Ca-Pd/Pt-Ge Compounds on the Zintl Border

Abstract: Zintl compounds form a category in intermetallic chemistry somewhere between metallic and ionic compounds. Classical Zintl compounds form between electropositive and electronegative elements, such as the alkaline earth metals and the p-block elements, i.e. elements with a relatively large electronegativity difference. The concept is based on the idea of a complete charge transfer from the electropositive to the electronegative element, leaving the electropositive element with a complete valence shell. The electronegative element will behave like its isoelectronic counterpart and complete its valence shell by forming a covalent anionic network, if necessary. Due to this charge separation, Zintl compounds are semiconductors. This classification is a useful tool for predicting and describing a structure, but what happens when a third element with similar electronegativity as the p-block element is added? The work covered in this thesis is based on how well the structures formed in the Ca-Pd/Pt-Ge system can be rationalised by the Zintl concept. The system has shown a large structural variability ranging from Laves phases to Zintl-like cluster compounds. What they all have in common is the network formation between the transition metal and Ge, and the compounds are all more metallic rather than semiconducting. Despite this metallicity there is a tendency toward a Zintl-like behaviour. That is, when there are more electrons available for palladium, platinum and germanium, the tendency to form extended anionic network becomes less.