Structurally Triggered Metal-Insulator Transition in Rare-Earth Nickelates

Rare-earth nickelates form an intriguing series of correlated perovskite oxides. Apart from LaNiO3, they exhibit on cooling a sharp metal-insulator electronic phase transition, a concurrent structural phase transition and a magnetic phase transition toward an unusual antiferromagnetic spin order. Appealing for various applications, full exploitation of these compounds is still hampered by the lack of global understanding of the interplay between their electronic, structural and magnetic properties. Here, we show from first-principles calculations that the metal-insulator transition of nickelates arises from the softening of an oxygen breathing distortion, structurally triggered by oxygen-octahedra rotation motions. The origin of such a rare triggered mechanism is traced back in their electronic and magnetic properties, providing a united picture. We further develop a Landau model accounting for the evolution of the metal-insulator transition in terms of the $R cations and rationalising how to tune this transition by acting on oxygen rotation motions.Comment: Submitted in Nature Communicatio

Electron-Lattice Interplays in LaMnO3 from Canonical Jahn-Teller Distortion Notations

LaMnO$_3$ is considered as a prototypical Jahn-Teller perovskite compound, exhibiting a metal to insulator transition at $T_{JT} = 750K$ related to the joint appearance of an electronic orbital ordering and a large lattice Jahn-Teller distortion. From first-principles, we revisit the behavior of LaMnO$_3$ and show that it is not only prone to orbital ordering but also to charge ordering. Both charge and orbital orderings appear to be enabled by rotations of the oxygen octahedra and the subtle competition between them is monitored by a large tetragonal compressive strain, that is itself a Jahn-Teller active distortion. Equally, the competition of ferromagnetic and antiferromagnetic orders is slave of the same tetragonal strain. Our results further indicate that the metal to insulator transition can be thought as a Peierls transition that is enabled by spin symmetry breaking. Therefore, dynamical spin fluctuations in the paramagnetic state stabilize the insulating phase by the instantaneous symmetry breaking they produce and which is properly captured from static DFT calculations. As a basis to our discussion, we introduce canonical notations for lattice distortions in perovskites that distort the oxygen octhedra and are connected to charge and orbital orderings

From charge- to orbital-ordered metal-insulator transition in alkaline-earth ferrites

While CaFeO3 exhibits upon cooling a metal-insulator transition linked to charge ordering, SrFeO3 and BaFeO3 keep metallic behaviors down to very low temperatures. Moreover, alkaline-earth ferrites do not seem prone to orbital ordering in spite of the d4 formal occupancy of Fe4+. Here, from first-principles simulations, we show that the metal-insulator transition of CaFeO3 is structurally triggered by oxygen rotation motions as in rare-earth nickelates. This not only further clarifies why SrFeO3 and BaFeO3 remain metallic but allows us to predict that an insulating charge-ordered phase can be induced in SrFeO3 from an appropriate engineering of oxygen rotation motions. Going further, we unveil the possibility to switch from the usual charge-ordered to an orbital-ordered insulating ground state under moderate tensile strain in CaFeO3 thin films. We rationalize the competition between charge and orbital orderings, highlighting alternative possible strategies to produce such a change of ground state, also relevant to manganite and nickelate compounds. © 2018 American Physical Society