Crystal Structure and Chemical Bonding of β-FeSi₂ and ε-FeSi

The semiconductor silicide β-FeSi2 has garnered significant attention due to its potential applications across photovoltaics, optoelectronics, and thermoelectric devices. The emergence of ε-FeSi during the synthesis of β-FeSi2 is a common phenomenon. The intricacies of the crystal structure and chemical bonds of β-FeSi2 and ε-FeSi are imperative for understanding their physical properties, yet these characteristics have not been thoroughly investigated. This study presents an in-depth examination of the temperature-dependent evolution of the crystal structures and chemical bonding of β-FeSi2 and ε-FeSi, utilizing high-resolution synchrotron powder X-ray diffraction and density functional theory calculations with Bader's quantum theory of atoms in molecules. The research spans a temperature range from 117.2 to 771.2 K, during which the crystal structures exhibit significant stability with no evidence of structural collapse or phase transitions. In terms of chemical bonding, the Si–Si bonds in β-FeSi2 are typically covalent, while the Fe–Si bonds display a polar covalent character with partial ionicity. ε-FeSi exhibits a similar bonding motif with Fe–Si bonds being either covalent or polar covalent. In addition, large Debye temperatures and a marginal weakening of bond strength with rising temperatures elucidate the strong bonding interactions as an origin of good thermal stability.