TbNb₆Sn₆: the first ternary compound from the rare earth–niobium–tin system

The title compound, terbium hexa­niobium hexastannide, TbNb₆Sn₆, is the first ternary compound from the rare earth–niobium–tin system. It has the HfFe₆Ge₆ structure type, which can be analysed as an inter­growth of the Zr₄Al₃ and CaCu₅ structures. All the atoms lie on special positions; their coordination geometries and site symmetries are: Tb (dodeca­hedron) 6/mmm; Nb (distorted icosa­hedron) 2mm; Sn (Frank–Caspar polyhedron, CN = 14–15) 6mm and m2; Sn (distorted icosa­hedron) m2. The structure contains a graphite-type Sn network, Kagome nets of Nb atoms, and Tb atoms alternating with Sn2 dumbbells in the channels.

Related literature

For background to niobium alloys, see: Ateev & Shamrai (1966). For related structures and background to inter­metallics, see: Nowotny (1942); Raeuchle & Rundle (1952); Schobinger-Papamantellos et al. (1998); Wilson et al. (1960). A statistical test of the distribution of the E values using the program E-STATS from WinGX system (Farrugia, 1999) suggested that the structure is centrosymmetric. For MgCo₆Ge₆, see: Gieck et al. (2006).

Experimental

Crystal data

• TbNb₆Sn₆

• M_r = 1428.52

• Hexagonal, P 6/m m m

• a = 5.7650 (1) Å

• c = 9.5387 (4) Å

• V = 274.55 (1) ų

• Z = 1

• Mo Kα radiation

• μ = 25.66 mm⁻¹

• T = 293 K

• 0.050 × 0.020 × 0.004 mm

Data collection

• Oxford Diffraction Xcalibur3 diffractometer with CCD detector

• Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2008) T_min = 0.592, T_max = 1.000

• 2493 measured reflections

• 208 independent reflections

• 181 reflections with I > 2σ(I)

• R_int = 0.027

Refinement

• R[F² > 2σ(F²)] = 0.017

• wR(F²) = 0.038

• S = 1.29

• 208 reflections

• 15 parameters

• Δρ_max = 1.68 e Å⁻³

• Δρ_min = −2.35 e Å⁻³

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Niobium compounds have very useful properties for superconductive materials: high critical fields and plasticity which gives an opportunity to make superconductive windings. The main disadvantage of these compounds is the low critical temperature (Ateev & Shamrai, 1966). Superconductive Nb₃Sn with T_c=18.2 K is a high performance superconductor and the gold standard of the world's superconductor industry.

So far, no ternary compounds in the RE–Nb–Sn (RE - rare-earth metals) systems are known. TbNb₆Sn₆ is the first ternary Rare earth – Niobium – Tin compound. According to the X-ray single-crystal data the TbNb₆Sn₆ compound crystallizes with hexagonal symmetry (space group P6/mmm, HfFe₆Ge₆ structure type). In TbNb₆Sn₆ planar graphite-type layers of Sn atoms and Kagome nets of Nb atoms alternate along the c axis, similar to the recently reported MgCo₆Ge₆ compound (Gieck et al., 2006). The resulting columns of vertex-sharing trigonal bipyramids form a three-dimensional Nb–Sn network with hexagonal tunnels (Figure 1). These tunnels are alternately centered by Tb atoms and Sn2 dumbbells (with Sn-Sn distances of 3.24 A). The coordination polyhedra of the atoms are: Tb1 — 20-vertex polyhedron (CN=20), Sn2 — Frank-Casper polyhedron (CN=15), Sn3 — distorted icosahedron (CN=12), Sn4 — Frank-Casper polyhedron (CN=14) and Nb5 — distorted icosahedron (CN=12).

The HfFe₆Ge₆ structure type (Schobinger-Papamantellos et al.,1998), also referred to MgFe₆Ge₆ or LiCo₆Ge₆, can be described as an intergrowth of the Zr₄Al₃ (Wilson et al., 1960) and CaCu₅ (Nowotny, 1942) structure types (see Fig.2). Another possibility to describe the HfFe₆Ge₆ structure is a transformation of the CaCu₅ structure via multiple substitution and ordering of atoms. The first step is the doubling of the CaCu₅ unit cell along the c axis. The substitution of every second Ca atom (R) along the c axis by a pair of atoms (2X) transforms this structure into the hexagonal modification of TiBe₁₂ (Raeuchle & Rundle, 1952). As a result, the c/a ratio increases to 1.733. Further ordering of X atoms in the TiBe₁₂ leads to the HfFe₆Ge₆ structure. Perhaps the presence of atoms of different radii leads to closer packing of the layers in the ternary HfFe₆Ge₆ compared to the binary TiBe₁₂. Therefore, the c/a ratio of 1.591 for ternary HfFe₆Ge₆ is much closer to the ideal value of 1.596 (c/a = 1.6546 for TbNb₆Sn₆).

Related literature

For background to niobium alloys, see: Ateev & Shamrai (1966). For related structures and background to intermetallics, see: Nowotny (1942); Raeuchle & Rundle (1952); Schobinger-Papamantellos et al. (1998); Wilson et al. (1960). A statistical test of the distribution of the E values using the program E-STATS from WinGX system (Farrugia, 1999) suggested that the structure is centrosymmetric.

S2. Experimental

Single crystals of the title compound were first found in a sample with the composition 2Tb:3Zn:5Sn, which was synthesized by induction heating of the pure elements in a niobium crucible. The sample was heated at 1100° C in an induction furnace (Hüttinger Elektronik, Freiburg, Type TIG 2.5/300) under continuous argon flow for 1 h followed by cooling to 700° C at a rate of 10 degrees/min. Finally it was quenched by switching off the furnace. A reaction between the sample and the Nb container was observed. Good-quality hexagonal plate-like crystals were selected from annealed sample by mechanical fragmentation. Single-crystal intensity data of TbNb₆Sn₆ were collected at room temperature on an Oxford-Xcalibur3 CCD area detector diffractometer. After the measurement, the single crystal was analyzed with a JEOL SEM 5900LV scanning electron microscope. No impurity elements heavier than sodium were observed. The EDX analysis of well-shaped single crystal reveals the composition (in atomic percentages) Tb 8(3), Nb 45 (6), and Sn 47 (7), which is in good agreement with the compositions resulting from XRD data refinement. Further, a sample with the composition of Tb:6Nb:6Sn was prepared by arc melting and examined by powder X-ray diffraction. As-cast sample does not contain TbNb₆Sn₆ phase. However, after grinding, pressing and annealing it at 900°C for 12 h a significant amount of the TbNb₆Sn₆ phase was observed. Magnetic measurements of the annealed sample were performed using a MPMS XL5 SQUID magnetometer (Quantum Design). Cooling the sample without magnetic field and rising the temperature in the presence of a field of 15 Oe from 1.8 K to 50 K revealed at approximately 18 K only the superconducting behavior of the Nb₃Sn impurity.

S3. Refinement

A statistical test of the distribution of the E values using the program E-STATS from WinGX system (Farrugia, 1999) suggested that the structure is centrosymmetric. The analysis of systematic extinctions yielded the space group P6/mmm, and it was confirmed by the following structure refinement. The structure was solved by the direct methods.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. View of the TbNb6Sn6 structure. The graphite-type Sn layers, Ge2 dumbbells and condensed Nb3Sn2 bipyramids are emphasized.
	Fig. 2. Relationships between the HfFe6Ge6, Zr4Al3 and CaCu5 structures.

terbium hexaniobium hexatin

Crystal data

TbNb6Sn6	Dx = 8.640 Mg m^−3
Mr = 1428.52	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P6/mmm	Cell parameters from 181 reflections
Hall symbol: -P 6 2	θ = 4.1–30.0°
a = 5.7650 (1) Å	µ = 25.66 mm^−1
c = 9.5387 (4) Å	T = 293 K
V = 274.55 (1) Å^3	Hexagonal plate, metallic gray
Z = 1	0.05 × 0.02 × 0.004 mm
F(000) = 611

Data collection

Oxford Diffraction Xcalibur3 diffractometer with CCD detector	208 independent reflections
Radiation source: Enhance (Mo) X-ray Source	181 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
Detector resolution: 16.0238 pixels mm^-1	θmax = 30.0°, θmin = 4.1°
ω and π scans	h = −8→7
Absorption correction: multi-scan (CrysAlis RED, Oxford Diffraction, 2008)	k = −5→8
Tmin = 0.592, Tmax = 1.000	l = −13→13
2493 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)] = 0.017	w = 1/[σ^2(Fo^2) + (0.0113P)^2 + 2.3749P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.038	(Δ/σ)max < 0.001
S = 1.29	Δρmax = 1.68 e Å^−3
208 reflections	Δρmin = −2.35 e Å^−3
15 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0333 (12)

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
Tb1	0.0000	0.0000	0.0000	0.0101 (3)
Sn2	0.3333	0.6667	0.5000	0.0074 (2)
Sn3	0.3333	0.6667	0.0000	0.0066 (2)
Sn4	0.0000	0.0000	0.33003 (11)	0.0091 (2)
Nb5	0.0000	0.5000	0.24932 (6)	0.0053 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tb1	0.0082 (3)	0.0082 (3)	0.0140 (5)	0.00408 (16)	0.000	0.000
Sn2	0.0084 (3)	0.0084 (3)	0.0054 (4)	0.00421 (14)	0.000	0.000
Sn3	0.0070 (3)	0.0070 (3)	0.0058 (4)	0.00349 (14)	0.000	0.000
Sn4	0.0063 (3)	0.0063 (3)	0.0148 (5)	0.00314 (15)	0.000	0.000
Nb5	0.0047 (3)	0.0050 (3)	0.0060 (3)	0.00235 (17)	0.000	0.000

Geometric parameters (Å, º)

Tb1—Sn4	3.1481 (10)	Sn3—Nb5^ix	2.9027 (5)
Tb1—Sn4^i	3.1481 (10)	Sn3—Nb5	2.9027 (5)
Tb1—Sn3^ii	3.3284	Sn3—Tb1^xiii	3.3284
Tb1—Sn3^iii	3.3284	Sn3—Tb1^xiv	3.3284
Tb1—Sn3^i	3.3284	Sn4—Nb5	2.9835 (3)
Tb1—Sn3	3.3284	Sn4—Nb5^xv	2.9835 (3)
Tb1—Sn3^iv	3.3285	Sn4—Nb5^ix	2.9835 (3)
Tb1—Sn3^v	3.3285	Sn4—Nb5^xvi	2.9835 (3)
Sn2—Nb5	2.9133 (5)	Sn4—Nb5^v	2.9835 (3)
Sn2—Nb5^vi	2.9133 (5)	Sn4—Nb5^xvii	2.9835 (3)
Sn2—Nb5^vii	2.9133 (5)	Sn4—Sn4^xviii	3.243 (2)
Sn2—Nb5^viii	2.9133 (5)	Nb5—Nb5^viii	2.8825
Sn2—Nb5^ix	2.9133 (5)	Nb5—Nb5^xvi	2.8825
Sn2—Nb5^x	2.9133 (5)	Nb5—Nb5^ix	2.8825
Sn3—Nb5^viii	2.9027 (5)	Nb5—Nb5^xix	2.8825
Sn3—Nb5^iv	2.9027 (5)	Nb5—Sn3^iv	2.9027 (5)
Sn3—Nb5^xi	2.9027 (5)	Nb5—Sn2^vii	2.9133 (5)
Sn3—Nb5^xii	2.9027 (5)	Nb5—Sn4^xiii	2.9835 (3)
Sn4—Tb1—Sn4^i	180.0	Nb5—Sn3—Tb1	73.341 (3)
Sn4—Tb1—Sn3^ii	90.0	Tb1^xiii—Sn3—Tb1	120.0
Sn4^i—Tb1—Sn3^ii	90.0	Tb1^xiv—Sn3—Tb1	120.0
Sn4—Tb1—Sn3^iii	90.0	Nb5—Sn4—Nb5^xv	113.586 (17)
Sn4^i—Tb1—Sn3^iii	90.0	Nb5—Sn4—Nb5^ix	57.772 (6)
Sn3^ii—Tb1—Sn3^iii	180.0	Nb5^xv—Sn4—Nb5^ix	150.09 (4)
Sn4—Tb1—Sn3^i	90.0	Nb5—Sn4—Nb5^xvi	57.772 (6)
Sn4^i—Tb1—Sn3^i	90.0	Nb5^xv—Sn4—Nb5^xvi	57.772 (6)
Sn3^ii—Tb1—Sn3^i	120.0	Nb5^ix—Sn4—Nb5^xvi	113.586 (17)
Sn3^iii—Tb1—Sn3^i	60.0	Nb5—Sn4—Nb5^v	150.09 (4)
Sn4—Tb1—Sn3	90.0	Nb5^xv—Sn4—Nb5^v	57.772 (6)
Sn4^i—Tb1—Sn3	90.0	Nb5^ix—Sn4—Nb5^v	113.586 (17)
Sn3^ii—Tb1—Sn3	60.0	Nb5^xvi—Sn4—Nb5^v	113.586 (17)
Sn3^iii—Tb1—Sn3	120.0	Nb5—Sn4—Nb5^xvii	113.586 (17)
Sn3^i—Tb1—Sn3	180.0	Nb5^xv—Sn4—Nb5^xvii	113.586 (17)
Sn4—Tb1—Sn3^iv	90.0	Nb5^ix—Sn4—Nb5^xvii	57.772 (6)
Sn4^i—Tb1—Sn3^iv	90.0	Nb5^xvi—Sn4—Nb5^xvii	150.09 (4)
Sn3^ii—Tb1—Sn3^iv	120.0	Nb5^v—Sn4—Nb5^xvii	57.772 (6)
Sn3^iii—Tb1—Sn3^iv	60.0	Nb5—Sn4—Tb1	75.05 (2)
Sn3^i—Tb1—Sn3^iv	120.0	Nb5^xv—Sn4—Tb1	75.05 (2)
Sn3—Tb1—Sn3^iv	60.0	Nb5^ix—Sn4—Tb1	75.05 (2)
Sn4—Tb1—Sn3^v	90.0	Nb5^xvi—Sn4—Tb1	75.05 (2)
Sn4^i—Tb1—Sn3^v	90.0	Nb5^v—Sn4—Tb1	75.05 (2)
Sn3^ii—Tb1—Sn3^v	60.0	Nb5^xvii—Sn4—Tb1	75.05 (2)
Sn3^iii—Tb1—Sn3^v	120.0	Nb5—Sn4—Sn4^xviii	104.95 (2)
Sn3^i—Tb1—Sn3^v	60.0	Nb5^xv—Sn4—Sn4^xviii	104.95 (2)
Sn3—Tb1—Sn3^v	120.0	Nb5^ix—Sn4—Sn4^xviii	104.95 (2)
Sn3^iv—Tb1—Sn3^v	180.0	Nb5^xvi—Sn4—Sn4^xviii	104.95 (2)
Nb5—Sn2—Nb5^vi	146.807 (6)	Nb5^v—Sn4—Sn4^xviii	104.95 (2)
Nb5—Sn2—Nb5^vii	110.325 (14)	Nb5^xvii—Sn4—Sn4^xviii	104.95 (2)
Nb5^vi—Sn2—Nb5^vii	59.302 (11)	Tb1—Sn4—Sn4^xviii	180.0
Nb5—Sn2—Nb5^viii	59.302 (11)	Nb5^viii—Nb5—Nb5^xvi	180.0
Nb5^vi—Sn2—Nb5^viii	110.325 (14)	Nb5^viii—Nb5—Nb5^ix	60.0
Nb5^vii—Sn2—Nb5^viii	146.808 (6)	Nb5^xvi—Nb5—Nb5^ix	120.0
Nb5—Sn2—Nb5^ix	59.302 (11)	Nb5^viii—Nb5—Nb5^xix	120.0
Nb5^vi—Sn2—Nb5^ix	146.808 (6)	Nb5^xvi—Nb5—Nb5^xix	60.0
Nb5^vii—Sn2—Nb5^ix	146.808 (6)	Nb5^ix—Nb5—Nb5^xix	180.0
Nb5^viii—Sn2—Nb5^ix	59.302 (11)	Nb5^viii—Nb5—Sn3	60.229 (6)
Nb5—Sn2—Nb5^x	146.807 (6)	Nb5^xvi—Nb5—Sn3	119.770 (6)
Nb5^vi—Sn2—Nb5^x	59.302 (11)	Nb5^ix—Nb5—Sn3	60.229 (6)
Nb5^vii—Sn2—Nb5^x	59.302 (11)	Nb5^xix—Nb5—Sn3	119.770 (6)
Nb5^viii—Sn2—Nb5^x	146.808 (6)	Nb5^viii—Nb5—Sn3^iv	119.770 (6)
Nb5^ix—Sn2—Nb5^x	110.325 (14)	Nb5^xvi—Nb5—Sn3^iv	60.229 (6)
Nb5^viii—Sn3—Nb5^iv	146.683 (6)	Nb5^ix—Nb5—Sn3^iv	119.771 (6)
Nb5^viii—Sn3—Nb5^xi	110.033 (14)	Nb5^xix—Nb5—Sn3^iv	60.229 (6)
Nb5^iv—Sn3—Nb5^xi	59.541 (11)	Sn3—Nb5—Sn3^iv	69.967 (14)
Nb5^viii—Sn3—Nb5^xii	146.683 (6)	Nb5^viii—Nb5—Sn2	60.349 (6)
Nb5^iv—Sn3—Nb5^xii	59.541 (12)	Nb5^xvi—Nb5—Sn2	119.652 (6)
Nb5^xi—Sn3—Nb5^xii	59.541 (11)	Nb5^ix—Nb5—Sn2	60.349 (6)
Nb5^viii—Sn3—Nb5^ix	59.541 (11)	Nb5^xix—Nb5—Sn2	119.652 (6)
Nb5^iv—Sn3—Nb5^ix	146.683 (6)	Sn3^iv—Nb5—Sn2	179.854 (14)
Nb5^xi—Sn3—Nb5^ix	146.683 (6)	Nb5^viii—Nb5—Sn2^vii	119.651 (6)
Nb5^xii—Sn3—Nb5^ix	110.033 (14)	Nb5^xvi—Nb5—Sn2^vii	60.349 (6)
Nb5^viii—Sn3—Nb5	59.541 (12)	Nb5^ix—Nb5—Sn2^vii	119.652 (6)
Nb5^iv—Sn3—Nb5	110.033 (14)	Nb5^xix—Nb5—Sn2^vii	60.349 (6)
Nb5^xi—Sn3—Nb5	146.683 (6)	Sn3—Nb5—Sn2^vii	179.855 (14)
Nb5^xii—Sn3—Nb5	146.683 (6)	Sn3^iv—Nb5—Sn2^vii	110.179 (2)
Nb5^ix—Sn3—Nb5	59.541 (11)	Sn2—Nb5—Sn2^vii	69.676 (14)
Nb5^viii—Sn3—Tb1^xiii	73.341 (3)	Nb5^viii—Nb5—Sn4	118.886 (3)
Nb5^iv—Sn3—Tb1^xiii	73.341 (3)	Nb5^xvi—Nb5—Sn4	61.114 (3)
Nb5^xi—Sn3—Tb1^xiii	73.341 (3)	Nb5^ix—Nb5—Sn4	61.114 (3)
Nb5^xii—Sn3—Tb1^xiii	124.984 (7)	Nb5^xix—Nb5—Sn4	118.886 (3)
Nb5^ix—Sn3—Tb1^xiii	124.984 (7)	Sn3—Nb5—Sn4	102.205 (16)
Nb5—Sn3—Tb1^xiii	73.341 (3)	Sn3^iv—Nb5—Sn4	102.205 (17)
Nb5^viii—Sn3—Tb1^xiv	73.341 (3)	Sn2—Nb5—Sn4	77.773 (17)
Nb5^iv—Sn3—Tb1^xiv	124.984 (7)	Sn2^vii—Nb5—Sn4	77.773 (17)
Nb5^xi—Sn3—Tb1^xiv	73.341 (3)	Nb5^viii—Nb5—Sn4^xiii	61.114 (3)
Nb5^xii—Sn3—Tb1^xiv	73.341 (3)	Nb5^xvi—Nb5—Sn4^xiii	118.886 (3)
Nb5^ix—Sn3—Tb1^xiv	73.341 (3)	Nb5^ix—Nb5—Sn4^xiii	118.886 (3)
Nb5—Sn3—Tb1^xiv	124.983 (7)	Nb5^xix—Nb5—Sn4^xiii	61.114 (3)
Tb1^xiii—Sn3—Tb1^xiv	120.0	Sn3—Nb5—Sn4^xiii	102.205 (17)
Nb5^viii—Sn3—Tb1	124.984 (7)	Sn3^iv—Nb5—Sn4^xiii	102.205 (16)
Nb5^iv—Sn3—Tb1	73.341 (3)	Sn2—Nb5—Sn4^xiii	77.773 (17)
Nb5^xi—Sn3—Tb1	124.983 (7)	Sn2^vii—Nb5—Sn4^xiii	77.772 (17)
Nb5^xii—Sn3—Tb1	73.341 (3)	Sn4—Nb5—Sn4^xiii	150.09 (4)
Nb5^ix—Sn3—Tb1	73.341 (3)

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

 

Footnotes

‡Also at Institute of Chemistry, Environment Protection and Biotechnology, Jan Dlugosz University, al. Armii Krajowej 13/15, 42-200 Czestochowa, Poland.