The stannides La3Pd4Sn6, Ce3Pd4Sn6, and Pr3Pd4Sn6: A new structure type with a complex three-dimensional [Pd4Sn6] polyanion

The new stannides La3Pd4Sn6, Ce3Pd4Sn6, and Pr3Pd4Sn6 have been synthesized in quantitative yield by reacting the elements in an are-melting furnace a nd subsequent annealing at 970 K. Their structures were determined from sin gle-crystal X-ray data: Pnma, a = 1685.5(2) pm, b = 462.37(9) pm, c = 1562. 6(2) pm, wR2 = 0.0788, 1561 F-2 values, 80 variables for La3Pd4Sn6 a = 1678 .2(3) pm, b = 458.9(1) pm, c = 1556.1(3) pm, wR2 = 0.0800, 1539 F-2 values, 81 variables for Ce3Pd4Sn6, and a = 1673.8(4) pm, b = 457.3(1) pm, c = 155 4.1(3) pm, wR2 = 0.0954, 1529 F-2 values, 81 variables for Pr3Pd4Sn6 Striki ng structural motifs in these structures are distorted PdSn5 square pyramid s which are condensed via common tin atoms and via Sn-Sn bonds forming a th ree-dimensional infinite [Pd4Sn6] polyanion that is characterized by strong Pd-Sn (256-285 pm) as well as Sn-Sn (302-336 pm) interactions. Six tin sit es occur in the Ce3Pd4Sn6 structure. Only the Sn4 atoms have no Sn-Sn conta cts. The rare earth atoms fill distorted pentagonal and hexagonal channels within the polyanion. The three crystallographically independent rare earth (RE) atoms have high coordination numbers: 5Ce + 7Pd + 9Sn for Ce1, 4Ce 6Pd + 9Sn for Ce2, and 5Ce + 7Pd + 9Sn for Ce3. Magnetic susceptibility mea surements indicate Pauli paramagnetism for La3Pd4Sn6 and Curie-Weiss behavi or for Ce3Pd4Sn6 (2.51(2) mu(B)/Ce) and Pr3Pd4Sn6 (3.70(5) mu(B)/Pr) NO mag netic ordering is detected down to 2 K. La3Pd4Sn6, Ce3Pd4Sn6, and Pr3Pd4Sn6 are metallic conductors with specific resistivities at room temperature of 80 +/- 20 mu Omega cm (La3Pd4Sn6), 65 +/- 20 mu Omega cm (Ce3Pd4Sn6), and 110 +/- 20 mu Omega cm (Pr3Pd4Sn6) The specific resistivity of Ce3Pd4Sn6 sh ows a broad minimum near 16 K, possibly suggesting some Kondo-type interact ions. The Sn-119 Mossbauer spectrum of La3Pd4Sn6 shows two superimposed sig nals: a singlet at delta(2) = 1.88(2) mm/s with a line width of Gamma(2) = 0.88(2) mm/s and a second signal at delta(1) = 1.94(2) mm/s with a line wid th of Gamma(1) = 0.87(2) mm/s, subject to quadrupole splitting of Delta E-Q 1 = 1.11(2) mm/s. These two signals occur in a ratio of about 5:1 in agreem ent with the six different tin sites. The cerium and praseodymium stannides show very similar behavior.