Europium thio­silicate at 100 K

The crystal structure of Eu₂SiS₄ was determined from single-crystal data. The title compound crystallizes as a monoclinic Sr₂GeS₄-type in the space group P2₁/m, both at room temperature and at 100 K. The main structural features are almost undistorted SiS₄^4- tetrahedra and Eu²⁺ in 6(+2) coordination by sulfur. The thio­silicate is isotypic with the paraelectric phase of Eu₂GeS₄, stable above T_C = 335 K. Unlike the thio­germanate, down to 100 K Eu₂SiS₄ shows no structural transition into a ferroelectric phase with space group P2₁.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](S-Si) = 0.003 Å
• R factor = 0.045
• wR factor = 0.072
• Data-to-parameter ratio = 25.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



RINTA_01  Alert C The value of Rint is greater than 0.10
          Rint given   0.106

General Notes



ABSTM_02  The ratio of expected to reported Tmax/Tmin(RR') is < 0.90
          Tmin and Tmax reported:      0.075     0.143
          Tmin' and Tmax expected:      0.154     0.249
          RR' =      0.852
          Please check that your absorption correction is appropriate.


 0 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 1 Alert Level C = Please check

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S1. Comment

Recently, we reported on the structural phase transition of monoclinic europium thiogermanate (Tampier & Johrendt, 2001). Eu₂GeS₄ crystallizes in the space group P2₁ at room temperature (α-Eu₂GeS₄), but above the Curie point T_C = 335 K, the structure is centrosymmetric with the space group P2₁/m (β-Eu₂GeS₄). Main contributions of the transition are an antidistorsive tilt of the tetrahedral GeS₄^4- groups and small shifts of the Eu²⁺ ions, whose coordination numbers change from 7 to 6(+2). The mechanism is similar to what has been found for isostructural ferroelectric K₂ZnBr₄ (Bärnighausen, 1992). From the structural viewpoint, we concluded that Eu₂GeS₄ should be ferroelectric below T_C = 335 K and paraelectric above this temperature.

Europium thiosilicate Eu₂SiS₄ was already mentioned in the literature (Olivier-Fourcade et al., 1978), but up to now no detailed structural data were published. From our results about Eu₂GeS₄ the question arises, whether or not the isostructural thiosilicate runs through a ferroelectric phase transition too. In order to clarify this, we have determined the room-temperature structure of Eu₂SiS₄ at first. This confirmed the monoclinic structure type in the space group P2₁/m. This structure did not differ significantly from the low temperature structure except that the displacement parameters U²² of the S1 and S2 on the mirror plane (2 e) were 0.030%A² in contrast to S3 on the general position (4f) with U²² = 0.010%A². This indicated only a small degree of disorder, if at all, of the S atoms on the special position. In going to lower temperatures, a structural transition would manifest itself by an ordering of the S1 and S2 positions and small shifts of the Eu atoms away from y = 1/4, tantamount with a change of the space group to noncentrosymmetric P2₁.

The structure determination of Eu₂SiS₄ at 100 K resulted in different behaviour. Important interatomic distances are compiled in Table 1. The displacemant parameters U²² of the sulfur atoms in question decrease to 0.016 Ų. This means that the deviation of the S1 and S2 atoms from their ideal position y = 1/4 decreases with temperature and would probably vanish as T → 0. A ttempts to refine the structure with space group P2₁ were not successful. Thus, Eu₂SiS₄ is isotypic with β-Eu₂GeS₄, but shows no transition to the noncentrosymmetric β-phase, it rather changes continuously towards the ideal Sr₂GeS₄-type.

Fig. 1 shows the unit cell of Eu₂SiS₄ emphasizing the SiS₄-tetrahedra and the coordination of europium. Six S atoms form a trigonal prism and two extra S atoms are located over the square faces (dashed bonds). Fig. 2 shows the connection of the EuS₆ polyhedra and the additional S atoms between them. The S1 and S2 atoms are slightly disordered in the space group P2₁/m and move towards the mirror plane as the temperature decreases. If the P2₁/m → P2₁ would occur, the dashed Eu—S bonds would have unequal lengths due to the ordering of S1 and S2 and the opposing shifts of europium. This would lead to a ferroelectric structure as is the case for europium thiogermanate, but not for the thiosilicate.

S2. Experimental

Eu₂SiS₄ was prepared from the elements by two steps. First, europium metal and silicon powder was heated to 1173 K for 15 h in a quartz tube under an argon atmosphere. The resulting alloy was grounded in an argon-filled glove-box and then oxidized by the stochiometric amount of sulfur at 1123 K for 24 h, again homogenized and reheated to 1073 K for 50 h. This results in a dark-yellow powder containing small single crystals suitable for the X-ray experiments.

S3. Refinement

The data collection was performed on a Stoe IPDS diffractometer, equipped with an Oxford Cryostream cooler. The measuring temperature was 100±2 K. 118 exposures were taken in the ϕ-range of 0–284° with a crystal to detector distance of 50 mm and an exposure time of 3 min. Dynamic profiles (9–25 pixels) without allowing overlapping were used for integration. 96.7% completeness of data has been achieved in the θ-range 2.6–30.35°. Absorption effects were corrected numerically by using the experimental crystal description. The atomic coordinates of Eu₂GeS₂ were initially used and refined by least-squares cycles using SHELXL97 (Sheldrick 1997).

Computing details

Data collection: IPDS Software (Stoe & Cie, 1998); cell refinement: IPDS Software; data reduction: IPDS Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1998); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Unit cell of the crystal structure of Eu2SiS4.
	Fig. 2. Crystal structure of Eu2SiS4. The 6(+2)-coordination of the Eu atoms and the SiS4-tetrahedra are emphasized.

Europiumtetrathiosilicate(IV)

Crystal data

Eu2SiS4	F(000) = 408
Mr = 460.25	Dx = 4.563 Mg m^−3
Monoclinic, P21/m	Mo Kα radiation, λ = 0.71073 Å
a = 6.524 (1) Å	Cell parameters from 3437 reflections
b = 6.591 (1) Å	θ = 3–30°
c = 8.205 (2) Å	µ = 19.84 mm^−1
β = 108.29 (3)°	T = 100 K
V = 334.99 (11) Å^3	Irregular, dark yellow
Z = 2	0.09 × 0.08 × 0.07 mm

Data collection

Stoe IPDS diffractometer	1062 independent reflections
Radiation source: fine-focus sealed tube	890 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.106
Detector resolution: 50 pixels mm^-1	θmax = 30.4°, θmin = 2.6°
ω scans	h = −9→9
Absorption correction: gaussian (X-RED; Stoe & Cie, 1998)	k = −9→9
Tmin = 0.075, Tmax = 0.143	l = −11→11
5587 measured reflections

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)] = 0.045	w = 1/[σ^2(Fo^2) + (0.035P)^2 + 1.9742P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.072	(Δ/σ)max < 0.001
S = 1.13	Δρmax = 1.82 e Å^−3
1062 reflections	Δρmin = −1.55 e Å^−3
41 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0051 (7)

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.

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

	x	y	z	Uiso*/Ueq
Eu1	0.71526 (6)	0.2500	0.04825 (5)	0.00950 (15)
Eu2	0.77759 (7)	0.2500	0.57195 (5)	0.00969 (15)
Si1	0.2807 (4)	0.2500	0.2040 (3)	0.0084 (4)
S1	0.1140 (3)	0.2500	−0.0638 (3)	0.0122 (4)
S2	0.0833 (3)	0.2500	0.3663 (3)	0.0111 (4)
S3	0.4948 (2)	−0.0010 (2)	0.26449 (18)	0.0099 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Eu1	0.0081 (2)	0.0127 (2)	0.0084 (2)	0.000	0.00357 (15)	0.000
Eu2	0.0087 (2)	0.0124 (2)	0.0089 (2)	0.000	0.00405 (15)	0.000
Si1	0.0076 (9)	0.0084 (9)	0.0101 (10)	0.000	0.0043 (8)	0.000
S1	0.0102 (9)	0.0168 (9)	0.0106 (9)	0.000	0.0048 (7)	0.000
S2	0.0093 (9)	0.0166 (9)	0.0083 (9)	0.000	0.0041 (7)	0.000
S3	0.0107 (6)	0.0085 (6)	0.0115 (6)	0.0012 (5)	0.0046 (5)	0.0013 (5)

Geometric parameters (Å, º)

Eu1—S2^i	2.940 (2)	Eu2—S2^x	3.4134 (7)
Eu1—S3^ii	2.9914 (16)	Eu2—S2^vii	3.4134 (7)
Eu1—S3^iii	2.9914 (16)	Eu2—Eu1^xi	4.0541 (11)
Eu1—S1^i	3.018 (2)	Si1—S3^iv	2.121 (2)
Eu1—S3	3.0912 (16)	Si1—S3	2.121 (2)
Eu1—S3^iv	3.0912 (16)	Si1—S2	2.123 (3)
Eu1—Si1	3.456 (2)	Si1—S1	2.124 (3)
Eu1—S1^v	3.4683 (8)	Si1—Eu1^xii	3.504 (3)
Eu1—S1^ii	3.4683 (8)	S1—Eu1^xii	3.018 (2)
Eu1—Si1^i	3.504 (3)	S1—Eu2^xiii	3.104 (2)
Eu1—Eu2^vi	4.0541 (11)	S1—Eu1^v	3.4683 (8)
Eu1—Eu2	4.1872 (12)	S1—Eu1^ii	3.4683 (8)
Eu2—S2^i	2.990 (2)	S2—Eu1^xii	2.940 (2)
Eu2—S3^vii	3.0247 (15)	S2—Eu2^xii	2.990 (2)
Eu2—S3^viii	3.0247 (15)	S2—Eu2^x	3.4134 (7)
Eu2—S3	3.0958 (17)	S2—Eu2^vii	3.4134 (7)
Eu2—S3^iv	3.0958 (17)	S3—Eu1^ii	2.9914 (16)
Eu2—S1^ix	3.104 (2)	S3—Eu2^vii	3.0247 (15)
S2^i—Eu1—S3^ii	142.01 (4)	S3^viii—Eu2—S1^ix	84.78 (5)
S2^i—Eu1—S3^iii	142.01 (4)	S3—Eu2—S1^ix	147.70 (3)
S3^ii—Eu1—S3^iii	66.54 (6)	S3^iv—Eu2—S1^ix	147.70 (3)
S2^i—Eu1—S1^i	74.24 (6)	S2^i—Eu2—S2^x	83.71 (4)
S3^ii—Eu1—S1^i	86.87 (5)	S3^vii—Eu2—S2^x	128.59 (5)
S3^iii—Eu1—S1^i	86.87 (5)	S3^viii—Eu2—S2^x	64.92 (4)
S2^i—Eu1—S3	83.57 (5)	S3—Eu2—S2^x	134.39 (4)
S3^ii—Eu1—S3	91.62 (4)	S3^iv—Eu2—S2^x	70.59 (5)
S3^iii—Eu1—S3	127.99 (3)	S1^ix—Eu2—S2^x	77.43 (4)
S1^i—Eu1—S3	140.84 (4)	S2^i—Eu2—S2^vii	83.71 (4)
S2^i—Eu1—S3^iv	83.57 (5)	S3^vii—Eu2—S2^vii	64.92 (4)
S3^ii—Eu1—S3^iv	127.99 (3)	S3^viii—Eu2—S2^vii	128.59 (5)
S3^iii—Eu1—S3^iv	91.62 (4)	S3—Eu2—S2^vii	70.59 (5)
S1^i—Eu1—S3^iv	140.84 (4)	S3^iv—Eu2—S2^vii	134.39 (4)
S3—Eu1—S3^iv	64.71 (5)	S1^ix—Eu2—S2^vii	77.43 (4)
S2^i—Eu1—Si1	102.01 (6)	S2^x—Eu2—S2^vii	149.79 (7)
S3^ii—Eu1—Si1	96.26 (5)	S2^i—Eu2—Eu1^xi	146.16 (5)
S3^iii—Eu1—Si1	96.26 (5)	S3^vii—Eu2—Eu1^xi	47.29 (3)
S1^i—Eu1—Si1	176.25 (6)	S3^viii—Eu2—Eu1^xi	47.29 (3)
S3—Eu1—Si1	37.28 (3)	S3—Eu2—Eu1^xi	124.83 (3)
S3^iv—Eu1—Si1	37.28 (3)	S3^iv—Eu2—Eu1^xi	124.83 (3)
S2^i—Eu1—S1^v	78.69 (4)	S1^ix—Eu2—Eu1^xi	47.63 (4)
S3^ii—Eu1—S1^v	127.46 (5)	S2^x—Eu2—Eu1^xi	87.67 (4)
S3^iii—Eu1—S1^v	64.25 (5)	S2^vii—Eu2—Eu1^xi	87.67 (4)
S1^i—Eu1—S1^v	73.35 (4)	S2^i—Eu2—Eu1	44.60 (4)
S3—Eu1—S1^v	133.59 (5)	S3^vii—Eu2—Eu1	125.03 (3)
S3^iv—Eu1—S1^v	70.88 (4)	S3^viii—Eu2—Eu1	125.03 (3)
Si1—Eu1—S1^v	106.13 (4)	S3—Eu2—Eu1	47.36 (3)
S2^i—Eu1—S1^ii	78.69 (4)	S3^iv—Eu2—Eu1	47.36 (3)
S3^ii—Eu1—S1^ii	64.25 (5)	S1^ix—Eu2—Eu1	143.12 (4)
S3^iii—Eu1—S1^ii	127.46 (5)	S2^x—Eu2—Eu1	95.05 (4)
S1^i—Eu1—S1^ii	73.35 (4)	S2^vii—Eu2—Eu1	95.05 (4)
S3—Eu1—S1^ii	70.88 (4)	Eu1^xi—Eu2—Eu1	169.243 (17)
S3^iv—Eu1—S1^ii	133.59 (5)	S3^iv—Si1—S3	102.51 (13)
Si1—Eu1—S1^ii	106.13 (4)	S3^iv—Si1—S2	109.74 (9)
S1^v—Eu1—S1^ii	143.68 (7)	S3—Si1—S2	109.74 (9)
S2^i—Eu1—Si1^i	37.18 (6)	S3^iv—Si1—S1	109.14 (9)
S3^ii—Eu1—Si1^i	117.34 (5)	S3—Si1—S1	109.14 (9)
S3^iii—Eu1—Si1^i	117.34 (5)	S2—Si1—S1	115.75 (14)
S1^i—Eu1—Si1^i	37.06 (6)	S3^iv—Si1—Eu1	61.97 (7)
S3—Eu1—Si1^i	114.63 (5)	S3—Si1—Eu1	61.97 (7)
S3^iv—Eu1—Si1^i	114.63 (5)	S2—Si1—Eu1	163.99 (12)
Si1—Eu1—Si1^i	139.19 (8)	S1—Si1—Eu1	80.25 (9)
S1^v—Eu1—Si1^i	72.38 (3)	S3^iv—Si1—Eu1^xii	128.74 (6)
S1^ii—Eu1—Si1^i	72.38 (3)	S3—Si1—Eu1^xii	128.74 (6)
S2^i—Eu1—Eu2^vi	123.68 (5)	S2—Si1—Eu1^xii	56.82 (8)
S3^ii—Eu1—Eu2^vi	47.98 (3)	S1—Si1—Eu1^xii	58.94 (9)
S3^iii—Eu1—Eu2^vi	47.98 (3)	Eu1—Si1—Eu1^xii	139.19 (8)
S1^i—Eu1—Eu2^vi	49.44 (5)	Si1—S1—Eu1^xii	84.00 (10)
S3—Eu1—Eu2^vi	139.36 (3)	Si1—S1—Eu2^xiii	166.93 (12)
S3^iv—Eu1—Eu2^vi	139.36 (3)	Eu1^xii—S1—Eu2^xiii	82.92 (6)
Si1—Eu1—Eu2^vi	134.31 (5)	Si1—S1—Eu1^v	84.71 (5)
S1^v—Eu1—Eu2^vi	84.67 (4)	Eu1^xii—S1—Eu1^v	106.65 (4)
S1^ii—Eu1—Eu2^vi	84.67 (4)	Eu2^xiii—S1—Eu1^v	99.05 (4)
Si1^i—Eu1—Eu2^vi	86.50 (5)	Si1—S1—Eu1^ii	84.71 (5)
S2^i—Eu1—Eu2	45.56 (5)	Eu1^xii—S1—Eu1^ii	106.65 (4)
S3^ii—Eu1—Eu2	138.69 (3)	Eu2^xiii—S1—Eu1^ii	99.05 (4)
S3^iii—Eu1—Eu2	138.69 (3)	Eu1^v—S1—Eu1^ii	143.68 (7)
S1^i—Eu1—Eu2	119.80 (5)	Si1—S2—Eu1^xii	86.00 (9)
S3—Eu1—Eu2	47.45 (3)	Si1—S2—Eu2^xii	175.84 (11)
S3^iv—Eu1—Eu2	47.45 (3)	Eu1^xii—S2—Eu2^xii	89.84 (6)
Si1—Eu1—Eu2	56.45 (5)	Si1—S2—Eu2^x	84.71 (4)
S1^v—Eu1—Eu2	92.05 (4)	Eu1^xii—S2—Eu2^x	103.69 (4)
S1^ii—Eu1—Eu2	92.05 (4)	Eu2^xii—S2—Eu2^x	96.29 (4)
Si1^i—Eu1—Eu2	82.74 (5)	Si1—S2—Eu2^vii	84.71 (4)
Eu2^vi—Eu1—Eu2	169.243 (17)	Eu1^xii—S2—Eu2^vii	103.69 (4)
S2^i—Eu2—S3^vii	147.07 (3)	Eu2^xii—S2—Eu2^vii	96.29 (4)
S2^i—Eu2—S3^viii	147.07 (3)	Eu2^x—S2—Eu2^vii	149.79 (7)
S3^vii—Eu2—S3^viii	65.72 (6)	Si1—S3—Eu1^ii	97.84 (8)
S2^i—Eu2—S3	82.69 (5)	Si1—S3—Eu2^vii	95.25 (8)
S3^vii—Eu2—S3	77.80 (4)	Eu1^ii—S3—Eu2^vii	84.73 (4)
S3^viii—Eu2—S3	111.63 (3)	Si1—S3—Eu1	80.75 (7)
S2^i—Eu2—S3^iv	82.69 (5)	Eu1^ii—S3—Eu1	88.38 (4)
S3^vii—Eu2—S3^iv	111.63 (3)	Eu2^vii—S3—Eu1	171.50 (5)
S3^viii—Eu2—S3^iv	77.80 (4)	Si1—S3—Eu2	87.37 (7)
S3—Eu2—S3^iv	64.61 (6)	Eu1^ii—S3—Eu2	170.97 (5)
S2^i—Eu2—S1^ix	98.52 (6)	Eu2^vii—S3—Eu2	102.20 (4)
S3^vii—Eu2—S1^ix	84.78 (5)	Eu1—S3—Eu2	85.18 (4)

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