Beta-Polymorphs of Uranium and Thorium Disilicides

Abstract

THORIUM disilicide was first described by Brauer and Mitius¹, who isolated single crystals and reported the crystal structure to be tetragonal with the space group I4/amd. Later, Zachariasen² described an isostructural uranium disilicide observed initially during metallurgical examinations by Kaufmann, Cullity and Bitsianes³. Another phase, thought by Kaufmann et al. to be U₂Si₃ and also studied by Zachariasen, was found to have a hexagonal crystal structure of the AlB₂-type, space group C6/mmm. The crystal structure results led Zachariasen to suggest that this compound was a second form of uranium disilicide and he designated it β-USi₂. The tetragonal uranium and thorium disilicides accordingly became the alpha forms. More recently, Jacobson, Freeman, Tharp and Searcy⁴ obtained a thorium silicide isostructural with Zachariasen's β-USi₂, but observed that the silicon content was lower than that of α-ThSi₂. They used the description ‘β-ThSi₂’ but suggested that the true composition was ThSi_1.5±0.2.