Optical properties of RCd3P3 (R: Ce or La) compounds: insulator–metal transition induced by displacement of atoms in the unit cell

We examined the electronic structures and optical properties of single crystals of RCd3P3 (R = Ce or La). Our first-principles analysis indicates that CeCd3P3 and LaCd3P3 exhibit semiconductor characteristics with narrow energy gaps of ~0.51 and 0.70 eV, respectively. Notably, a slight displacement of the Cd and P atoms within the unit cell significantly transforms the electronic structure from insulating to metallic state. Optical spectroscopy of both compounds reveals a metallic state with a low charge carrier density, suggesting a finite density of states at the Fermi level. A comparison between the theoretical electronic structures and experimental optical properties elucidates the observed metallic behavior. Additionally, the notable modification of the infrared-active phonons strongly indicates a structural phase transition in these compounds. Our findings also suggest that CeCd3P3 serves as a suitable platform for investigating the photoinduced Kondo effect due to its metallic ground state with limited charge carriers. This study explores the unique properties of certain materials called RCd3P3 compounds, which have a special arrangement of atoms that can lead to interesting electronic behaviors. Researchers used a combination of experiments and theoretical calculations to investigate these compounds. They grew single crystals of the materials and measured their optical properties at various temperatures. They also performed detailed calculations to predict how the atoms in the material might move and affect its electronic properties. The key finding is that small changes in the positions of certain atoms (Cd1 and P1) can cause the material to switch between acting like a metal and a semiconductor. This behavior is linked to a structural phase transition, where the arrangement of atoms changes slightly. Researchers conclude that understanding these transitions could help in designing new materials with specific electronic properties. This summary was initially drafted using artificial intelligence, then revised and fact-checked by the author. Broadband optical spectroscopy is used to obtain optical spectra of RCd₃P₃ (R: Ce or La), which exhibit a Fermi-liquid behavior with a very low charge carrier density. Notably, our first-principles calculations suggest that subtle displacements of Cd1 and P1 atoms within the unit cell can induce a semiconductor-to-metal transition, emphasizing the sensitivity of electronic structures to atomic positioning. The temperature-dependent anomalies of infrared-active phonons suggest a structural phase transition in these compounds. Our findings will offer a fundamental understanding of structural distortions leading to electronic transitions, which may be relevant for broader applications in correlated electron systems.