Potassium silver tin selenide, K₂Ag₂Sn₂Se₆

The title compound was synthesized by a reactive salt reaction at 773 K over a period of 5 d. It has a one-dimensional chain structure consisting of K⁺ cations and one-dimensional [Ag₂Sn₂Se₆]^2- anions. The chain is constructed by edge-sharing bitetrahedral [Sn₂Se₆] units connected in a 1:2 ratio via linear Ag⁺ ions.

S1. Comment

Multi-component metal chalcogenides are of great interest due to their low-dimensional structures and unusual properties. Since Ibers and co-workers first synthesized K₄Ti₃S₁₄ crystals using a molten salt (alkali metal polysulfide flux) reaction at 648 K (Sunshine et al., 1987), great progress has been made in the flux growth of solid-state chalcogenides at intermediate temperatures. A number of Sn-containing quaternary systems have been reported so far, for example, KGaSnS₄ (Wu et al., 1992), A₂Hg₃Sn₂S₈ (A = Rb, Cs; Marking et al., 1998), K₂MnSn₂Se₆, K₂MnSnSe₄ and K₂Ag₂SnSe₄ (Chen et al., 2000). For the A₂M₂Sn₂Q₆ family (A = alkali metal; M = Cu, Ag or Au; Q = S or Se), the members with M = Cu and Au, including A₂Cu₂Sn₂S₆ (A = Na, K, Rb, Cs), A₂Cu₂Sn₂Se₆ (A = K, Rb), K₂Au₂Sn₂S₆ and K₂Au₂Sn₂Se₆, have been investigated (Liao et al., 1993). We report here a new member to the family, namely K₂Ag₂Sn₂Se₆.

K₂Ag₂Sn₂Se₆ has a one-dimensional structure containing [Ag₂Sn₂Se₆]^2- chain anions separated by K⁺ ions. The packing view along the c axis is shown in Fig. 1(a). The [Ag₂Sn₂Se₆]^2- chain is constructed by edge-sharing bitetrahedral [Sn₂Se₆] units and Ag⁺ ions in a 1:2 ratio (see Fig. 1 b). In the [Sn₂Se₆] dimer, the bridging Se1 atoms form Sn—Se1 bonds of 2.5839 (7) Å, which are longer than the bonds formed between the terminal Se2 and Sn atoms [2.5075 (7) Å]. This is due to the stress of the [SnSe1₂Sn] four-membered ring. There are two nearly linear Se—Ag—Se bridging bonds between adjacent [Sn₂Se₆] units, forming Sn(SeAgSe)₂Sn eight-membered rings. The Se—Ag—Se fragments of the ring are not parallel to each other, while an Ag—Ag bond occurs inside the ring with a distance of 3.0717 (19) Å. The [Ag₂Sn₂Se₆]^2- chains extend along the crystallographic c axis direction and are separated by K⁺ ions. The shortest inter-chain Se—Se distance is 3.61 Å. There are three crystallographically distinct K⁺ ions. Each K⁺ is eight-coordinated by Se atoms in a square-antiprismatic arrangement, with K—Se distances ranging from 3.3244 (13) to 3.550 (3) Å. K3 is statistically distributed among the available sites with a 50% probability.

The title compound, K₂Ag₂Sn₂Se₆, is isostructural with K₂Au₂Sn₂S₆ and K₂Au₂Sn₂Se₆, but has a different structure type from A₂Cu₂Sn₂Q₆ (A = Na, K, Rb, Cs; Q = S, Se; Liao et al., 1993). Cu⁺ prefers tetrahedral coordination in A₂Cu₂Sn₂Q₆, while Ag⁺ and Au⁺ tend to adopt a linear coordination, as in K₂Ag₂Sn₂Se₆, K₂Au₂Sn₂S₆ and K₂Au₂Sn₂Se₆.

Related literature

For related literature, see: Chen et al. (2000); Liao & Kanatzidis (1993); Marking et al. (1998); Sunshine et al. (1987); Wu et al. (1992).

S2. Experimental

A mixture of K₂Se (0.0640 g, 0.417 mmol), Ag (0.0450 g, 0.417 mmol), Sn (0.0459 g, 0.417 mmol) and Se (0.0998 g, 1.264 mmol) was loaded into a Pyrex tube in a glove-box under an argon atmosphere and then sealed under vacuum conditions (about 10 ^-1 Pa). The tube was gradually heated to 773 K and was kept at this temperature for 5 d. It was then cooled at a rate of 4 K h^-1 to 473 K, followed by cooling naturally to room temperature. Orange–red block-like crystals were isolated from the reaction product, washed with dimethylformamide and ethanol, and finally dried with anhydrous ether. Semi-quantitative elemental analysis for the crystal, performed on an electron probe micro-analyzer (EPM-810Q, SHIMADZU) using energy dispersive spectroscopy (EDS), indicated the composition to be KAgSnSe_2.5. A single-crystal was selected for X-ray crystal structure determination.

S3. Refinement

Direct phase determination yielded the positions of the Ag, Sn and Se atoms. The remaining K atoms were located from the subsequent difference Fourier synthesis. The highest residual electronic density peaks were located at 1.08 Å from K3.

Computing details

Data collection: CAD-4-PC Software (Enraf-Nonius, 1992); cell refinement: CAD-4-PC Software; data reduction: XCAD4/PC (Harms, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SCHAKAL97 (Keller, 1997).

Figures

Fig. 1. Views of K2Ag2Sn2Se6 (a) along the c axis, with double-shaded circles for K, solid circles for Sn, single-shaded circles for Ag and open circles for Se atoms. The unit cell is outlined. (b) An illustration of the [Ag2Sn2Se6]^2- chain extending along the c axis, showing 70% probability displacement ellipsoids.

Potassium Silver Tin Selenide

Crystal data

K2Ag2Sn2Se6	Dx = 4.929 Mg m^−3
Mr = 1005.08	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4/mcc	Cell parameters from 25 reflections
a = 8.173 (1) Å	θ = 5.9–11.7°
c = 20.278 (4) Å	µ = 23.18 mm^−1
V = 1354.5 (4) Å^3	T = 293 K
Z = 4	Block, orange-red
F(000) = 1744	0.08 × 0.05 × 0.05 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	546 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.036
Graphite monochromator	θmax = 30.0°, θmin = 2.0°
ω scans	h = 0→11
Absorption correction: ψ scan (Kopfman & Huber, 1968)	k = 0→11
Tmin = 0.259, Tmax = 0.390	l = 0→28
1946 measured reflections	3 standard reflections every 300 reflections
1029 independent reflections	intensity decay: −1.6%

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)] = 0.031	w = 1/[σ^2(Fo^2)]
wR(F^2) = 0.048	(Δ/σ)max < 0.001
S = 0.99	Δρmax = 1.68 e Å^−3
546 reflections	Δρmin = −1.61 e Å^−3
35 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00190 (4)

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	Occ. (<1)
Sn1	0.0000	0.5000	0.087649 (19)	0.01738 (16)
Ag1	0.0000	0.31209 (11)	0.2500	0.0455 (3)
Se1	−0.15436 (13)	0.33021 (13)	0.0000	0.0191 (2)
Se2	0.19114 (9)	0.32614 (9)	0.15433 (2)	0.02159 (18)
K1	0.0000	0.0000	0.09381 (13)	0.0277 (7)
K2	0.5000	0.5000	0.2500	0.0419 (13)
K3	0.5000	0.5000	0.0531 (3)	0.0338 (15)	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0230 (4)	0.0186 (4)	0.01051 (18)	0.0022 (3)	0.000	0.000
Ag1	0.0609 (7)	0.0511 (7)	0.0244 (3)	0.000	0.0185 (5)	0.000
Se1	0.0221 (6)	0.0202 (6)	0.0149 (3)	−0.0015 (4)	0.000	0.000
Se2	0.0226 (5)	0.0254 (5)	0.0168 (2)	0.0038 (3)	−0.0022 (3)	0.0005 (3)
K1	0.0225 (11)	0.0225 (11)	0.0380 (14)	0.000	0.000	0.000
K2	0.031 (2)	0.031 (2)	0.063 (3)	0.000	0.000	0.000
K3	0.025 (2)	0.025 (2)	0.052 (3)	0.000	0.000	0.000

Geometric parameters (Å, º)

Sn1—Se2	2.5075 (7)	K1—K1^vi	3.805 (5)
Sn1—Se2^i	2.5075 (7)	K1—Ag1^viii	4.067 (2)
Sn1—Se1	2.5839 (8)	K1—Ag1^vii	4.067 (2)
Sn1—Se1^ii	2.5839 (8)	K2—Se2	3.4864 (8)
Sn1—K1^iii	4.0884 (5)	K2—Se2^xi	3.4864 (8)
Sn1—K1	4.0884 (5)	K2—Se2^xii	3.4864 (8)
Sn1—K3^iv	4.1460 (11)	K2—Se2^xiii	3.4864 (8)
Sn1—K3	4.1460 (11)	K2—Se2^xiv	3.4864 (8)
Ag1—Se2^v	2.4935 (7)	K2—Se2^xv	3.4864 (8)
Ag1—Se2	2.4935 (7)	K2—Se2^xvi	3.4864 (8)
Ag1—Ag1^i	3.0717 (19)	K2—Se2^xvii	3.4864 (8)
Ag1—K1	4.067 (2)	K2—K3^xiii	3.992 (6)
Ag1—K1^v	4.067 (2)	K2—K3	3.992 (6)
Se1—Sn1^ii	2.5839 (8)	K3—K3^xviii	2.155 (12)
Se1—K3^ii	3.327 (2)	K3—Se1^x	3.327 (2)
Se1—K3^iv	3.327 (2)	K3—Se1^xix	3.327 (2)
Se1—K1^vi	3.5347 (17)	K3—Se1^xx	3.327 (2)
K1—Se1	3.5347 (18)	K3—Se1^ii	3.327 (2)
K1—Se2	3.3244 (13)	K3—Se2	3.550 (3)
K1—Se2^vii	3.3244 (13)	K3—Se2^xv	3.550 (3)
K1—Se2^viii	3.3244 (13)	K3—Se2^xvi	3.550 (3)
K1—Se2^ix	3.3244 (13)	K3—Se2^xii	3.550 (3)
K1—Se1^ix	3.5347 (18)	K3—Sn1^xx	4.1460 (11)
K1—Se1^vi	3.5347 (17)	K3—Sn1^xix	4.1460 (11)
K1—Se1^x	3.5347 (18)
Se2—Sn1—Se2^i	114.74 (3)	Se2^v—Ag1—Se2	174.72 (5)
Se2—Sn1—Se1	111.76 (3)	Se2^v—Ag1—Ag1^i	87.36 (3)
Se2^i—Sn1—Se1	111.78 (3)	Se2—Ag1—Ag1^i	87.36 (3)
Se2—Sn1—Se1^ii	111.78 (3)	Sn1—Se1—Sn1^ii	86.92 (3)
Se2^i—Sn1—Se1^ii	111.76 (3)	Ag1—Se2—Sn1	93.17 (3)
Se1—Sn1—Se1^ii	93.08 (3)

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