EuGa2±xGe4∓x:: preparation, crystal chemistry and properties

The title compound was prepared from a mixture of the elements by melting in a glassy carbon crucible (HF furnace, argon atmosphere). EuGa2+/-x Ge-4divided byx crystallizes in a new structure type (orthorhombic symmetry, space group Cmcm (no. 63), a=4.1571(3) Angstrom, b=11.268(1) Angstrom, c=13.155(1) Angstrom, V=616.2(2) Angstrom(3), Z=4, Pearson symbol oC28) and is characterized by a 3D framework of 4-fold bonded (4b) E atoms (E=Ga,Ge) with channels parallel to the short a-axis. The europium atoms lie in the E-18 holes of those channels. The structure can also be described as an intergrowth of segments from BaAl2Si2 and from a reconstructed diamond type of structure. The (Ga,Ge)(24) framework of the title compound is equivalent to that of the Si(Al) atoms in the zeolite structure of CsAlSi5O12. Magnetic susceptibility measurements show that EuGa2+/-xGe4-/+x undergoes a transition from a paramagnetic to an antiferromagnetic ordered state at about 9 K. In the paramagnetic region, an oxidation state of 2+ for europium (mu(eff)approximate to8 mu(B)/Eu) was obtained. The crystal chemical formula of EuGa2Ge4 is Eu2+[(4b)Ga1-](2)[(4b)Ge-0](4) according to a charge balanced Zintl phase. This was confirmed by quantum chemical calculations (TB-LMTO, ELF). However, like several other similar compounds, e.g. alkaline earth metal clathrates, EuGa2+/-xGe4-/+x has a metal-like temperature dependence of the electrical resistivity with a low charge-carrier concentration in agreement with the low value of the calculated density of states. (C) 2002 Elsevier Science B.V. All rights reserved.