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.

So far, no ternary compounds in the RE-Nb-Sn (RE -rare-earth metals) systems are known. TbNb 6 Sn 6 is the first ternary Rare earth -Niobium -Tin compound. According to the X-ray single-crystal data the TbNb 6 Sn 6 compound crystallizes with hexagonal symmetry (space group P6/mmm, HfFe 6 Ge 6 structure type). In TbNb 6 Sn 6 planar graphite-type layers of Sn atoms and Kagome nets of Nb atoms alternate along the c axis, similar to the recently reported MgCo 6 Ge 6 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- The HfFe 6 Ge 6 structure type (Schobinger-Papamantellos et al.,1998), also referred to MgFe 6 Ge 6 or LiCo 6 Ge 6 , can be described as an intergrowth of the Zr 4 Al 3 (Wilson et al., 1960) and CaCu 5 (Nowotny, 1942) structure types (see Fig.2).
Another possibility to describe the HfFe 6 Ge 6 structure is a transformation of the CaCu 5 structure via multiple substitution and ordering of atoms. The first step is the doubling of the CaCu 5 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 TiBe 12 (Raeuchle & Rundle, 1952). As a result, the c/a ratio increases to 1.733. Further ordering of X atoms in the TiBe 12 leads to the HfFe 6 Ge 6 structure. Perhaps the presence of atoms of different radii leads to closer packing of the layers in the ternary HfFe 6 Ge 6 compared to the binary TiBe 12 . Therefore, the c/a ratio of 1.591 for ternary HfFe 6 Ge 6 is much closer to the ideal value of 1.596 (c/a = 1.6546 for TbNb 6 Sn 6 ).

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 TbNb 6 Sn 6 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 sup-2 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 TbNb 6 Sn 6 phase. However, after grinding, pressing and annealing it at 900°C for 12 h a significant amount of the TbNb 6 Sn 6 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 Nb 3 Sn impurity.

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. Fig. 1. View of the TbNb 6 Sn 6 structure. The graphite-type Sn layers, Ge 2 dumbbells and condensed Nb 3 Sn 2 bipyramids are emphasized.