DIMORPHIC YBPDSN WITH ZRNIAL AND TINISI TYPE-STRUCTURE

Two modifications of YbPdSn were prepared from the elements in sealed tantalum tubes at 870 K (alpha-YbPdSn) and 1400 K (beta-YbPdSn). Both structures were refined from single crystal X-ray data: alpha-YbPdSn ( ZrNiAl type structure), space group <P(6)over bar>2m, a = 759.0(2) pm, c = 376.98(9) pm, wR2 = 0.0541, 344 F-2 values, 14 variables; beta-Yb PdSn (TiNiSi type structure), space group Pnma, a = 718.7(1) pm, b = 4 58.3(1) pm, c = 796.1(1) pm, wR2 = 0.0455, 391 F-2 values, 20 variable s. Structural features of both modifications are palladium-centered tr igonal prisms formed by the ytterbium and tin atoms. The structural si milarities between alpha- and beta-YbPdSn and the relationship with th e aristotype AlB2 is discussed in the framework of a group-subgroup sc heme. Magnetic susceptibility data show Curie-Weiss behaviour for alph a-YbPdSn with mu(exp) = 4.2(1) mu(B) and theta = -75(2) K, indicating trivalent ytterbium. The susceptibility of beta-YbPdSn can be fitted t o a modified Curie-Weiss law with mu(exp) = 1.6(1) mu(B), theta = -5(1 ) K and chi(0) = 3.0 x 10(-9) m(3)/mol, suggesting a tendency towards divalent ytterbium. Temperature dependent resistivity measurements ind icate metallic behaviour with room temperature values of 50 +/- 20 mu Omega cm for alpha-YbPdSn and 1300 +/- 100 mu Omega cm for beta-YbPdSn . Sn-119 Mossbauer spectroscopic data show only one signal at delta = 1.84(2)mm/s (alpha-YbPdSn) and delta = 1.96(4)mm/s (beta-YbPdSn) subje cted to quadrupole splitting of Delta Eq = 0.73(4) mm/s and Delta Eq = 0.97(5) mm/s, respectively.