Crystal Structure\nand Chemical Bonding of β‑FeSi<sub>2</sub> and ε‑FeSi

The\nsemiconductor silicide β-FeSi<sub>2</sub> has\ngarnered\nsignificant attention due to its potential applications across photovoltaics,\noptoelectronics, and thermoelectric devices. The emergence of ε-FeSi\nduring the synthesis of β-FeSi<sub>2</sub> is a common phenomenon.\nThe intricacies of the crystal structure and chemical bonds of β-FeSi<sub>2</sub> and ε-FeSi are imperative for understanding their physical\nproperties, yet these characteristics have not been thoroughly investigated.\nThis study presents an in-depth examination of the temperature-dependent\nevolution of the crystal structures and chemical bonding of β-FeSi<sub>2</sub> and ε-FeSi, utilizing high-resolution synchrotron powder\nX-ray diffraction and density functional theory calculations with\nBader’s quantum theory of atoms in molecules. The research\nspans a temperature range from 117.2 to 771.2 K, during which the\ncrystal structures exhibit significant stability with no evidence\nof structural collapse or phase transitions. In terms of chemical\nbonding, the Si–Si bonds in β-FeSi<sub>2</sub> are typically\ncovalent, while the Fe–Si bonds display a polar covalent character\nwith partial ionicity. ε-FeSi exhibits a similar bonding motif\nwith Fe–Si bonds being either covalent or polar covalent. In\naddition, large Debye temperatures and a marginal weakening of bond\nstrength with rising temperatures elucidate the strong bonding interactions\nas an origin of good thermal stability.