The new compounds SEPdP (SE = La—Gd), which were prepared by heating the elements, crystallize with a modified Ni2In-type structure. The phosphides of SE = Dy—Lu attain the orthorhombic TiNiSi-type structure. A comparison of the cell volumes suggests that europium has intermediate valence in EuPdP.
The electronic structure and bonding of some ternary AlB2-type compounds AMX (A=2a-, X=5b-element, M=transition metal) were studied by means of Extended-Hückel and TBLMTO- ASA calculations. Previous geometric stability criteria of the AIB2 structure have been extended, and reasons for some structural peculiarities and distortions are given. A simple orbital interaction scheme based on local symmetry is used as starting point to investigate the electronic properties of these compounds, whereas the band structures of the ZrBeSi-type SrPdAs and the Peierls-distorted variants CaPdAs and BaPdP are described in more detail. The results of calculations and resistivity measurements are discussed within the scope of Zintl’s concept under consideration of the transition metal d-orbitals. It turns out that these solids can be described as metallic Zintl compounds.
BaCu6Ge2 S8 was synthesized by direct reaction of the elements at 750°C and characterized by X-ray single crystal techniques. The thiogermanate crystallizes in a new orthorhombic structure type (a = 6.122(1) Å, b = 12.084(3) Å, c = 17.614(5) Å; Pbcm, Z = 4). Isolated [GeS4]4- tetrahedra form a slightly distorted cubic face-centered (fee) arrangement. Baand Cu-atoms each occupy half the octahedral gaps (OG) of this “tetrahedra packing” . Further Cu atoms fill the tetrahedral gaps ( TG) completely. The compound can be written as Ba2(1/2OG)(Cu2)2(1/2OG)Cu8(TG)(GeS4)4 . Thus the structure of BaCu6Ge2S8 can be derived from the Li3Bi type. The sulfur coordination of the metal atoms (Cu tetrahedral or trigonal, Ba with CN 8) are realized by the spatial orientation of the [GeS4]4- tetrahedra. Their centers nearly maintain the fcc-arrangement. This structure interpretation of BaCu6 Ge2S8 from the viewpoint of a “filled tetrahedra packing” is discussed for further known thiogermanate compounds.