Ternary Intermetallic Palladium Compounds with Anionic Partial Structures

149 The description of chemical bonding in intermetallic compounds is a demanding task since the well known 8-N rule, which holds for insulating or semiconducting valence compounds, is in general not valid for metallic systems. In order to study the effects of factors like valence electron concentration or atomic volume on pattern formation, we started to systematically investigate atomic arrangements, homogeneity ranges and polymorphism of transition metal compounds comprising a main group element like germanium or gallium. Ternary equiatomic intermetallic compounds have been studied in some detail for systems M1 – M2 – L, where M1 = rare-earth metal, M2 = transition metal and L = aluminium, gallium, silicon or germanium. Most of these compounds adopt crystal structures which are distorted substitution varieties of the AlB2 type and some of these are interesting as perspective materials, for example ZrRuSi [1], LaRhAl and YRhAl [2] exhibit superconducting properties at low temperatures. In order to study chemical bonding systematically, we investigated new compounds M1 – Pd – {Ga,Ge} where M1 = Ti, Zr and Hf. The new ternary compounds are prepared from the elements by arc melting. Thermal treatment of the samples is performed in sealed alumina crucibles which are placed in evacuated silica ampoules and finally quenched in cold water. The crystal structures are studied by X-ray single-crystal diffraction or Rietveld refinement of powder diffraction data (TiPdGa). The compositions obtained from crystal structure refinements are in agreement with those determined by EDAX for all synthesized compounds with typical differences being less than one at. % for each element. With respect to physical properties, samples of TiPdGa and ZrPdGa exhibit high overall values of the electrical resistivity (Fig. 1). In combination with an almost linear increase with temperature the finding indicates that the compounds are bad metals. Among the investigated compounds, only Ti1+xPd1–xGa exhibits a significantly broad homogeneity range from TiPdGa to about Ti1.25Pd0.75Ga evidenced by varying unit cell parameters and analytically determined composition differences. The relation between volume and composition of samples Ti1+xPd1–xGa is linear so that Vegard’s rule is fulfilled. The crystal structure of TiPdGa (P63/mmc, a = 4.3924(1) Å, c = 5.4593(1) Å) is isotypic to that of ZrBeSi [3] and represents an undistorted, ordered decoration variety of the AlB2 type. The corresponding germanium compound, TiPdGe exhibits temperature polymorphism with a transition temperature of about 1240 °C. Thus, the low-temperature (LT) modification (Pnma, a = 6.3707(7) Å, b = 3.8574(3) Å, c = 7.5372(6) Å) was synthesized at 800 °C and exhibits a TiNiSi type atomic pattern [4]. The high temperature (HT) modification (P62m, a = 6.6030(4) Å, c = 3.6998(3) Å) is obtained by heat treatment at 1275°C and realizes a ZrNiAl type pattern [5]. Both atomic arrangements are deformation varieties of the AlB2 type. The atomic pattern of HfPdGa (P62c, a = 7.1572(2) Å, c = 6.8945(4) Å, HfRhSn type [6]) is a variety of the ZrNiAl type while zirconium palladium gallide ZrPdGa (Pnma, a = 6.9182(5) Å, b = 3.7186(4) Å, c = 15.975(2) Å) is isotypic to LaNiAl [7]. The crystal structure of HfPdGe represents a new crystal structure type. Analysis of reflections intensities in the initially detected hexagonal metric of Ternary Intermetallic Palladium Compounds with Anionic Partial Structures