Scientific publication

Nickelates : conductor or insulator?

In Research

Depending on whether or not they conduct electricity, materials are typically classified as either conductors or insulators. But is it that simple? Can the same material eventually change its behaviour depending on the environment in which it is placed? This is the case of some compounds such as nickelates, a series of complex oxides of perovskite structure. Depending on the temperature, these materials can change from electrical conductor to insulator in an abrupt and reversible way, a transformation that is as unusual as it is intriguing.  In an Article published in Nature Communications (1), Alain Mercy and Philippe Ghosez of the Group of Theoretical Materials Physics, within the Q-MAT Center of  the CESAM Research Unit, reveal the microscopic mechanism behind this change of state.

"No materials, no devices ! " tells us Prof. Philippe Ghosez, Director of the CESAM Research Unit and specialist of functional materials. « Just as culinary innovation is based on the discovery and combination of ingredients with specific flavours, device innovation relies on the discovery and combination of materials with original properties." The theoretical study of the properties of materials at the atomic scale, as performed within his team, is in itself a fundamental research usually ignored from the general public. It lays nevertheless the necessary foundations for future technological innovation and is even the cornerstone of the latter: not only do most of the devices that surround us (computers, tablets, smartphones,...) require the combination of materials with specific and varied properties, but the discovery of new materials with unexpected behaviours will be at the origin of new devices, the existence of which is not yet suspected simply because we cannot imagine that what they can achieve is possible!

Synthesized for the first time in 1971, rare earth nickelates (of chemical formula ReNiO3 where Re is a rare earth element appearing in the lower part of the famous Mendeleev Table) have attracted the interest of researchers since their discovery because of their ability to change abruptly from a conducting to an insulating state at a specific temperature ranging from 0 to 600 Kelvin, depending on the compound under consideration. Although widely debated, the origin of this transformation has remained until now relatively enigmatic, making its concrete use in devices premature.  The most commonly accepted model was relating the metal-insulating transition to a purely electronic instability induced by strong electron interactions (a so-called Mott transition). 

Ghosez Mercy Nickelates

When the heat and cold blow on nickelates. At high temperatures, all nickel atoms (green atoms) of nickelates are equivalent and possess one valence electron that is delocalized within the crystal producing a conductive state. Below a critical temperature, these electrons come together and are located in pairs on every second nickel atom, producing a sudden and reversible transition to an insulating state. Often associated with strong electron interactions, this transformation is explained today as the result of a specific and progressive distortion of the crystal with temperature.  This atomic distortion corresponds to a "breathing" of the oxygen cages (red atoms forming cages) that surround these electrons and traps them within the large cages (blue cages). Acting on this distortion opens the door to effective manipulation and exploitation of the metal-insulating transition. (Figure: Alain Mercy, copyright: ©PhyTheMa-ULiège)

The study lead by Alain Mercy and Philippe Ghosez reveals that this is not the case! They demonstrate that the transition is primarily of structural origin and driven by a specific distortion of the crystal resulting from an unusual coupling between different atomic vibration of the crystal. They also explain the origin of this coupling in connection with the very specific electronic properties of these compounds (a so-called Peierls transition, but of a new type). This discovery is the output of a "first-principles" study, an extremely powerful theoretical approach based on the equations of quantum mechanics and electromagnetism, allowing the exploration and understanding of the properties of materials at the atomic scale.

Alain Mercy, first author of the publication, is particularly happy: “This study required more than three years of intensive researches and thousands of hours of computation on the big and powerful computers of the CECI (Consortium of Equipments for Intensive Computing). But the game was worth it! Not only does our study provide for the first time a unified picture of the metal-insulating transition in the intriguing family of nickelates, but it also provides the keys to manipulate this transition in thin films (materials in the form of ultra-thin layers) and nanostructures (materials structured at the atomic scale), opening the way to future concrete applications. We are currently finalising another article with experimentalists of the University of Twente (The Netherlands), which highlights this potential in more concrete terms. »

For Philippe Ghosez, this work is also part of a broader context." Not only has the mechanism identified in nickelates never been mentioned before, but it is also relevant in other families of compounds such as ferrites (AFeO3) or manganites (AMnO3). This discovery is therefore also a major step forward in the long term perspective of achieving a unified understanding of the behaviour of perovskite oxides, one of the families of inorganic compounds that are the most widely used in functional devices today.”

Scientific reference

(1) Structurally triggered metal-insulator transition in rare-earth nickelates. A. Mercy, J. Bieder, J. Iñiguez and Ph. Ghosez, Nature Communications 8, 1677 (2017)


Alain MERCY I UR CESAM I Phythema I +32(0)4 366 37 22

Pr. Philippe GHOSEZ I URM CESAM I Phythema I +32(0)4 366 36 11

This study was conducted by Alain MERCY (doctoral student) and Philippe GHOSEZ (director of UR CESAM) within the group of Theoretical Physics of Materials, Q-MAT pole, CESAM Research unit.

This research has been conducted with the collaboration of Jordan Bieder, permanent researcher at the CEA (Paris, France) and a former post-doctoral fellow at ULiège and Dr.Jorge Iniguez, researcher at the LIST (Luxembourg). The research was supported by the ARC AIMED project, the FNRS PDR-HiT4FiT project and required intensive access to Céci's IT resources (Tier 1 and Tier 2) and the PRACE MEGAPASTA project.

Share this news