Structure and properties of the compounds LnAl(2)X(2) (Ln = Eu, Yb; X = Si, Ge)

EuAl2Si2, EuAl2Ge2 and YbAl2Ge2 were synthesized by heating the elements at 1070-1270 K and characterized by single-crystal X-ray methods. They are isotypic and crystallize in the CaAl2Si2-type structure (space group P (3) over bar m1) with the lattice constants (Angstrom): YbAl2Ge2: a = 4.179(2), c = 7.069(3). EuAl2Ge2: a = 4.214(1), c = 7.320(1). EuAl2Si2: a = 4.181(1), c = 7.259(1). Magnetic susceptibility measurements of EuAl2Si2 and EuAl(2)G(2) show paramagnetic behavior above 50 K with experimental magnetic moments of 7.82(5) mu(B)/Eu and 7.90(5) mu(B)/Eu indicating divalent europium. Antiferromagnetic ordering is detected at T-N = 35.5(5) K for EuAl2Si2 and at T-N = 27.5(5) K for EuAl2Ge2. Both compounds undergo metamagnetic transitions at low temperatures. Previously described YbAl2Si2 shows the typical behavior of an intermediate-valent compound. Between 100 and 300 K the inverse susceptibility linearly depends on temperature with a reduced moment of 2.57(5) mu(B)/Yb and a strongly negative paramagnetic Curie temperature of -382(5) K. Below 100 K the degree of divalent ytterbium increases. YbAl2Ge2 is a Pauli paramagnet with a room temperature susceptibility of 1.2(1) x 10(-9) m(3) mol(-1). All compounds are metallic conductors between 8 and 320 K. Eu-151 Mossbauer spectroscopic measurements of EuAl2Si2 and EuAl2Ge2 show isomer shifts of -10.3(1) and -10.8(2) mm s(-1), respectively, at 4.2 K in accordance with divalent europium. Full magnetic hyperfine field splitting is detected at 4.2 K. LMTO band structure calculations confirm the metallic properties for all compounds and result a fully polarized 4f(7) state for EuAl2Ge2 and EuAl2Si2. For the Yb-comnpounds nonmagnetic 4f(14) ground states were predicted, but the high 4f-contribution at the fermi level indicates the tendency to intermediate valency in YbAl2Si2. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.