Iron antimonide (FeSb2) is a prototypical correlated narrow-gap system that exhibits a metal−insulator-like crossover and a concomitant diamagnetic-to-paramagnetic transition. However, the microscopic physical origin of these anomalous electrical and magnetic behaviors has been the subject of a long-standing debate. Two competing physical pictures have been proposed: the Kondo insulator (KI) framework, which attributes the gap formation to Kondo hybridization, and the spin-state excitation (SSE) mechanism, which emphasizes a thermally activated transition from a low-spin state to a high-spin state. In this review, we provide a critical overview of key experimental results from electrical transport, magnetic properties, and a broad range of spectroscopic probes, assessing how each supports or challenges these competing physical pictures. Particular attention is paid to recent advances in element- and orbital-sensitive spectroscopies, such as X-ray absorption spectroscopy (XAS), which together offer direct insights into the evolution of the Fe spin state and electronic structure. Additional anomalous properties, such as the colossal thermoelectric effect and spin-structure instabilities, are also discussed in the end. These emerging features not only underscore the complexity of this correlated narrow-gap system but also suggest rich, unresolved physics beyond existing models, inviting further investigation into its fundamental behavior and potential applications.
Li et al. (Thu,) studied this question.