At last! The single-crystal X-ray structure of a naturally occurring sample of the ilmenite-type oxide FeCrO₃

A natural single crystal of the ferrimagnetic oxide FeCrO₃, which was found in an opencast mine situated in the San Luis Potosí State in Mexico, has been characterized in order to elucidate some outstanding issues about the actual structure of this material. The single-crystal X-ray analysis unambiguously shows that transition metal cations are segregated in alternating layers normal to the threefold crystallographic axis, affording a structure isomorphous to that of ilmenite (FeTiO₃), in the space group . The possible occurrence of cation antisite and vacancy defects is below the limit of detection available from X-ray data. Structural and magnetic results are in agreement with the coherent slow intergrowth of magnetic phases provided by the two antiferromagnetic corundum-type parent oxides Fe₂O₃ (hematite) and Cr₂O₃ (eskolaite). Our results are consistent with the most recent density functional theory (DFT) studies carried out on digital FeCrO₃ [Sadat Nabi & Pentcheva (2011). Phys. Rev. B, 83, 214424], and suggest that synthetic samples of FeCrO₃ might present a cation distribution different to that of the ilmenite structural type.

Computing details

Data collection: XSCANS (Bruker, 1997); cell refinement: XSCANS; data reduction: XPREP (Bruker, 1997); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: SHELXTL (Bruker, 1997) and CaRIne Crystallography (Boudias & Monceau, 1998); software used to prepare material for publication: SHELXTL.

Figures

(I)

Crystal data

CrFeO3	Dx = 4.982 Mg m^−3
Mr = 155.85	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3:H	Cell parameters from 50 reflections
a = 5.0770 (4) Å	θ = 7.5–19.4°
c = 13.9621 (14) Å	µ = 11.87 mm^−1
V = 311.67 (6) Å^3	T = 298 K
Z = 6	Prism, metallic silver
F(000) = 444	0.10 × 0.06 × 0.06 mm

Data collection

Bruker P4 diffractometer	373 independent reflections
Radiation source: fine-focus sealed tube	324 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
2θ/ω scans	θmax = 37.5°, θmin = 4.4°
Absorption correction: ψ scan XSCANS (Bruker, 1997)	h = −8→8
Tmin = 0.303, Tmax = 0.489	k = −8→8
2211 measured reflections	l = −23→23

Refinement

Refinement on F^2	0 constraints
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0301P)^2 + 1.1564P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.020	(Δ/σ)max < 0.001
wR(F^2) = 0.059	Δρmax = 0.65 e Å^−3
S = 1.11	Δρmin = −1.18 e Å^−3
373 reflections	Extinction correction: SHELXL2014/7 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
17 parameters	Extinction coefficient: 0.0254 (19)
0 restraints

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

	x	y	z	Uiso*/Ueq
Cr1	0.0000	0.0000	0.35412 (2)	0.00537 (13)
Fe2	0.0000	0.0000	0.14523 (2)	0.00713 (13)
O1	0.2986 (2)	−0.0155 (2)	0.25302 (7)	0.00683 (19)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.00548 (15)	0.00548 (15)	0.00513 (18)	0.00274 (7)	0.000	0.000
Fe2	0.00757 (15)	0.00757 (15)	0.00626 (18)	0.00379 (8)	0.000	0.000
O1	0.0075 (4)	0.0076 (4)	0.0063 (4)	0.0044 (3)	0.0010 (3)	0.0010 (3)

Geometric parameters (Å, º)

Cr1—O1^i	1.9003 (10)	Fe2—O1^vii	2.0348 (10)
Cr1—O1^ii	1.9003 (10)	Fe2—O1^viii	2.0348 (10)
Cr1—O1^iii	1.9003 (10)	Fe2—O1^v	2.1653 (10)
Cr1—O1^iv	2.1013 (10)	Fe2—O1^iv	2.1653 (10)
Cr1—O1^v	2.1013 (10)	Fe2—O1	2.1653 (10)
Cr1—O1	2.1013 (10)	O1—Cr1^iii	1.9003 (10)
Cr1—Fe2	2.9166 (6)	O1—Fe2^vii	2.0348 (10)
Fe2—O1^vi	2.0348 (10)
O1^i—Cr1—O1^ii	102.31 (4)	O1^vi—Fe2—O1^v	162.41 (3)
O1^i—Cr1—O1^iii	102.30 (4)	O1^vii—Fe2—O1^v	89.39 (5)
O1^ii—Cr1—O1^iii	102.30 (4)	O1^viii—Fe2—O1^v	89.21 (4)
O1^i—Cr1—O1^iv	83.51 (4)	O1^vi—Fe2—O1^iv	89.21 (4)
O1^ii—Cr1—O1^iv	162.57 (4)	O1^vii—Fe2—O1^iv	162.41 (3)
O1^iii—Cr1—O1^iv	92.31 (6)	O1^viii—Fe2—O1^iv	89.39 (5)
O1^i—Cr1—O1^v	92.31 (6)	O1^v—Fe2—O1^iv	77.01 (4)
O1^ii—Cr1—O1^v	83.51 (4)	O1^vi—Fe2—O1	89.39 (5)
O1^iii—Cr1—O1^v	162.57 (4)	O1^vii—Fe2—O1	89.21 (4)
O1^iv—Cr1—O1^v	79.82 (4)	O1^viii—Fe2—O1	162.41 (3)
O1^i—Cr1—O1	162.57 (4)	O1^v—Fe2—O1	77.01 (4)
O1^ii—Cr1—O1	92.31 (6)	O1^iv—Fe2—O1	77.01 (4)
O1^iii—Cr1—O1	83.51 (4)	O1^vi—Fe2—Cr1	116.45 (3)
O1^iv—Cr1—O1	79.82 (4)	O1^vii—Fe2—Cr1	116.45 (3)
O1^v—Cr1—O1	79.82 (4)	O1^viii—Fe2—Cr1	116.45 (3)
O1^i—Cr1—Fe2	115.93 (3)	O1^v—Fe2—Cr1	45.96 (3)
O1^ii—Cr1—Fe2	115.93 (3)	O1^iv—Fe2—Cr1	45.96 (3)
O1^iii—Cr1—Fe2	115.93 (3)	O1—Fe2—Cr1	45.96 (3)
O1^iv—Cr1—Fe2	47.80 (3)	Cr1^iii—O1—Fe2^vii	119.94 (5)
O1^v—Cr1—Fe2	47.80 (3)	Cr1^iii—O1—Cr1	96.49 (4)
O1—Cr1—Fe2	47.80 (3)	Fe2^vii—O1—Cr1	129.23 (5)
O1^vi—Fe2—O1^vii	101.68 (3)	Cr1^iii—O1—Fe2	134.33 (5)
O1^vi—Fe2—O1^viii	101.68 (3)	Fe2^vii—O1—Fe2	90.79 (4)
O1^vii—Fe2—O1^viii	101.68 (3)	Cr1—O1—Fe2	86.24 (4)

Symmetry codes: (i) x−y−2/3, x−1/3, −z+2/3; (ii) y+1/3, −x+y+2/3, −z+2/3; (iii) −x+1/3, −y−1/3, −z+2/3; (iv) −x+y, −x, z; (v) −y, x−y, z; (vi) x−y−1/3, x−2/3, −z+1/3; (vii) −x+2/3, −y+1/3, −z+1/3; (viii) y−1/3, −x+y+1/3, −z+1/3.