Samson Phases: Interplay of Theoretical Ideas and the Synthesis of New Phases

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This project addresses the factors stabilizing a series of complex (1000 atom/unit cell) intermetallic phases in the sodium-cadmium (NaCd2 ), copper-cadmium (Cu4 Cd3 ), and aluminum-magnesium (Al3 Mg2 ) systems. Preliminary quantum mechanical calculations suggest their unusual complexity results from the insertion of defect planes into a simple crystal structure breaking it up into nanometer-scale fragments. As the boundaries between these fragments are periodically arrayed, they are amenable to study by X-ray crystallography. This provides a rare glimpse at grain boundary phenomena that influence material properties. Theoretical calculations will be used to examine how these boundaries confer stability to Samson's structures. This project will also target the synthesis of new phases exhibiting these boundaries, and reevaluate Samson's original structures which have hints of undetected super-structuring. The field of intermetallics is specialized due largely to the shear number of phases with no theory to interrelate them. This project aims to develop this connective theory within the same language that makes organic and inorganic chemistry accessible for students, namely molecular orbital theory.%%%This project supports a collaboration between the PI, as an NSF MPS Distinguished Postdoctoral Fellow, and the research group of Professor Sven Lidin at Stockholm University, Sweden. It is aimed at uncovering the factors stabilizing solid-state compounds formed between metallic elements. It will address why some ratios of metallic elements lead to simple alloy structures (with two to four atoms/unit cell) while others give the extremely complex structures discovered by Sten Samson in the 1960s (1000 atom/unit cell). Preliminary quantum mechanical calculations suggest these complex structures result from the insertion of defect planes into a simple crystal structure. Unlike typical material defects, in which the defects occur randomly, in Samson's phases the defects make regular patterns. This regularity allows the defects to be seen at atom-by-atom resolution with the use of X-ray crystallography, providing a rare glimpse at the defect phenomena so important to material properties. Theoretical calculations will examine how these defect planes confer stability to Samson's structures. Experimental work will target the synthesis of new compounds exhibiting these structural features, and the reevaluation of Samson's original compounds, which are probably even more complicated than could be seen with the technology of the time. The field of compounds between metallic elements is specialized, largely due to the shear number of compounds existing with no theory to interrelate them. This project aims to develop this connective theory within the language that makes organic and inorganic chemistry accessible for students - that of molecular orbital theory.