Cerium iron sulfide, Ce₃Fe_1.94S₇

Tricerium(III) diiron(II,III) hepta­sulfide, Ce₃Fe_1.94S₇, crystallizes in the polar hexagonal space group P6₃ and adopts the Ce₆Al_3.33S₁₄ structure type. The Fe atoms occupy both tetrahedral and octahedral sites. Isolated FeS₄ tetrahedra, all pointing in the same direction, are stacked along the threefold rotation axes. Chains of face-sharing FeS₆ octahedra propagate along the 6₃ axis. Vacancies resulting from the partial oxidation of Fe²⁺ to Fe³⁺ occur exclusively in the octahedral Fe sites. The Ce atoms are coordinated by [7+1] S atoms, which form bicapped trigonal prisms.

S1. Comment

In an early investigation of the pseudo-binary systems R₂S₃—TS (R is La—Nd and T is Mn—Ni), a series of compounds originally described with the formula R₄TS₇ were discovered (Collin et al., 1968). The cell parameters of the compounds, determined from X-ray powder diffraction patterns indexed in the hexagonal space group P6₃, were reported (for Ce₄FeS₇, a = 10.202 and c = 5.657 Å). Since then, more detailed structural information has been lacking for these compounds. Here, we report the single-crystal structure determination of one member of the series, which has the actual formula Ce₃Fe_1.94S₇.

The title compound adopts the Ce₆Al_3.33S₁₄ structure type (de Saint-Giniez et al., 1968), which is also sometimes referred to as the La₃CuSiS₇ structure type (Flahaut & Laruelle, 1970). Thus, Ce₃Fe_1.94S₇ belongs to a large family of compounds of general formula R₃MM'Q₇, where R is a rare-earth metal, M and M' are metals or metalloids, and Q is a chalcogen (Villars, 1997; Flahaut & Laruelle, 1970). Some other recently reported R₃MM'Q₇ compounds include Y₃NaSiS₇ (Hartenbach & Schleid, 2003), La₃CuGeQ₇ (Q is S or Se; Poduska et al., 2002) and La₃Al_0.44Si_0.93S₇ (Yang & Ibers, 2000).

Views of the Ce₃Fe_1.94S₇ structure, highlighting the Fe-centred coordination polyhedra, are presented in Figs. 1 and 2. The Fe atoms occupy two types of sites, with tetrahedral and octahedral geometries, respectively. Isolated Fe1S₄ tetrahedra, all pointing in the polar [001] direction, are stacked along the threefold rotation axes (Fig. 2). The tetrahedra are trigonally compressed, with one shorter Fe1—S3 distance of 2.224 (6) Å and three longer Fe1—S2 distances of 2.265 (3) Å (Table 1). These distances are somewhat longer than those found in the FeS₄ tetrahedra of La₃MnFeS₇ (2.11–2.22 Å; Nanjundaswamy & Gopalakrishnan, 1983), but shorter than those found in the tetrahedra of La₂Fe₂S₅ (2.30–2.37 Å; Besrest & Collin, 1977). The S—Fe1—S angles of 106.67 (12) and 112.14 (11)° are close to the ideal tetrahedral value of 109.5°.

The Fe2S₆ octahedra share opposite faces to form linear FeS_6/2 chains that propagate along the 6₃ axis (Fig. 2). The Fe atoms within the slightly elongated Fe2S1₆ octahedra are shifted closer to one shared face than the other, yielding Fe2—S1 distances of 2.532 (4) and 2.578 (4) Å (Table 1). These distances are similar to those observed in the more distorted FeS₆ octahedra of Ce₂Fe_1.82S₅ (2.466–2.702 Å; Harms et al., 2004). The cis S—Fe2—S angles range from 88.40 to 90.58° and the trans angles are 178.6 (2)°.

As in the Ce₆Al_3.33S₁₄ structure (de Saint-Giniez et al., 1968), vacancies occur in the octahedral site of Ce₃Fe_1.94S₇ [Fe2, with a refined occupancy of 0.942 (16)]. The defects in Ce₃Fe_1.94S₇ are presumed to result from the partial oxidation of Fe²⁺ to Fe³⁺, according to the charge-balanced formula (Ce³⁺)₃(Fe³⁺)_1.12(Fe²⁺)_0.82□_0.06(S²⁻)₇. In support of this hypothesis, a recent Mößbauer investigation of the nonstoichiometric cerium iron sulfide Ce₂Fe_1.82S₅ established that the oxidation of Fe²⁺ to Fe³⁺ accompanies the formation of vacancies in this compound (Harms et al., 2004).

The Ce atoms, located between the Fe-centred polyhedra, are coordinated by [7 + 1] S atoms: one S3, two S2, and three S1 atoms, at distances of 2.843–3.037 Å, form a trigonal prism, capped by one S2 atom at a distance of 3.000 (3) Å and one S1 atom at a significantly longer distance of 3.414 (4) Å (Fig. 2, Table 1). The shorter distances are within the range observed in the CeS₈ bicapped trigonal prisms of Ce₂Fe_1.82S₅ (2.887–3.142 Å; Harms et al., 2004). A similar but more pronounced [7 + 1] coordination is observed in Ce₆Al_3.33S₁₄, where the corresponding short and long Ce—S distances are 2.83–3.03 and 3.58 (1) Å, respectively (de Saint-Giniez et al., 1968).

S2. Experimental

Hexagonal prisms of Ce₃Fe_1.94S₇ were isolated from a reaction of the elements in an alkali chloride flux. Starting reagents were cerium (rod, 99.85%, Treibacher; freshly filed prior to use), iron (powder, 99.99%, ABCR), and sulfur [powder, >99%, VEB Laborchemie; recrystallized from CS₂, then purified of C according to the method of von Wartenberg (1956)]. A 1:1 mixture of LiCl (p.a., Merck) and KCl (p.a., J. T. Baker) was used as a flux after being heated under dynamic vacuum to remove any moisture. The elements, in a ratio of 3Ce:2Fe:7S (0.25 g in total), were added to the LiCl/KCl flux (0.5 g) in a fused silica ampoule (6 cm in length, 0.8 cm in diameter), which was then sealed under vacuum (10⁻³ Torr; 1 Torr = 133.322 Pa). The reaction mixture was heated at 1170 K for 4 d and then cooled to room temperature at a rate of 10 K h⁻¹. The flux was removed by washing the sample several times with water and ethanol. The major component of the product was Ce₃Fe_1.94S₇. Energy-dispersive X-ray (EDX) analysis on a CamScan CS44 scanning electron microscope of the crystal chosen for the diffraction experiment confirmed the presence of Ce, Fe, and S; analysis (mol. %): Ce 24 (1), Fe 14 (1), S 62 (1).

S3. Refinement

Since the displacement ellipsoid of the Fe2 octahedral site was initially unusually large, the occupancy of the site was refined freely, resulting in a value of 0.942 (16). Refinement of the occupancy of the Fe1 tetrahedral site confirmed that it is fully occupied. Atomic positions were standardized using the program STRUCTURE TIDY (Gelato & Parthé, 1987).

Computing details

Data collection: IPDS (Stoe & Cie, 2000); cell refinement: IPDS; data reduction: IPDS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the structure of Ce3Fe1.94S7 along [001]. Displacement ellipsoids are drawn at the 95% probability level.
	Fig. 2. A view along [100] of part of the structure of Ce3Fe1.94S7, showing the stacked FeS4 tetrahedra and the FeS6/2 chains of face-sharing octahedra, which are connected by CeS7 + 1 bicapped trigonal prisms. Displacement ellipsoids are drawn at the 95% probability level. [Symmetry codes: (i) 1 − x, 1 − y, z − 1/2; (vii) x-y, x, z − 1/2.]

tricerium(III) diiron(II,III) heptasulfide

Crystal data

Ce3Fe1.94S7	Dx = 4.754 Mg m^−3
Mr = 753.13	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63	Cell parameters from 7967 reflections
Hall symbol: P 6c	θ = 2.3–28.1°
a = 10.096 (2) Å	µ = 16.65 mm^−1
c = 5.961 (1) Å	T = 293 K
V = 526.2 (2) Å^3	Prism, black
Z = 2	0.16 × 0.15 × 0.12 mm
F(000) = 673

Data collection

Stoe IPDS-I diffractometer	865 independent reflections
Radiation source: fine-focus sealed tube	856 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.107
ϕ scans	θmax = 28.2°, θmin = 2.3°
Absorption correction: numerical [X-RED (Stoe & Cie, 2001) and X-SHAPE (Stoe & Cie, 1999)]	h = −13→13
Tmin = 0.108, Tmax = 0.246	k = −12→12
6903 measured reflections	l = −7→7

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0988P)^2 + 5.9571P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.051	(Δ/σ)max < 0.001
wR(F^2) = 0.135	Δρmax = 3.38 e Å^−3
S = 1.08	Δρmin = −1.54 e Å^−3
865 reflections	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
39 parameters	Extinction coefficient: 0.035 (3)
1 restraint	Absolute structure: Flack (1983), 390 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.02 (6)

Special details

Experimental. [X-RED (Stoe & Cie, 2001); crystal description optimized based on equivalent reflections using X-SHAPE (Stoe & Cie, 1999)]

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	Occ. (<1)
Ce	0.37467 (6)	0.14279 (7)	0.22240 (18)	0.0147 (3)
Fe1	0.3333	0.6667	0.1395 (5)	0.0145 (6)
Fe2	0.0000	0.0000	0.0000 (7)	0.0166 (11)	0.942 (16)
S1	0.1451 (3)	0.2352 (3)	0.2566 (6)	0.0163 (6)
S2	0.5782 (3)	0.0959 (3)	0.4962 (5)	0.0147 (6)
S3	0.3333	0.6667	0.5125 (9)	0.0155 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ce	0.0122 (4)	0.0150 (4)	0.0167 (5)	0.0066 (2)	−0.0014 (3)	−0.0017 (3)
Fe1	0.0159 (8)	0.0159 (8)	0.0118 (13)	0.0079 (4)	0.000	0.000
Fe2	0.0144 (11)	0.0144 (11)	0.0210 (19)	0.0072 (6)	0.000	0.000
S1	0.0154 (11)	0.0149 (11)	0.0195 (15)	0.0084 (10)	−0.0011 (10)	0.0006 (9)
S2	0.0148 (12)	0.0149 (13)	0.0152 (13)	0.0080 (9)	0.0005 (10)	0.0014 (10)
S3	0.0150 (14)	0.0150 (14)	0.016 (2)	0.0075 (7)	0.000	0.000

Geometric parameters (Å, º)

Ce—S2	2.843 (3)	Fe2—S1^viii	2.532 (4)
Ce—S3^i	2.878 (3)	Fe2—S1	2.578 (4)
Ce—S1^ii	2.897 (3)	Fe2—S1^ii	2.578 (4)
Ce—S1	2.906 (3)	Fe2—S1^x	2.578 (4)
Ce—S2^iii	2.935 (3)	Fe2—Ce^x	3.5623 (18)
Ce—S2^iv	3.000 (3)	Fe2—Ce^ii	3.5623 (18)
Ce—S1^v	3.037 (3)	S1—Fe2^xi	2.532 (4)
Ce—S1^vi	3.414 (4)	S1—Ce^x	2.897 (3)
Ce—Fe2	3.5623 (18)	S1—Ce^xii	3.037 (3)
Fe1—S3	2.224 (6)	S2—Fe1^xiii	2.265 (3)
Fe1—S2^i	2.265 (3)	S2—Ce^xiv	2.935 (3)
Fe1—S2^vii	2.265 (3)	S2—Ce^xv	3.000 (3)
Fe1—S2^viii	2.265 (3)	S3—Ce^xvi	2.878 (3)
Fe2—S1^v	2.532 (4)	S3—Ce^xiii	2.878 (3)
Fe2—S1^ix	2.532 (4)	S3—Ce^xii	2.878 (3)
S2—Ce—S3^i	77.10 (10)	S1^ix—Fe2—S1	178.6 (2)
S2—Ce—S1^ii	103.26 (8)	S1^viii—Fe2—S1^ii	178.6 (2)
S3^i—Ce—S1^ii	150.64 (10)	S1^v—Fe2—S1^x	178.6 (2)
S2—Ce—S1	140.53 (10)	S1^ix—Fe2—Ce	126.30 (7)
S3^i—Ce—S1	122.05 (7)	S1^viii—Fe2—Ce	126.52 (7)
S1^ii—Ce—S1	76.54 (11)	S1^v—Fe2—Ce	56.80 (8)
S2—Ce—S2^iii	77.92 (12)	S1—Fe2—Ce	53.64 (7)
S3^i—Ce—S2^iii	75.66 (10)	S1^ii—Fe2—Ce	53.43 (7)
S1^ii—Ce—S2^iii	133.54 (9)	S1^x—Fe2—Ce	121.77 (16)
S1—Ce—S2^iii	75.02 (8)	S1^ix—Fe2—Ce^x	126.52 (7)
S2—Ce—S2^iv	72.91 (5)	S1^viii—Fe2—Ce^x	56.80 (8)
S3^i—Ce—S2^iv	79.68 (7)	S1^v—Fe2—Ce^x	126.30 (7)
S1^ii—Ce—S2^iv	72.64 (8)	S1—Fe2—Ce^x	53.43 (7)
S1—Ce—S2^iv	139.62 (8)	S1^ii—Fe2—Ce^x	121.77 (16)
S2^iii—Ce—S2^iv	145.22 (8)	S1^x—Fe2—Ce^x	53.64 (7)
S2—Ce—S1^v	143.71 (9)	Ce—Fe2—Ce^x	107.00 (7)
S3^i—Ce—S1^v	87.13 (12)	S1^ix—Fe2—Ce^ii	56.80 (8)
S1^ii—Ce—S1^v	75.42 (8)	S1^viii—Fe2—Ce^ii	126.30 (7)
S1—Ce—S1^v	75.29 (7)	S1^v—Fe2—Ce^ii	126.52 (7)
S2^iii—Ce—S1^v	129.71 (8)	S1—Fe2—Ce^ii	121.77 (16)
S2^iv—Ce—S1^v	72.20 (8)	S1^ii—Fe2—Ce^ii	53.64 (7)
S2—Ce—Fe2	148.85 (7)	S1^x—Fe2—Ce^ii	53.43 (7)
S3^i—Ce—Fe2	129.36 (12)	Ce—Fe2—Ce^ii	107.00 (7)
S1^ii—Ce—Fe2	45.61 (7)	Ce^x—Fe2—Ce^ii	107.00 (7)
S1—Ce—Fe2	45.59 (7)	Fe2^xi—S1—Fe2	71.35 (7)
S2^iii—Ce—Fe2	120.57 (7)	Fe2^xi—S1—Ce^x	85.57 (8)
S2^iv—Ce—Fe2	94.04 (6)	Fe2—S1—Ce^x	80.95 (7)
S1^v—Ce—Fe2	44.24 (9)	Fe2^xi—S1—Ce	85.38 (8)
S2^i—Fe1—S2^viii	106.67 (12)	Fe2—S1—Ce	80.78 (8)
S2^vii—Fe1—S2^i	106.67 (12)	Ce^x—S1—Ce	161.41 (11)
S2^vii—Fe1—S2^viii	106.67 (12)	Fe2^xi—S1—Ce^xii	78.96 (12)
S3—Fe1—S2^i	112.14 (11)	Fe2—S1—Ce^xii	150.31 (15)
S3—Fe1—S2^vii	112.14 (11)	Ce^x—S1—Ce^xii	97.18 (9)
S3—Fe1—S2^viii	112.14 (11)	Ce—S1—Ce^xii	96.98 (9)
S1—Fe2—S1^ii	88.40 (15)	Fe1^xiii—S2—Ce	88.60 (11)
S1—Fe2—S1^x	88.40 (15)	Fe1^xiii—S2—Ce^xiv	86.35 (10)
S1^ii—Fe2—S1^x	88.40 (15)	Ce—S2—Ce^xiv	101.97 (10)
S1^ix—Fe2—S1^v	90.43 (15)	Fe1^xiii—S2—Ce^xv	122.94 (13)
S1^viii—Fe2—S1^v	90.43 (15)	Ce—S2—Ce^xv	144.08 (11)
S1^ix—Fe2—S1^viii	90.43 (15)	Ce^xiv—S2—Ce^xv	97.19 (9)
S1^v—Fe2—S1	90.58 (5)	Fe1—S3—Ce^xvi	115.76 (10)
S1^viii—Fe2—S1	90.58 (5)	Fe1—S3—Ce^xiii	115.76 (10)
S1^ix—Fe2—S1^ii	90.58 (5)	Ce^xvi—S3—Ce^xiii	102.51 (12)
S1^v—Fe2—S1^ii	90.58 (5)	Fe1—S3—Ce^xii	115.76 (10)
S1^ix—Fe2—S1^x	90.58 (5)	Ce^xvi—S3—Ce^xii	102.51 (12)
S1^viii—Fe2—S1^x	90.58 (5)	Ce^xiii—S3—Ce^xii	102.51 (12)

Symmetry codes: (i) −x+1, −y+1, z−1/2; (ii) −x+y, −x, z; (iii) −x+y+1, −x+1, z; (iv) −x+1, −y, z−1/2; (v) y, −x+y, z−1/2; (vi) y, −x+y, z+1/2; (vii) y, −x+y+1, z−1/2; (viii) x−y, x, z−1/2; (ix) −x, −y, z−1/2; (x) −y, x−y, z; (xi) −x, −y, z+1/2; (xii) x−y, x, z+1/2; (xiii) −x+1, −y+1, z+1/2; (xiv) −y+1, x−y, z; (xv) −x+1, −y, z+1/2; (xvi) y, −x+y+1, z+1/2.