Metal-insulator transition in FeSex originating in an anomalous lattice effect

We present a comprehensive investigation of the structural, electrical transport, and magnetic properties of ${\mathrm{FeSe}}_{x}$ ($x$ = 1.14, 1.18, 1.23, 1.28, and 1.32) to unravel the mechanism of the metal-insulator transition observed in these systems. For this, we systematically evaluated the structural parameters of ${\mathrm{FeSe}}_{\mathit{x}}$ as a function of Se concentration and temperature. We observe increased lattice constants and cell volume with increased Se concentration. On the other hand, the temperature-dependent x-ray diffraction studies suggest unusual lattice change around the metal-insulator (MI) transition temperature of the respective compositions. This remarkable observation suggests that the MI transition originates in the anomalous lattice effect in these systems. Additionally, our density of states (DOS) calculations on ${\mathrm{FeSe}}_{1.14}$ qualitatively explain the MI transition, as the low-temperature (50 K) structure DOS suggests a metallic nature and the high-temperature (300 K) structure DOS shows a gap near the Fermi level.