First-principles investigation of the insulator–metal transition in layered NaNiO2: coupled electronic and lattice effects

Abstract

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 e_g 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.