Potassium sodium tin selenide, K₃NaSn₃Se₈

The title compound, tripotassium sodium tritin octaselenide, K₃NaSn₃Se₈, has a molecular (zero-dimensional) structure containing trimeric [Sn₃Se₈]^4- units which consist of three edge-sharing SnSe₄ tetrahedra. The [Sn₃Se₈]^4- anions and the tetrahedrally coordinated Na⁺ cations are arranged in an alternating fashion along the c axis to form SiS₂-like chains, which are then separated by eight-coordinate K⁺ cations. The Sn-Se bond distances are normal, being in the range 2.477 (1)-2.612 (1) Å.

S1. Comment

In recent years, a number of intermetallic compounds in the A—Sn—Se system have been detected and structurally characterized. These include A₄SnSe₄ (A is Na or K; Klepp, 1992a) with isolated SnSe₄ tetrahedra, K₄Sn₂Se₆ (Eisenmann & Hansa, 1993a) and K₄Sn₃Se₈ (Sheldrick, 1988) with dimers and trimers of edge-sharing tetrahedra, Na₂SnSe₃ (Klepp, 1995) with one-dimensional chains of corner-sharing SnSe₄ tetrahedra, K₂Sn₂Se₅ (Klepp, 1992b) with chains of edge-sharing SnSe₅-bipyramids which are connected via apical Se atoms to form an infinite anionic framework, and K₂Sn₄Se₈ (Klepp & Fabian, 1992) containing layers of corner-sharing Sn^IVSe₄-tetrahedra which are connected by Sn^II atoms to form an open three-dimensional network. During our investigation of the quaternary K—Ni—Sn—Se system using the molten alkali-metal polychalcogenide flux method, we unexpectedly obtained single crystals of the title new trimeric compound, K₃NaSn₃Se₈. In this paper we report its crystal structure determination.

The crystal structure of K₃NaSn₃Se₈ projected approximately on the ab plane is shown in Fig. 1. Interestingly, Na⁺ cations and [Sn₃Se₈]⁴⁻ anions are aligned in an alternating fashion along the [001] direction to form a chainlike structure. These chains are then separated by K⁺ cations. The [NaSn₃Se₈]³⁻ chain (Fig. 2), which looks very similar to those observed in KFeS₂ (Boon & MacGillavry, 1942) and K₂MnSn₂Se₆ (Chen et al., 2000), may be described as a substitutional variant of the SiS₂-type (Peters & Krebs, 1982). However, because Na—Se interactions are ionic in character, it is more appropriate to describe the title compound as having a molecular (zero-dimensional) structure containing highly charged trimeric [Sn₃Se₈]⁴⁻ units which consist of three edge-sharing SnSe₄ tetrahedra. In the [Sn₃Se₈]⁴⁻ unit, the central atom Sn1 has a site symmetry of −42m, giving a single Sn—Se bond distance [4 × 2.517 (1) Å], while the external atom Sn2 lies on a twofold axis, resulting in two sets of bond lengths [Sn2—Se1 2 × 2.612 (1) and Sn2—Se2 2 × 2.477 (1) Å]. The terminal Sn2—Se2 bonds are markedly shorter than the bridging Sn—Se1 bonds, as expected, and both are close to those observed in discrete di- and tritetrahedral species, K₄Sn₂Se₆ (Eisenmann & Hansa, 1993a) and K₄Sn₃Se₈ (Sheldrick, 1988). The Se—Sn—Se angles range from 97.01 (5) to 116.04 (3)° for Sn1 and 92.42 (6) to 114.16 (7)° for Sn2, with their respective smallest angles being associated with the constrained Sn₂Se₂ four-membered rings.

There are two crystallographically independent K⁺ ions and one unique Na⁺ ion surrounding the [Sn₃Se₈]⁴⁻ anions. Each K⁺ is coordinated by eight Se atoms in a square antiprismatic arrangement, with K—Se distances of 3.333 (2)–3.5955 (8) Å, which are comparable with those observed in K₂MnSn₂Se₆ [3.352 (2)–3.642 (3) Å; Chen et al., 2000 query?]. In contrast, each Na⁺ is four-coordinate to Se atoms, with shorter Na—Se distances of 4 × 2.895 (1) Å. Tetrahedrally coordinated Na⁺ is rare. Several limited examples include Na₂Se (Na—Se 2.948 Å; Zintl et al., 1934) and Na₆Sn₂Se₇ [Na—Se 2.880 (6)–2.926 (7) Å; Eisenmann & Hansa, 1993b).

It is noted that K₄Sn₃Se₈ is related to the title compound in its stoichiometry but differs in its structure. This compound crystallizes in the orthorhombic space group Ccca. The adjacent [Sn₃Se₈]⁴⁻ groups are not arranged into linear arrays as in K₃NaSn₃Se₈, but are shifted with respect to each other by (a+b)/2. As a result, no tetrahedral voids are formed and the trimers are separated by six- and eight-coordinate K⁺ cations. To the best of our knowledge, K₃NaSn₃Se₈ represents a new structure type and it is also the first structurally characterized A—Sn—Q ternary chalcogenide containing mixed alkali metal cations.

Thermal analysis via differential scanning calorimetry showed that K₃NaSn₃Se₈ melts congruently at about 741 K. The optical diffuse reflectance spectrum of K₃NaSn₃Se₈ displays a steep absorption edge with an estimated band gap of about 2.2 eV, confirming its expected semiconducting nature.

S2. Experimental

Single crystals of the title compound were extracted from an experiment attempting to prepare K—Ni—Sn—Se quaternary chalcogenides. K₂Se (0.080 g, 0.5 mmol), Ni (0.015 g, 0.25 mmol), Sn (0.060 g; 0.5 mmol) and Se (0.158 g, 2 mmol) were weighed in a glove box under an atmosphere of argon. The mixture was introduced into a thin-walled Pyrex tube and sealed under vacuum (about 10⁻³ Torr; 1 Torr = 133.322 Pa). The tube was gradually heated to 773 K, where it was kept for 6 d, then cooled at a rate of 4 K h⁻¹ to 423 K. Several orange block-like crystals were observed after washing the reaction product with dimethylformamide and anhydrous ethanol and drying with anhydrous diethyl ether. Structural refinements indicated the composition of these crystals to be K₃NaSn₃Se₈. The sodium constituent may come from the impure potassium resource. Subsequently, direct reaction of a stoichiometric mixture of K₂Se, Na₂Se, Sn, and Se at 723 K for one week yielded an almost single phase polycrystalline sample with small amounts of Se and SnSe₂ impurities that were confirmed by powder X-ray analysis.

S3. Refinement

Direct phase determination yielded the positions of the Sn and Se atoms. The K and Na atoms were located from the subsequent difference Fourier synthesis. All atoms were refined anisotropically. The highest residual electronic density peaks were located 0.74 Å from the Sn2 atoms.

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, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SCHAKAL92 (Keller, 1992); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The unit cell of K3NaSn3Se8 projected approximately along the c axis. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A fragment of the [NaSn3Se8]^3− chain, with Na—Se interactions indicated by dashed lines. Displacement ellipsoids are drawn at the 50% probability level.

Potassium sodium tin selenide

Crystal data

K3NaSn3Se8	Dx = 4.155 Mg m^−3
Mr = 1128.04	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4/nbm	Cell parameters from 25 reflections
Hall symbol: -P 4a 2b	θ = 7.2–13.9°
a = 8.121 (1) Å	µ = 20.96 mm^−1
c = 13.672 (3) Å	T = 293 K
V = 901.7 (3) Å^3	Block-like, orange
Z = 2	0.10 × 0.08 × 0.05 mm
F(000) = 980

Data collection

Enraf-Nonius CAD4 diffractometer	387 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.039
Graphite monochromator	θmax = 26.0°, θmin = 3.0°
ω scans	h = 0→10
Absorption correction: ψ-scan (Kopfman & Huber, 1968)	k = 0→10
Tmin = 0.121, Tmax = 0.348	l = 0→16
879 measured reflections	3 standard reflections every 150 reflections
491 independent reflections	intensity decay: −1.5%

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)] = 0.029	Calculated w = 1/[σ^2(Fo^2) + 12P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.071	(Δ/σ)max < 0.001
S = 1.23	Δρmax = 1.83 e Å^−3
491 reflections	Δρmin = −0.75 e Å^−3
27 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0026 (2)

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
Sn1	3/4	1/4	1/2	0.0204 (4)
Sn2	3/4	1/4	0.24580 (7)	0.0196 (3)
Se1	0.91418 (11)	0.08582 (11)	0.37801 (8)	0.0222 (3)
Se2	0.56892 (11)	0.06892 (11)	0.14732 (8)	0.0225 (4)
K1	1/4	1/4	0	0.0396 (15)
K2	1/4	1/4	0.2773 (3)	0.0309 (9)
Na	3/4	1/4	0	0.033 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn1	0.0258 (6)	0.0258 (6)	0.0095 (6)	0	0	0
Sn2	0.0233 (4)	0.0233 (4)	0.0121 (5)	−0.0052 (5)	0	0
Se1	0.0250 (4)	0.0250 (4)	0.0165 (5)	0.0037 (5)	0.0002 (3)	−0.0002 (3)
Se2	0.0244 (4)	0.0244 (4)	0.0187 (6)	−0.0039 (5)	−0.0012 (3)	−0.0012 (3)
K1	0.041 (2)	0.041 (2)	0.036 (3)	0	0	0
K2	0.0268 (12)	0.0268 (12)	0.039 (2)	0	0	0
Na	0.046 (4)	0.046 (4)	0.007 (4)	0	0	0

Geometric parameters (Å, º)

Sn1—Se1^i	2.517 (1)	K1—Se2^xiii	3.5955 (8)
Sn1—Se1^ii	2.517 (1)	K1—K2^xiv	3.792 (4)
Sn1—Se1	2.517 (1)	K1—K2	3.792 (4)
Sn1—Se1^iii	2.517 (1)	K1—Na^xv	4.0605 (5)
Sn2—Se2	2.477 (1)	K1—Na	4.0605 (5)
Sn2—Se2^ii	2.477 (1)	K2—Se1^ii	3.333 (2)
Sn2—Se1^ii	2.612 (1)	K2—Se1^xvi	3.333 (2)
Sn2—Se1	2.612 (1)	K2—Se1^xvii	3.333 (2)
Sn2—Na	3.3606 (12)	K2—Se1^xviii	3.333 (2)
Sn2—K2^iv	4.0833 (7)	K2—Se2	3.468 (2)
Sn2—K2^v	4.0833 (7)	K2—Se2^xiii	3.468 (2)
Sn2—K2	4.0833 (7)	K2—Se2^viii	3.468 (2)
Sn2—K2^vi	4.0833 (7)	K2—Se2^xii	3.468 (2)
Se1—K2^vi	3.3332 (18)	K2—Sn2^xviii	4.0833 (7)
Se1—K2^v	3.3332 (18)	K2—Sn2^xvii	4.0833 (7)
Se2—K2^vi	3.468 (2)	Na—Se2	2.895 (1)
Se2—K1^vii	3.5955 (8)	Na—Se2^ii	2.895 (1)
K1—Se2	3.5955 (8)	Na—Se2^x	2.895 (1)
K1—Se2^viii	3.5955 (8)	Na—Se2^xix	2.895 (1)
K1—Se2^ix	3.5955 (8)	Na—Sn2^x	3.3606 (12)
K1—Se2^x	3.5955 (8)	Na—K1^xv	4.0605 (5)
K1—Se2^xi	3.5955 (8)	Na—K1^v	4.0605 (5)
K1—Se2^vii	3.5955 (8)	Na—K1^vii	4.0605 (5)
K1—Se2^xii	3.5955 (8)
Se1^i—Sn1—Se1^ii	116.04 (3)	Se2^xi—K1—Se2^xiii	131.72 (4)
Se1^i—Sn1—Se1	116.04 (3)	Se2^vii—K1—Se2^xiii	87.83 (4)
Se1^ii—Sn1—Se1	97.01 (5)	Se2^xii—K1—Se2^xiii	71.710 (16)
Se1^i—Sn1—Se1^iii	97.01 (5)	Se1^ii—K2—Se1^xvi	80.18 (5)
Se1^ii—Sn1—Se1^iii	116.04 (3)	Se1^ii—K2—Se1^xvii	131.21 (13)
Se1—Sn1—Se1^iii	116.04 (3)	Se1^xvi—K2—Se1^xvii	80.18 (5)
Se2—Sn2—Se2^ii	114.16 (7)	Se1^ii—K2—Se1^xviii	80.18 (5)
Se2—Sn2—Se1^ii	112.09 (2)	Se1^xvi—K2—Se1^xviii	131.21 (13)
Se2^ii—Sn2—Se1^ii	112.09 (2)	Se1^xvii—K2—Se1^xviii	80.18 (5)
Se2—Sn2—Se1	112.09 (2)	Se1^ii—K2—Se2^xiii	149.01 (4)
Se2^ii—Sn2—Se1	112.09 (2)	Se1^xvi—K2—Se2^xiii	130.77 (3)
Se1^ii—Sn2—Se1	92.42 (6)	Se1^xvii—K2—Se2^xiii	64.28 (3)
Sn1—Se1—Sn2	85.28 (4)	Se1^xviii—K2—Se2^xiii	76.72 (3)
Se2^viii—K1—Se2^ix	154.56 (4)	Se1^ii—K2—Se2^viii	64.28 (3)
Se2^viii—K1—Se2	71.710 (16)	Se1^xvi—K2—Se2^viii	76.72 (3)
Se2^ix—K1—Se2	131.72 (4)	Se1^xvii—K2—Se2^viii	149.01 (4)
Se2^viii—K1—Se2^x	74.01 (3)	Se1^xviii—K2—Se2^viii	130.77 (3)
Se2^ix—K1—Se2^x	111.86 (3)	Se2^xiii—K2—Se2^viii	118.35 (12)
Se2—K1—Se2^x	87.83 (4)	Se1^ii—K2—Se2^xii	130.77 (3)
Se2^viii—K1—Se2^xi	87.83 (4)	Se1^xvi—K2—Se2^xii	64.28 (3)
Se2^ix—K1—Se2^xi	71.710 (16)	Se1^xvii—K2—Se2^xii	76.72 (3)
Se2—K1—Se2^xi	154.56 (4)	Se1^xviii—K2—Se2^xii	149.01 (4)
Se2^x—K1—Se2^xi	71.710 (16)	Se2^xiii—K2—Se2^xii	74.78 (5)
Se2^viii—K1—Se2^vii	131.72 (4)	Se2^viii—K2—Se2^xii	74.78 (5)
Se2^ix—K1—Se2^vii	71.710 (16)	Se1^ii—K2—Se2	76.72 (3)
Se2—K1—Se2^vii	74.01 (3)	Se1^xvi—K2—Se2	149.01 (4)
Se2^x—K1—Se2^vii	71.710 (16)	Se1^xvii—K2—Se2	130.77 (3)
Se2^xi—K1—Se2^vii	111.86 (3)	Se1^xviii—K2—Se2	64.28 (3)
Se2^viii—K1—Se2^xii	71.710 (16)	Se2^xiii—K2—Se2	74.78 (5)
Se2^ix—K1—Se2^xii	87.83 (4)	Se2^viii—K2—Se2	74.78 (5)
Se2—K1—Se2^xii	111.86 (3)	Se2^xii—K2—Se2	118.35 (12)
Se2^x—K1—Se2^xii	131.72 (4)	Se2^ii—Na—Se2^x	118.95 (3)
Se2^xi—K1—Se2^xii	74.01 (3)	Se2^ii—Na—Se2	91.83 (5)
Se2^vii—K1—Se2^xii	154.56 (4)	Se2^x—Na—Se2	118.95 (3)
Se2^viii—K1—Se2^xiii	111.86 (3)	Se2^ii—Na—Se2^xix	118.95 (3)
Se2^ix—K1—Se2^xiii	74.01 (3)	Se2^x—Na—Se2^xix	91.83 (5)
Se2—K1—Se2^xiii	71.710 (16)	Se2—Na—Se2^xix	118.95 (3)
Se2^x—K1—Se2^xiii	154.56 (4)

Symmetry codes: (i) −y+1, x−1/2, −z+1; (ii) −x+3/2, −y+1/2, z; (iii) y+1/2, −x+1, −z+1; (iv) x+1/2, −y+1, z; (v) x+1, y, z; (vi) x+1/2, −y, z; (vii) −x+1, −y, −z; (viii) −y+1/2, x, z; (ix) −y, x−1/2, −z; (x) y+1/2, −x+1, −z; (xi) x−1/2, y+1/2, −z; (xii) −x+1/2, −y+1/2, z; (xiii) y, −x+1/2, z; (xiv) −x+1/2, y, −z; (xv) −x+1, −y+1, −z; (xvi) y, −x+3/2, z; (xvii) x−1, y, z; (xviii) −y+1/2, x−1, z; (xix) −y+1, x−1/2, −z.