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.