THE BINARY SILICIDES EU5SI3 AND YB3SI5 - SYNTHESIS, CRYSTAL-STRUCTURE, AND CHEMICAL BONDING

The binary silicides Eu5Si3 and Yb3Si5 were prepared from the elements in sealed tantalum tubes and their crystal structures were determined from single crystal X-ray data: I4/mcm, a = 791.88(7)pm, c = 1532.2(2 )pm, Z = 4, wR2 = 0.0545, 600 F-2 values, 16 variables for EU5Si3 (Cr5 B3-type ) and P (6) over bar 2m, a = 650.8(2)pm, c = 409.2(1)pm, Z = 1 , wR2 = 0.0427, 375 F-2 values, 12 variables for Yb3Si5 (Th3Pd5 type). The new silicide Eu5Si3 contains isolated silicon atoms and silicon p airs with a Si-Si distance of 242.4 pm. This silicide may be described as a Zintl phase with the formula [5EU(2+)](10+)[Si](4-)[Si-2](6-). T he silicon atoms in Yb3Si5 form a two-dimensional planar network with two-connected and three-connected silicon atoms. According to the Zint l-Klemm concept the formula of homogeneous mixed-valent Yb3Si5 may to a first approximation be written as [3Yb](8+)[2Si(-)](2-)[3Si(2-)](6-) . Magnetic susceptibility investigations of Eu5Si3 show Curie-Weiss be haviour above 100K with a magnetic moment of 7.85(5) mu(B) which is cl ose to the free ion value of 7.94 mu(B) for EU2+. Chemical bonding in Eu5Si3 and Yb3Si5 was investigated by semi-empirical band structure ca lculations using an extended Huckel hamiltonian. The strongest bonding interactions are found for the Si-Si contacts followed by Eu-Si and Y b-Si, respectively. The main bonding characteristics in EU5Si3 are ant ibonding Si1(2)-pi and bonding Eu-Si1 states at the Fermi level. The same holds true for the silicon polyanion in Yb3Si5.