implemented in PLATON (Spek, 2003); program(s) used to solve structure: coordinates taken from a previous refinement; program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: SHELXL97.

Mercury(II) chromium(III) tetroxide

Crystal data

HgCr2O4	Dx = 7.580 Mg m^−3
Mr = 368.59	Mo Kα radiation, λ = 0.71073 Å
Cubic, Fd3m	Cell parameters from 25 reflections
Hall symbol: -F 4vw 2vw	θ = 10.3–15.6°
a = 8.6443 (3) Å	µ = 53.86 mm^−1
V = 645.94 (4) Å^3	T = 293 K
Z = 8	Octahedron, black
F(000) = 1280	0.06 × 0.05 × 0.05 mm

Data collection

Enraf–Nonius CAD-4 diffractometer	114 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.099
Graphite monochromator	θmax = 40.0°, θmin = 4.1°
ω/2θ scans	h = −15→15
Absorption correction: numerical (HABITUS; Herrendorf, 1997)	k = −15→15
Tmin = 0.093, Tmax = 0.216	l = −15→15
3703 measured reflections	3 standard reflections every 200 min
122 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: isomorphous structure methods
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + 1.36P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.011	(Δ/σ)max < 0.001
wR(F^2) = 0.023	Δρmax = 1.18 e Å^−3
S = 1.15	Δρmin = −1.66 e Å^−3
122 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
8 parameters	Extinction coefficient: 0.00043 (12)
0 restraints

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
Hg	0.3750	0.3750	0.3750	0.00884 (9)
Cr	0.0000	0.0000	0.0000	0.00544 (10)
O1	0.22968 (14)	0.22968 (14)	0.22968 (14)	0.0072 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg	0.00884 (9)	0.00884 (9)	0.00884 (9)	0.000	0.000	0.000
Cr	0.00544 (10)	0.00544 (10)	0.00544 (10)	−0.00027 (11)	−0.00027 (11)	−0.00027 (11)
O1	0.0072 (3)	0.0072 (3)	0.0072 (3)	−0.0011 (3)	−0.0011 (3)	−0.0011 (3)

Geometric parameters (Å, º)

Hg—O1	2.176 (2)	Cr—O1^vii	2.0009 (10)
Hg—O1^i	2.176 (2)	Cr—O1^viii	2.0009 (10)
Hg—O1^ii	2.176 (2)	Cr—O1^ix	2.0009 (10)
Hg—O1^iii	2.176 (2)	O1—Cr^ix	2.0009 (10)
Cr—O1^iv	2.0009 (10)	O1—Cr^vii	2.0009 (10)
Cr—O1^v	2.0009 (10)	O1—Cr^v	2.0009 (10)
Cr—O1^vi	2.0009 (10)
O1—Hg—O1^i	109.5	O1^vi—Cr—O1^viii	79.52 (8)
O1—Hg—O1^ii	109.5	O1^vii—Cr—O1^viii	180.000 (9)
O1^i—Hg—O1^ii	109.5	O1^iv—Cr—O1^ix	180.000 (9)
O1—Hg—O1^iii	109.5	O1^v—Cr—O1^ix	79.52 (8)
O1^i—Hg—O1^iii	109.5	O1^vi—Cr—O1^ix	100.48 (8)
O1^ii—Hg—O1^iii	109.5	O1^vii—Cr—O1^ix	79.52 (8)
O1^iv—Cr—O1^v	100.48 (8)	O1^viii—Cr—O1^ix	100.48 (8)
O1^iv—Cr—O1^vi	79.52 (8)	Cr^ix—O1—Cr^vii	99.58 (7)
O1^v—Cr—O1^vi	180.000 (9)	Cr^ix—O1—Cr^v	99.58 (7)
O1^iv—Cr—O1^vii	100.48 (8)	Cr^vii—O1—Cr^v	99.58 (7)
O1^v—Cr—O1^vii	79.52 (8)	Cr^ix—O1—Hg	118.13 (5)
O1^vi—Cr—O1^vii	100.48 (8)	Cr^vii—O1—Hg	118.13 (5)
O1^iv—Cr—O1^viii	79.52 (8)	Cr^v—O1—Hg	118.13 (5)
O1^v—Cr—O1^viii	100.48 (8)

Symmetry codes: (i) x, −y+3/4, −z+3/4; (ii) −x+3/4, −y+3/4, z; (iii) −x+3/4, y, −z+3/4; (iv) −x, y−1/4, z−1/4; (v) −x+1/4, −y+1/4, z; (vi) x−1/4, y−1/4, −z; (vii) −x+1/4, y, −z+1/4; (viii) x−1/4, −y, z−1/4; (ix) x, −y+1/4, −z+1/4.