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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.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101014299/iz1017sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101014299/iz1017Isup2.hkl
Contains datablock I

Comment top

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 top

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

Experimental top

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 KAgSnSe2.5. A single-crystal was selected for X-ray crystal structure determination.

Refinement top

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 top

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 top
[Figure 1] 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 top
Crystal data top
K2Ag2Sn2Se6Dx = 4.929 Mg m3
Mr = 1005.08Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4/mccCell parameters from 25 reflections
a = 8.173 (1) Åθ = 5.9–11.7°
c = 20.278 (4) ŵ = 23.18 mm1
V = 1354.5 (4) Å3T = 293 K
Z = 4Block, orange-red
F(000) = 17440.08 × 0.05 × 0.05 mm
Data collection top
Enraf-Nonius CAD-4
diffractometer
546 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.036
Graphite monochromatorθmax = 30.0°, θmin = 2.0°
ω scansh = 011
Absorption correction: ψ scan
(Kopfman & Huber, 1968)
k = 011
Tmin = 0.259, Tmax = 0.390l = 028
1946 measured reflections3 standard reflections every 300 reflections
1029 independent reflections intensity decay: 1.6%
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.031 w = 1/[σ2(Fo2)]
wR(F2) = 0.048(Δ/σ)max < 0.001
S = 0.99Δρmax = 1.68 e Å3
546 reflectionsΔρmin = 1.61 e Å3
35 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.00190 (4)
Crystal data top
K2Ag2Sn2Se6Z = 4
Mr = 1005.08Mo Kα radiation
Tetragonal, P4/mccµ = 23.18 mm1
a = 8.173 (1) ÅT = 293 K
c = 20.278 (4) Å0.08 × 0.05 × 0.05 mm
V = 1354.5 (4) Å3
Data collection top
Enraf-Nonius CAD-4
diffractometer
546 reflections with I > 2σ(I)
Absorption correction: ψ scan
(Kopfman & Huber, 1968)
Rint = 0.036
Tmin = 0.259, Tmax = 0.3903 standard reflections every 300 reflections
1946 measured reflections intensity decay: 1.6%
1029 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.03135 parameters
wR(F2) = 0.0480 restraints
S = 0.99Δρmax = 1.68 e Å3
546 reflectionsΔρmin = 1.61 e Å3
Special details top

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 (Å2) top
xyzUiso*/UeqOcc. (<1)
Sn10.00000.50000.087649 (19)0.01738 (16)
Ag10.00000.31209 (11)0.25000.0455 (3)
Se10.15436 (13)0.33021 (13)0.00000.0191 (2)
Se20.19114 (9)0.32614 (9)0.15433 (2)0.02159 (18)
K10.00000.00000.09381 (13)0.0277 (7)
K20.50000.50000.25000.0419 (13)
K30.50000.50000.0531 (3)0.0338 (15)0.50
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Sn10.0230 (4)0.0186 (4)0.01051 (18)0.0022 (3)0.0000.000
Ag10.0609 (7)0.0511 (7)0.0244 (3)0.0000.0185 (5)0.000
Se10.0221 (6)0.0202 (6)0.0149 (3)0.0015 (4)0.0000.000
Se20.0226 (5)0.0254 (5)0.0168 (2)0.0038 (3)0.0022 (3)0.0005 (3)
K10.0225 (11)0.0225 (11)0.0380 (14)0.0000.0000.000
K20.031 (2)0.031 (2)0.063 (3)0.0000.0000.000
K30.025 (2)0.025 (2)0.052 (3)0.0000.0000.000
Geometric parameters (Å, º) top
Sn1—Se22.5075 (7)K1—K1vi3.805 (5)
Sn1—Se2i2.5075 (7)K1—Ag1viii4.067 (2)
Sn1—Se12.5839 (8)K1—Ag1vii4.067 (2)
Sn1—Se1ii2.5839 (8)K2—Se23.4864 (8)
Sn1—K1iii4.0884 (5)K2—Se2xi3.4864 (8)
Sn1—K14.0884 (5)K2—Se2xii3.4864 (8)
Sn1—K3iv4.1460 (11)K2—Se2xiii3.4864 (8)
Sn1—K34.1460 (11)K2—Se2xiv3.4864 (8)
Ag1—Se2v2.4935 (7)K2—Se2xv3.4864 (8)
Ag1—Se22.4935 (7)K2—Se2xvi3.4864 (8)
Ag1—Ag1i3.0717 (19)K2—Se2xvii3.4864 (8)
Ag1—K14.067 (2)K2—K3xiii3.992 (6)
Ag1—K1v4.067 (2)K2—K33.992 (6)
Se1—Sn1ii2.5839 (8)K3—K3xviii2.155 (12)
Se1—K3ii3.327 (2)K3—Se1x3.327 (2)
Se1—K3iv3.327 (2)K3—Se1xix3.327 (2)
Se1—K1vi3.5347 (17)K3—Se1xx3.327 (2)
K1—Se13.5347 (18)K3—Se1ii3.327 (2)
K1—Se23.3244 (13)K3—Se23.550 (3)
K1—Se2vii3.3244 (13)K3—Se2xv3.550 (3)
K1—Se2viii3.3244 (13)K3—Se2xvi3.550 (3)
K1—Se2ix3.3244 (13)K3—Se2xii3.550 (3)
K1—Se1ix3.5347 (18)K3—Sn1xx4.1460 (11)
K1—Se1vi3.5347 (17)K3—Sn1xix4.1460 (11)
K1—Se1x3.5347 (18)
Se2—Sn1—Se2i114.74 (3)Se2v—Ag1—Se2174.72 (5)
Se2—Sn1—Se1111.76 (3)Se2v—Ag1—Ag1i87.36 (3)
Se2i—Sn1—Se1111.78 (3)Se2—Ag1—Ag1i87.36 (3)
Se2—Sn1—Se1ii111.78 (3)Sn1—Se1—Sn1ii86.92 (3)
Se2i—Sn1—Se1ii111.76 (3)Ag1—Se2—Sn193.17 (3)
Se1—Sn1—Se1ii93.08 (3)
Symmetry codes: (i) x, y+1, z; (ii) x, y+1, z; (iii) x, y+1, z; (iv) x1, 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.

Experimental details

Crystal data
Chemical formulaK2Ag2Sn2Se6
Mr1005.08
Crystal system, space groupTetragonal, P4/mcc
Temperature (K)293
a, c (Å)8.173 (1), 20.278 (4)
V3)1354.5 (4)
Z4
Radiation typeMo Kα
µ (mm1)23.18
Crystal size (mm)0.08 × 0.05 × 0.05
Data collection
DiffractometerEnraf-Nonius CAD-4
diffractometer
Absorption correctionψ scan
(Kopfman & Huber, 1968)
Tmin, Tmax0.259, 0.390
No. of measured, independent and
observed [I > 2σ(I)] reflections
1946, 1029, 546
Rint0.036
(sin θ/λ)max1)0.702
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.031, 0.048, 0.99
No. of reflections546
No. of parameters35
Δρmax, Δρmin (e Å3)1.68, 1.61

Computer programs: CAD-4-PC Software (Enraf-Nonius, 1992), CAD-4-PC Software, XCAD4/PC (Harms, 1997), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), SCHAKAL97 (Keller, 1997).

Selected geometric parameters (Å, º) top
Sn1—Se22.5075 (7)Ag1—Se2iii2.4935 (7)
Sn1—Se2i2.5075 (7)Ag1—Se22.4935 (7)
Sn1—Se12.5839 (8)Ag1—Ag1i3.0717 (19)
Sn1—Se1ii2.5839 (8)
Se2—Sn1—Se2i114.74 (3)Se2i—Sn1—Se1ii111.76 (3)
Se2—Sn1—Se1111.76 (3)Se1—Sn1—Se1ii93.08 (3)
Se2i—Sn1—Se1111.78 (3)Se2iii—Ag1—Se2174.72 (5)
Se2—Sn1—Se1ii111.78 (3)
Symmetry codes: (i) x, y+1, z; (ii) x, y+1, z; (iii) x, y, z+1/2.
 

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