Quantifying the role of the lattice in metal-insulator phase transitions

For phase transitions involving concomitant structural and electronic changes, it has been challenging to determine which component provides the dominant contribution. Here, a general formalism capable of resolving this question is introduced and applied to metal-insulator transitions in two material families. Many materials exhibit phase transitions at which both the electronic properties and the crystal structure change. Some authors have argued that the change in electronic order is primary, with the lattice distortion a relatively minor side-effect, and others have argued that the lattice distortions play an essential role in the energetics of the transition. In this paper, we introduce a formalism that resolves this long-standing problem. The methodology works with any electronic structure method that produces solutions of the equation of state determining the electronic order parameter as a function of lattice distortion. We use the formalism to settle the question of the physics of the metal-insulator transitions in the rare-earth perovskite nickelates (RNiO3) and Ruddlesden-Popper calcium ruthenates (Ca2RuO4) in bulk, heterostructure, and epitaxially strained thin film forms, finding that electron-lattice coupling is key to stabilizing the insulating state in both classes of materials.

Automated summary

Determining the dominant contribution in phase transitions involving structural and electronic changes has been challenging. In this study, a general formalism is introduced to address this problem and is applied to metal-insulator transitions in two material families. The results reveal that electron-lattice coupling is key to stabilizing the insulating state in both rare-earth perovskite nickelates (RNiO3) and Ruddlesden-Popper calcium ruthenates (Ca2RuO4).