New Yb2Pt3Sn5 type stannides

The stannides CaErPt3Sn5 CaTmPt3Sn5, CaYbPt3Sn5, and CaLuPt3Sn5 were prepar ed from the elements in glassy carbon crucibles under an argon atmosphere i n a high-frequency furnace. Their Yb2Pt3Sn5 type structure (space group Pnm a) was refined from single-crystal X-ray data: a = 736.4(1) pm, b = 444.11( 9) pm, c = 2639.9(4) pm, wR = 0.0905, 1403 F-2 values, 63 variables for the erbium; a = 733.9(2) pm, b = 443.0(1) pm, 2634.8(5) pm, wR2 = 0.1430, 1372 F-2 values, 63 variables for the thulium; a = 733.9(1)pm, b = 443.3(1)pm, 2635.7(6) pm, wR2 = 0.0734, 2079 F-2 values, 64 variables for the ytterbium ; and a = 735.0(2)pm, b = 441.6(2)pm, 2634.1(7) pm, wR2 = 0.1202, 1344 F-2 values, 63 variables for the lutetium compound, The Yb2Pt3Sn5 structure is built up from a complex threedimensional [Pt3Sn5] polyanion in which the yt terbium atoms fill distorted hexagonal channels. The two crystallographical ly different ytterbium sites have coordination numbers (CN) 20 and 18, resp ectively. In the erbium, thulium, and lutetium compound, the CN 20 position is occupied exclusively by calcium atoms, while a mixed calcium-rare-earth occupancy is observed for the CN 18 position. In tbe ytterbium compound bo th positions show mixed occupancy. The refinements resulted in the composit ions Ca1.33Er0.67Pt3Sn5, Ca1.45Tm0.55Pt3Sn5, Ca1.28Yb0.72Pt3Sn5, and Ca1.63 Lu0.37Pt3Sn5 for the crystals investigated. These results were confirmed by magnetic susceptibility measurements. The erbium and the thulium compound show Curie-Weiss behavior with experimental effective magnetic moments of 9 .5(2) and 8.4(2) mu(B), respectively. At 4.8(1) K, the erbium compound orde rs ferro- or ferrimagnetically, while antiferromagnetic ordering at T-N = 5 .0(1) K is observed for the thulium compound. CaYbPt3Sn5 shows Curie-Weiss behavior above 100 K with an experimental magnetic moment of 2.8(2) mu(B) a nd Theta = -71(5) K, indicating mixed-valent behavior. CaLuPt3Sn5 shows a d iamagnetic signal of -7.8 x 10(-9) m(3)/mol. (C) 2000 Academic Press.