Exploring structural dynamics in a spintronic material

Rare-earth nickelates, represented as RNiO₃ (where R stands for a rare-earth cation ranging from La to Lu), are recognised as a fundamental perovskite system within applied materials science. Their broad range of accessible electronic and magnetic properties makes them suitable for applications in spintronics and thermoelectric devices. Nickelates, which contain nickel (Ni) in the relatively unusual 3+ valence state, exhibit a diverse array of characteristics, spanning from paramagnetic to antiferromagnetic behaviours, as well as between insulating or metallic states. These properties display significant variation with chemical substitutions at the R site and can be finely tuned by external parameters such as pressure, temperature, and magnetic fields. Among the nickelates, PrNiO₃ (where Pr stands for Praseodymium) plays a key role, bridging the properties of nickelates with large and small R cations. Understanding how external parameters impact the structural properties of this nickelate across different length scales, and how they subsequently affect its overall properties, is instrumental in comprehending the intricate interplay between chemistry and properties within the entire nickelate family.

With the aim of addressing this question, a study was conducted into the temperature-pressure-magnetic phase diagram of PrNiO₃ nickelate. Pure PrNiO₃ nickelate was synthesised using a high-pressure and high-temperature technique, followed by the application of a synergistic methodology utilising three distinct X-ray probe techniques at beamlines BM23, ID27 and ID22, respectively. Specifically, X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were employed to observe local and medium-range structural changes, as well as bulk structural variations of PrNiO₃ under extreme pressures, temperatures, and magnetic fields. The experimental findings were complemented by ab-initio calculations to gain insight into the electronic structure of PrNiO₃, along with laboratory-based magnetic measurements conducted under pressure.

This approach allowed the determination of the magnetic transition at 130.0(2) K. Surprisingly, it was observed that the bulk structural transition does not occur simultaneously. Instead, the local- and medium-range parameters adapt progressively (Figure 1). This clarification also resolved previous discrepancies in reported transition pressures. Additionally, it was observed that variations of the Ni–O–Ni bond-angle not only drive the bandgap narrowing at the insulator–metal transition (monoclinic P2₁/n → orthorhombic Pbnm), a well-known phenomenon, but also influence the less studied metal–metal transition (orthorhombic Pbnm → rhombohedral R3 ̅c).

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Fig. 1: Temperature evolution of structural parameters obtained for short- (EXAFS), medium- and long- (SXRD) range orders, namely the Debye-Waller exponent (σ²) for (a) the Ni−O bond, (b) the bond-angle Ni–O–Ni (Φ), and (c) the pseudo-cubic volume V_p. The vertical dashed line at 130.0(2) K represents the magnetic transition temperature. d) Pressure–temperature phase diagram for PrNiO₃ nickelate obtained from SXRD and XANES data. The phase boundaries as thick white lines were extrapolated based on the results of this work.

Furthermore, the completion of the orthorhombic to rhombohedral phase transition at high pressure was characterised and shown to evolve progressively over a wide pressure interval of approximately 7 GPa. This new finding enabled the establishment of the relationship between the extent of this phase coexistence and the size of the rare-earth cation. The compilation of these observations facilitated the construction of the pressure–temperature phase diagram of PrNiO₃, as depicted in Figure 1. The insights gained from this work pave the way for designing optimised spintronic devices for modern high-tech industries.

Principal publication and authors
Mapping Pressure- and Temperature-Induced Structural and Magnetic Transitions in Perovskite PrNiO₃ with Local and Long-Range Probes, J.E. Rodrigues (a), A.D. Rosa (a), J. Gainza (b), R.S. Silva (b), E. Mijit (a), G. Garbarino (a), T. Irifune (c), T. Shinmei (c), C. Dejoie (a), N.M. Nemes (b), J.L. Martínez (b), M. Mezouar (a), J.A. Alonso (b), O. Mathon (a), Chem. Mater. 36 (1), 596-608 (2024); https://doi.org/10.1021/acs.chemmater.3c02949
(a) ESRF
(b) Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid (Spain)
(c) Geodynamics Research Center (GRC), Ehime University, Matsuyama (Japan)