First-principles investigation of the insulator–metal transition in layered NaNiO 2 : coupled electronic and lattice effects

NaNiO2 is a layered material composed of alternating NaO6 and NiO6 octahedra, which undergoes an insulator-metal transition (IMT) from a monoclinic insulating phase to a metallic phase at approximately 480 K. Although this phase transition has been experimentally observed, its microscopic mechanism remains unclear, particularly regarding the role of Jahn-Teller (JT) distortion and the nature of the structural dynamics during the transition. In this work, we carry out a comprehensive first-principles study to address these issues. Our results show that the IMT is primarily driven by the gradual disappearance of the JT distortion of Ni3+, which restores the eg orbital degeneracy and enables electronic delocalization. Furthermore, potential energy surface analysis and phonon spectrum calculations reveal that this process follows a displacive phase transition pathway, consistent with experimental observations. These findings provide, for the first time, a theoretical explanation of the microscopic mechanism underlying the IMT in NaNiO2, thereby clarifying its structural-electronic interplay and offering new insights into the phase transition behavior of transition-metal oxides for future material applications.