Structure and properties of the compounds LnAl2X2 (Ln=Eu, Yb; X=Si, Ge)

Abstract

Abstract 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 P3m1) with the lattice constants (A): 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 EuAl2Ge2 show paramagnetic behavior above 50 K with experimental magnetic moments of 7.82(5) μB/Eu and 7.90(5) μB/Eu indicating divalent europium. Antiferromagnetic ordering is detected at TN=35.5(5) K for EuAl2Si2 and at TN=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) μB/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)×10−9 m3 mol−1. All compounds are metallic conductors between 8 and 320 K. 151Eu 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 4f7 state for EuAl2Ge2 and EuAl2Si2. For the Yb-compounds nonmagnetic 4f14 ground states were predicted, but the high 4f-contribution at the fermi level indicates the tendency to intermediate valency in YbAl2Si2.