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Exploring structural dynamics in a spintronic material


Rare-earth nickelates attract significant interest in materials science due to their diverse electronic and magnetic properties. Researchers have studied a rare-earth nickelate under extreme conditions using X-ray techniques, revealing insights into the structural dynamics and phase transitions of the material. The findings may contribute to designing optimised spintronic devices for modern industry.

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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).


Fig. 1.jpg

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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 Vp. 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);
(a) ESRF
(b) Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid (Spain)
(c) Geodynamics Research Center (GRC), Ehime University, Matsuyama (Japan)


About the beamlines
BM23 is the multi-purpose X-ray absorption spectroscopy (XAS) beamline of the ESRF. BM23 is optimised for high-quality EXAFS measurements in a large energy range (5 - 75 keV), in transmission or fluorescence mode, featuring stability, versatility, automation and online data analysis. The flexible beamline sample environments (viz., cryostats, ovens, gases, chemistry cells, high-pressure cell, DRIFTS/XAS/MS setup, user in-house setups, etc.) are in full operation, covering large scientific fields such as chemistry, earth sciences, geoscience and hard condensed matter physics. In addition, a new micro-XAS station has been implemented, allowing a higher degree of mechanical precision and more flexibility in a sample environment requiring micro-sized beam spot-sizes (cryo-DAC, large-volume press, external or internal RH-DAC, etc.). The focal spot size is below 5×5 µm2 (FWHM) with 109 photon/s. The micro-XAS station can be operated between 5 and 45 keV. Additionally, a Pilatus 1M detector is available for complementary XRD measurements.
The recently upgraded beamline ID27 addresses some of the most exciting and challenging questions in science at extremely high pressures and temperatures, such as exploring the conditions deep inside planets, searching for room-temperature superconductivity, and synthesising new super-hard materials. The beamline accommodates sample environments such as the double-sided laser-heating system, the Paris-Edinburgh press, the nano-stage, and the high-pressure helium cryostat. With a much higher photon flux and smaller beam sizes than its predecessor (0.3×0.3 µm2), as well as better detector systems, it enables a new class of ultra-high-pressure experiments (P > 4 Matm), time-resolved experiments with millisecond resolution, 2D micro-fluorescence mapping and in-situ X-ray imaging.
Key features of ID22 include exceptional angular resolution due to a highly collimated and monochromatic beam, alongside a 13-channel Si(111) multi-analyser stage positioned between the sample and a Dectris Eiger2 X 2M-W CdTe pixel detector, operational in the 6–75 keV range. The detector’s axial resolution enables automatic correction of recorded 2θ values for axial divergence effects, resulting in narrower and more symmetric peaks. Additionally, a complementary Perkin Elmer XRD1611 medical-imaging detector is available for faster, lower-resolution data acquisition, typically utilised at photon energies ranging from 60 to 70 keV for pair-distribution function analysis. Various sample environments are available, allowing for sample temperatures ranging from 4 K to 1600°C, a capillary cell for non-corrosive gas atmospheres within the 0–100 bar range, and a sample-changing robot capable of handling 75 capillary samples compatible with temperatures ranging from 80 K to 950°C.