Mapping Orthorhombic Domains with Geometrical Phase Analysis in Rare-Earth Nickelate Heterostructures

Most perovskite oxides belong to the Pbnm space group, composed by an anisotropic unit cell, A-site antipolar displacements and oxygen octahedral tilts. Mapping the orientation of the orthorhombic unit cell in epitaxial heterostructures that consist of at least one Pbnm compound is often required to understand and control the different degrees of coupling established at their coherent interfaces and, therefore, their resulting physical properties. However, retrieving this information from the strain maps generated with high-resolution scanning transmission electron microscopy can be challenging, because the three pseudocubic lattice parameters are very similar in these systems. Here, we present a novel methodology for mapping the crystallographic orientation in Pbnm systems. It makes use of the geometrical phase analysis algorithm, as applied to aberration-corrected scanning transition electron microscopy images, but in an unconventional way. The method is fast and robust, giving real-space maps of the lattice orientations in Pbnm systems, from both cross-sectional and plan-view geometries and across large fields of view. As an example, we apply our methodology to rare-earth nickelate heterostructures, in order to investigate how the crystallographic orientation of these films depends on various structural constraints that are imposed by the underlying single crystal substrates. We observe that the resulting domain distributions and associated defect landscapes mainly depend on a competition between the epitaxial compressive/tensile and shear strains, together with the matching of atomic displacements at the substrate/film interface. The results point towards strategies for controlling these characteristics by appropriate substrate choice.Comment: 32 pages, 5 figures, 2 table

Electronic coupling in nickelate-based superlattices

We discussed a particularly interesting interfacial effect-a coupling due to a phase boundary cost-that can be used to achieve fine control over the electronic properties of artificially-layered materials. The artificial model system, presented in this thesis, uses SmNiO3 and NdNiO3-two compounds belonging to the fascinating rare earth nickelate family RNiO3, well known for their sharp metal-to-insulator transition (MIT) observed as a function of temperature. The insulating ground state is also antiferromagnetic. When these two compounds are brought together at an interface the stability of a metal-insulator and magnetic phase separation can be controlled by the thickness of the individual layers. Such behaviors can be explained in terms of a phase boundary cost between metallic and insulating regions, and magnetic and non-magnetic regions, respectively. These results provide a better understanding of the coupling of the MIT and magnetic transitions in systems sharing identical order parameters

Electronic coupling of metal-to-insulator transitions in nickelate-based heterostructures

AbstractRaw resistance and x-ray diffraction data related to the Advanced Electronic Material paper titled "Electronic coupling of metal-to-insulator transitions in nickelate-based heterostructures" (2023

Near-Atomic-Scale Mapping of Electronic Phases in Rare Earth Nickelate Superlattices

Nanoscale mapping of the distinct electronic phases characterizing the metal-insulator transition displayed by most of the rare-earth nickelate compounds is fundamental for discovering the true nature of this transition and the possible couplings that are established at the interfaces of nickelate-based heterostructures. Here, we demonstrate that this can be accomplished by using scanning transmission electron microscopy in combination with electron energy-loss spectroscopy. By tracking how the O K and Ni L edge fine structures evolve across two different NdNiO3/SmNiO3 superlattices, displaying either one or two metal- insulator transitions depending on the individual layer thickness, we are able to determine the electronic state of each of the individual constituent materials. We further map the spatial configuration associated with their metallic/insulating regions, reaching unit cell spatial resolution. With this, we estimate the width of the metallic/insulating boundaries at the NdNiO3/SmNiO3 interfaces, which is measured to be on the order of four unit cells

Competition between carrier injection and structural distortions in electron-doped perovskite nickelate thin films

AbstractData repository for the paper published in Advanced Electronic Materials

Crossover between distinct symmetries in solid solutions of rare earth nickelates

AbstractA strong coupling of the lattice to functional properties is observed in many transition metal oxide systems such as the ABO3 perovskites. In the quest for tailor-made materials it is essential to be able to control the structural properties of the compound(s) of interest. Here, thin film solid solutions that combine NdNiO3 and LaNiO3, two materials with perovskite structure but distinct space-groups, are analyzed. Raman spectroscopy and scanning transmission electron microscopy are combined in a synergistic approach to fully determine the mechanism of the structural crossover with chemical composition. It is found that the symmetry transition is achieved by phase coexistence in a way that depends upon the substrate selected. These results carry implications for analog-tuning of physical properties in future functional materials based on these compounds