# Mapping Pressure- and Temperature-Induced Structural and Magnetic Transitions in Perovskite PrNiO3 with Local and Long-Range Probes

João Elias Rodrigues, Angelika D. Rosa, Javier Gainza, Romualdo S. Silva, Emin Mijit, Gaston Garbarino, Tetsuo Irifune, T. Shinmei, Catherine Dejoie, Norbert M. Nemes, et al.

► To cite this version:

João Elias Rodrigues, Angelika D. Rosa, Javier Gainza, Romualdo S. Silva, Emin Mijit, et al. Mapping Pressure- and Temperature-Induced Structural and Magnetic Transitions in Perovskite PrNiO3 with Local and Long-Range Probes. Chemistry of Materials, 2024, 36 (1), pp.596–608. 10.1021/acs.chemmater.3c02949. [https://doi.org/10.1021/acs.chemmater.3c02949](https://doi.org/10.1021/acs.chemmater.3c02949). [arXiv:2306.02949](https://arxiv.org/abs/2306.02949)

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# Mapping Pressure- and Temperature-Induced Structural and Magnetic Transitions in Perovskite PrNiO$_3$ with Local and Long-Range Probes

João Elias Rodrigues,* Angelika D. Rosa, Javier Gainza, Romualdo S. Silva, Emin Mijit, Gaston Garbarino, Tetsuo Irifune, T. Shinmei, Catherine Dejoie, Norbert M. Nemes, José Luis Martínez, Mohamed Mezouar, José Antonio Alonso,* and Olivier Mathon

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Cite This: *Chem. Mater.* 2024, 36, 596–608

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Supporting Information

**ABSTRACT:** The RNiO$_3$ nickelate perovskites represent a promising class of materials for spintronics applications such as in modern communication devices. This is related to the large diversity of properties shown by these compounds. Indeed, depending on the external parameters, such as temperature, pressure, or R cation size, they can exhibit para- or antiferromagnetic behavior and can transform from an insulator to a metal. Among them, PrNiO$_3$ is one of the most important members because it exhibits tunable electronic and magnetic properties. However, our understanding of how these properties can be modified through external parameters, such as pressure and temperature, remains under debate. In this work, we characterized the structural, electronic, and magnetic properties of PrNiO$_3$ in the range of 0–21 GPa and 10–973 K. For this purpose, we performed synchrotron X-ray diffraction (SXRD), X-ray absorption near-edge structure spectroscopy (XANES), and magnetic measurements. We compared the experimental XANES data with *ab initio* calculations to extract information about the electronic structure. The diffraction data demonstrated a sharp transition at the bulk level between monoclinic and orthorhombic PrNiO$_3$ (P2$_1$/n → Pbnm) at T$_{IM}$ ~ 130 K, while XANES data, which probe the medium-range and electronic structure, showed progressive changes between 90 and 130 K. The latter is in accordance with our previous EXAFS data, attesting that this transition occurs around 130 K and that it is driven at the local and medium-range scales. Between 700 and 800 K at ambient pressure, the Pbnm phase transforms into a rhombohedral phase (R3c), in agreement with a previous laboratory X-ray diffraction study. Our XANES data and *ab initio* simulations across this transition indicate a significant raising of the orbital overlap between Ni 3d and O 2p, suggesting superior electronic properties of the rhombohedral phase compared to the orthorhombic structure. Under isothermal cold compression, we found that the Pbnm to R3c transition is characterized by a large coexisting domain between 5.8 and 12.2 GPa. Based on the present and literature data, we proposed an extended pressure- and temperature-phase diagram of PrNiO$_3$ and provided first-order constraints on the interplay between external parameters and properties in nickelates.

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## 1. INTRODUCTION

Rare earth nickelates RNiO$_3$ (R = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Lu) have been recognized as one of the most important perovskite systems in condensed matter physics, due to their unique and great diversity of electronic and magnetic properties.¹⁻⁷ Recently, nickelates have experienced a renaissance of interest in view of their potential applications in spintronics and their recently discovered superconducting properties.⁸,⁹ Indeed, because the noncollinear spin–lattice of nickelates is almost insensitive to small amounts of electron doping, these materials can be easily used for fast spin switching and reliable data storage.¹⁰,¹¹ However, the structures of nickelates are sensitive to pressure, temperature, magnetic field, or composition (mainly R cation size).

Interestingly, in the group of nickelates, few common structures have been found that transform through a displacive mechanism as driven by a collective octahedral [NiO$_6$] tilting and charge (dis)ordering phenomena.¹²,¹³ For most of the nickelate compounds, a complete description of the temperature- and pressure-induced phase transitions is still missing, despite its importance to better predict and fine-tune their

Received: November 20, 2023

Revised: November 30, 2023

Accepted: December 1, 2023

Published: December 16, 2023

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structural, electronic, and magnetic properties for industrial applications.¹ Presently, most studies have focused only on the phase transitions occurring during heating or cooling and very few on the temperature- and pressure-induced transitions. Filling this gap of knowledge and providing a more complete phase diagram description for such transitions in PrNiO₃ is the scope of this work.

The nickelates can be categorized into three groups according to the R cation size: small (e.g., Lu, Tl, Y, and Sm), medium (e.g., Pr and Nd), and large (e.g., La). Most of nickelates, except for the small R cation size group, present an orthorhombic *Pbnm* or rhombohedral (R̃3c) lattice at room conditions with a metallic character.¹ For all of the nickelates, except for R = La, a charge ordering transition is observed under cooling at ambient pressure. This transition is induced at the local scale by the charge disproportionation of Ni valence states for neighboring [NiO₆] octahedra and results in a structural transition from the metallic *Pbnm* to the monoclinic insulator phase *P2₁/n*.¹⁴,¹⁵ Therefore, this transition is hereafter referred to as the insulator–metal transition (IMT). In the medium R cation size group, the metallic *Pbnm* to insulator *P2₁/n* transition is accompanied by an antiferromagnetic (AFM) ordering for the low-temperature structure. These medium R cation size nickelates undergo in addition a pressure- and temperature-induced transition from the *Pbnm* phase to a higher symmetry rhombohedral structure (R̃3c).¹⁶,¹⁷ This rhombohedral phase is reported for large R cation size nickelates at room conditions, but it has not been observed for the small R cation size nickelates.¹⁸,¹⁹

In this work, we focus on the nickelate PrNiO₃ because it not only displays a very peculiar scientific case within the group of medium R cation size but also because it is a potential link among small, medium, and large cation size nickelates. Indeed, in PrNiO₃, the three common nickelate structures (i.e., *P2*₁/*n*, *Pbnm*, and *R*̃3c) can be induced through pressure and temperature variations. These transitions exhibit strong changes in the electronic and magnetic structures, and in addition, in PrNiO₃ the AFM ordering temperature (*T*<sub>N</sub>) coincides with the IMT temperature (*T*<sub>IM</sub>) at ~130 K²⁰ and the structural transition from a monoclinic to an orthorhombic phase, hereafter referred to as I(AFM)MT. Interestingly, a significant positive unit-cell volume jump during cooling is reported for this transition in PrNiO₃, which is not observed in all the nickelates.⁵,²¹ Previous works have proposed that the charge disproportionation of Ni cations is essential to stabilizing the AFM ordering in monoclinic PrNiO₃.²,¹⁵ Several studies have proposed that the I(AFM)MT in PrNiO₃ may be phonon-mediated, which would inhibit orbital ordering effects due to Jahn–Teller distortions.⁵,²²,²³ However, tighter constraints on the role of local structural variations on the bulk, electronic, and magnetic properties, the influence of vibrational properties, across the transitions, or the pressure and temperature evolution of *T*<sub>N</sub> are required. In addition, the high-temperature- and/or pressure-induced *Pbnm* to *R*̃3c (metal–metal) transition is not yet fully characterized; only one work reporting its occurrence at 773 K²⁴ and another one demonstrated the onset of an analogous transition under pressure at 4 GPa.²⁵ However, the driving forces and the structural property evolution across the structural transitions at the short-, medium-, and bulk-length scales as well as the completion pressure are not yet well established for PrNiO₃.

To address those open and important questions, we investigated the temperature- and pressure-induced structural

and magnetic transitions in PrNiO₃ using synchrotron X-ray diffraction (SXRD), Ni K-edge X-ray absorption near-edge structure spectroscopy (XANES), and SQUID-based magnetic measurements. We took great care to unveil the pressure-induced structural phase transition and the phase coexistence at quasi-hydrostatic conditions using the diamond anvil cell technique and sample loading with a quasi-hydrostatic pressure medium. From SXRD, we obtained information on the bulk- and medium-range structural changes, while XANES provided insights into the chemical environment at the medium and local atomic scales around the absorber atom (Ni). The combination of these methods enabled a multiscale length study on the structural phase transitions in PrNiO₃ as induced by temperature and under pressure. Our complete data set provided deep insights into the effective interplay between structural, electronic, and magnetic changes across the temperature- and pressure-induced transitions and thus disclosing their underneath mechanisms.²³

## 2. METHODOLOGY

### 2.1. Synthesis of PrNiO₃ Nickelate.
We used a high-pressure synthesis method to prepare a high-quality powder of PrNiO₃. Stoichiometric mixtures of Pr₆O₁₁ and Ni(OH)₂ were ground in an agate mortar with 30% wt. KClO₄, which produces in-situ high oxygen pressure for promoting the Ni oxidation to the 3+ state (Ni³⁺). The mixture was inserted into a gold capsule of 5 mm diameter and set in a cylindrical graphite heater. A piston cylinder Rockland Research press was used to pressurize and heat the sample to 3.5 GPa and 800 °C for 30 min, before being quenched down to room temperature. Traces of KCl and unreacted oxides (Pr₆O₁₁ + NiO) were eliminated by washing the final powder in a diluted HNO₃ solution at 60 °C. The reproducibility of this method is reliable, providing a high-quality powder of PrNiO₃ with the orthorhombic unit-cell (*Pbnm*) at room temperature, as verified by laboratory X-ray diffraction (Bruker D5 diffractometer with Cu Kα; λ = 1.54053 Å).

### 2.2. SXRD Data Acquisition and Analysis.
Temperature-dependent high-resolution SXRD data were recorded at the beamline ID22 at the ESRF-EBS operating at λ = 0.35418 Å.²⁶,²⁷ The high angular resolution (=0.002° 2θ) of this instrument allows clearly identifying the splitting or deformation of the diffraction peaks associated with structural transitions in PrNiO₃. A Dectris EIGER2 × 2M-W CdTe pixel detector was placed in Position Sensitive mode to access a large 2θ angular range of 1–40°. The beam spot size was defined by slit dimensions set to 3 × 1 mm² (H × V). We sealed the PrNiO₃ powder sample in a quartz-glass capillary of 0.5 mm diameter. During the diffraction data acquisition, the capillary was in continuous rotation mode to minimize possible effects due to preferred orientations in the sample. Temperature-dependent data were acquired with a 10 min waiting time at each temperature point of the list [10, 29, 48, 71, 90, 109, 132, 151, 170, 193, 295, 400, 500, 600, 700, 800, and 900 K] using a cryogenic cooling (Dynaflow cryostat) and a heating system (hot-blower) deployed at ID22. Integrated diffraction patterns were indexed using previous literature data on PrNiO₃ structures, i.e., *P2*₁/*n*, *Pbnm*, and *R*̃3c. These data were subsequently analyzed via Rietveld refinement methods using the FullProf program and pseudo-Voigt function to fit the peak profiles.²⁸ The refined parameters included scale factors, zero-point error, background coefficients, asymmetry
<figure><img src="image_10.png" /><figcaption>**Figure 1.** Typical high-resolution SXRD profiles for PrNiO₃ (red open circles) and corresponding Rietveld refinements (black lines) at 10 K for the P2₁/n (monoclinic) phase (a), at 295 K for the Pbnm (orthorhombic) phase (b), and at 900 K for the R3c (rhombohedral) phase (c). Blue lines denote the fit residuals, and the green bars represent the expected Bragg reflections.</figcaption></figure>

correction factors, lattice parameters, atomic positions, occupancy factors, and isotropic displacement parameters.

Pressure-dependent SXRD data were collected in angle-dispersive mode at beamline ID27 at the ESRF-EBS. ID27 has an undulator for photon delivery and a double-crystal monochromator equipped with a set of Si crystals. A beam spot size of 1 × 1 µm² (FWHM) was achieved from the achromatic Kirkpatrick–Baez mirrors, and the measurements were performed using an incident beam wavelength of 0.3738 Å.²⁹,³⁰ Both beam properties are necessary to probe the micrometer-sized samples contained in high-pressure devices. An image plate detector Dectris EIGER2 × 9 M CdTe with pixel area of 75 × 75 µm² was placed at 368.265 mm behind the sample. This detector provides a large dynamic range and thus is ideal to discriminate weak peaks originating from low-symmetry phases of samples contained in the diamond anvil cell. The detector-to-sample distance and detector-tilt parameters were calibrated using a CeO₂ standard and the software Dioptas.³¹ Diffraction data of PrNiO₃ were acquired in oscillation mode over an angular range of 10° (−5° ≤ ω ≤ +5°) and in pressure steps of 3 GPa up to 20 GPa.

For pressure generation, we used a Letoullec-type membrane-driven diamond anvil cell (mDAC) equipped with

single-crystalline diamonds of a culet size of 300 µm. Preindented Re gaskets of 50 µm thickness with a sample chamber hole of 160 µm diameter were employed. PrNiO₃ powder with dimensions of 25 × 25 × 10 µm³ was placed in the center of the sample chamber. Two small ruby microspheres (diameter: 10 µm) were loaded at the side of the powder to monitor the pressure using the calibrated shift of its luminescence R₁ line.³² To ensure quasi-hydrostatic conditions, the sample was loaded together with He as the pressure-transmitting medium (PTM). The raw diffraction data were integrated using the Dioptas software.³¹ The integrated patterns were treated using the Le Bail method for extracting the lattice parameters.

**2.3. XANES Data Acquisition and Analysis.** All synchrotron X-ray absorption spectroscopic measurements were conducted in transmission mode at Ni K-edge at the beamline BM23 at the ESRF-EBS.³³,³⁴ BM23 is equipped with a fixed-exit double-crystal monochromator (DCM) to operate in energy-scanning mode. For all experiments, ionization chambers were filled with the appropriate gas mixtures to measure the photon flux before and after the sample. Prior to and during the entire experiment, the monochromator angle-
<figure><img src="image_11.png" /><figcaption>**Figure 2.** Temperature evolution of the pseudo cubic parameters $a_p$ (a), $b_p$ (b), and $c_p$ (c), the angle $\beta$ for the monoclinic, orthorhombic, and rhombohedral (described in the hexagonal setting) phases (d), and the unit-cell volume $V_p$ (e). Temperature dependence of the nickel bond valence sum from Brown's bond valence model as $3+\delta$ (Ni₁) and $3-\delta$' (Ni₂) (f), the average Ni–O bond distances (g), and the super exchange angles $\Phi$ (h). The error bars were estimated to be smaller than the data point symbols. The dashed vertical lines delimit the transition temperature at 130 K for $P2_1/n \rightarrow Pbnm$ and at 800 K for $Pbnm \rightarrow R\bar{3}c$, respectively. The color codes green, blue, and orange denote the monoclinic, orthorhombic, and rhombohedral phases, respectively.</figcaption></figure>

to-energy calibration was verified by acquiring XANES spectra of a metallic Ni reference foil (Goodfellow, 99.95%).

For all temperature-dependent XANES acquisitions, we employed a millimeter-size X-ray beam that was collimated by slits to a size of 3 × 1 mm² ($H \times V$). The harmonic rejection was facilitated by two parallel silicon mirrors with an incident angle of 3 mrad. For temperature-dependent measurements, cold-pressed PrNiO₃ pellets were prepared mixed with boron nitrite (BN) that provided an edge jump of ~0.6. High temperatures were generated using a furnace developed at the ESRF's sample environment laboratory that enables measurements up to 973 K. During the measurements, the air inside the furnace was flushed using synthetic air to avoid the self-reduction effects of the sample. Low temperatures were achieved using a liquid He cryostat that permits measurements down to 10 K. For a complete temperature run, we selected points in the list [10, 50, 100, 120, 130, 140, 150, 200, 250, 300, 373, 473, 573, 673, 723, 773, 823, 873, 923, and 973 K].

For the pressure-dependent XANES measurements, the X-ray beam was focused to 3 × 3 µm² (FWHM) using a pair of Pt-coated mirrors in the Kirkpatrick–Baez configuration with an incidence angle of 5.75 mrad for harmonic rejection. High-pressure was generated using membrane diamond anvil cells equipped with nanopolycrystalline diamonds of a culet size of 300 µm (one full and one partially perforated).³⁵,³⁶ The perforation served to reduce the signal absorption due to the diamonds in the Ni K-edge energy range. To ensure quasi-hydrostatic conditions, the sample was loaded together with Ne as PTM.³⁷ XANES data treatment was performed using Larch XAS Viewer software for the pre-edge background subtraction, edge jump normalization, pre-edge, and XANES features peak analyses.³⁸,³⁹ These peak fitting analyses provided an estimation of the error bars for the peak position of the XANES features ($\Delta A$, see Figure S4).

To interpret the experimental data, we also performed ab initio calculations to compute theoretical XANES spectra at Ni K-edge in the range −10 to 60 eV (units of $E - E_F$, eV; $E_F$ is the Fermi-level energy), using the FDMNES code.⁴⁰ Ni-centered cluster radii were set to 7 Å. Calculations employed Green's method to solve the relativistic extension of the Schrödinger equation considering expansions up to the quadrupole term. The self-consistency of the potential was evaluated prior to the calculation for a better estimation of the energy-cutoff level. A spectral broadening was simulated by an energy-dependent Lorentzian function to compute the XANES spectra. The simulated XANES spectra and all pertinent discussion are summarized in the Supporting Information.

**2.4. Magnetic Characterization.** The magnetic measurements were performed in a vibrating-coil SQUID magnetometer (MPMS-3) from Quantum Design (San Diego, USA) in a temperature range from 80 up to 150 K, with fine temperature steps of 0.3 K, under an applied magnetic field of 0.1 T. The nonmagnetic CuBe cylindrical pressure cell, with an internal diameter for the sample of 2.2 mm and a maximum applied pressure of 1.5 GPa, was considered to probe pressure-dependent magnetic properties. This cell is screw-based, and the increase in pressure follows the closure of the pressurized cylindrical capsule. The sample pressure was monitored by using the superconducting critical temperature of a tin manometer that was loaded with the sample.

## 3. RESULTS

### 3.1. Temperature-Induced Structural Transitions.

**3.1.1. Long- and Medium-Length Scales (SXRD).** In Figure 1, we show representative SXRD patterns and corresponding Rietveld refinements of the three observed PrNiO₃ structures that occurred in the temperature interval 10 to 900 K at ambient pressure. These include the monoclinic phase $P2_1/n$
<figure><img src="image_12.png" /><figcaption>**Figure 3.** Normalized Ni K-edge XANES spectra of PrNiO₃ as a function of temperature (a) and extracted energy shifts of spectral features ΔA (calculated relative to the values at 10 K) (b–e). The dashed vertical line delineates the potential transition temperatures of the P2₁/n → Pbnm and Pbnm → R3c transitions derived from the discontinuities in ΔA values and comparisons with those extracted from SXRD data. The color codes green, blue, and orange denote the monoclinic, orthorhombic, and rhombohedral phases, respectively. Error bars of shifts ΔA were estimated from the peak fitting procedure (see in Figure S4).</figcaption></figure>

that is stabilized at temperatures below $T_N = T_{IM} \sim 130$ K, the orthorhombic phase Pbnm that was observed in the range of $T_N$~700 K, and the rhombohedral phase R3c that occurred at temperatures above 700 K. A characteristic feature of the monoclinic phase is the diffraction peak doublet ( [224] and [−224] reflections) at around 15.1° (2θ) [see inset of Figure 1a]. The appearance of this doublet is linked to the charge disproportionation at Ni sites and, thus, to the generation of two distinct crystallographic Ni sites with different charges ($Ni_1^{3+}+\delta$ and $Ni_2^{3-}+\delta$).⁵,⁴¹ This doublet is not observed in the orthorhombic phase and for temperatures above $T_N = T_{IM}$ [see the inset of Figure 1b]. In the orthorhombic lattice, the charge disproportionation at the Ni site collapses, resulting in a single Ni site with a uniform valence of 3+. Above 700 K, we found that a high symmetry phase with a rhombohedral lattice is stabilized, as attested to by the disappearance of the orthorhombic characteristic peaks [Figure 1c]. All refined parameters for the three phases are listed in Tables S1–S3 in the Supporting Information, respectively.

The temperature evolution of structural parameters extracted from Rietveld refinements is presented in Figure 2. We used a pseudo cubic description of the lattice parameters (a, b, and c) to compare the temperature dependence across the structural transitions in a continuous manner. For this purpose, we followed the methodology outlined by Woodward.⁴² As shown in Figure 2, the temperature evolution of the pseudo cubic lattice parameters ($a_p, b_p$, and $c_p$) and the pseudo cubic unit-cell volume ($V_p$) [Figure 2a–c,e] displays two clear discontinuities in the range from 110 to 130 K and from 700 to 800 K. The first discontinuity coincides with the P2₁/n to Pbnm transition interval (125~130 K), already reported in literature,⁴³ referred to as I(AFM)TM transition. The second discontinuity includes the Pbnm → R3c transition that was previously reported to take place above 773 K,²⁴ which is here referred to as O↔R transition. We observed a clear reduction of the pseudo cubic lattice parameters and volume for both temperature-induced transitions under heating. Interestingly,

the volume reduction of the O↔R transition exceeds that of the I(AFM)MT transition and leads to a smaller pseudo cubic lattice volume of the rhombohedral phase [$V_p = 55.164(5)$ Å³ at 800 K] compared to the monoclinic one [$V_p = 55.483(1)$ Å³ at 10 K].

In Figure 2f, the temperature evolution of both $Ni_1$ and $Ni_2$ bond valences in PrNiO₃, estimated from the Brown's bond valence sum model,⁴⁴ is shown. This figure depicts an interesting behavior, as the Ni valence is slightly below 3+ in the orthorhombic phase between 170 and 700 K, while it retrieves a value of 3+ in the rhombohedral phase. In Figure 2g, the temperature evolution of the average Ni–O bond length, obtained from the refined crystal structures, is displayed. At low temperature and in the monoclinic phase, the charge disproportionation induces two different octahedral geometries, as illustrated in Figure 2g by the average <Ni₁₂–O> bond length. Upon temperature increase and above the I(AFM)MT, the charge disproportionation of Ni sites in the monoclinic phase vanishes and a more uniform octahedral geometry is formed, which attests the formation of the metallic orthorhombic phase. The orthorhombic phase is characterized by a single crystallographic site for Ni, with an average <Ni–O> bond length that is intermediate to <Ni₁₂–O>, and a single Ni valence of 3+. By increasing temperature, the average Ni–O bond distances remains constant in the orthorhombic phase but slightly decreases across the O↔R transition, in accordance with bulk structural parameters.

In Figure 2h, the temperature evolution of the average super exchange angle Φ is displayed, which describes the angle between Ni–O–Ni of adjacent [NiO₆] octahedra (more details can be found in the Supporting Information, Tables S1–S3). This angle is constant within the stability field of the monoclinic phase, and it progressively increases across the monoclinic to orthorhombic transition from 156.8(3)° (at 10 K) to 157.2(1)° (at 132 K). It stays nearly constant in the orthorhombic phase, and it shows a strong increase from 158.8(1)° (at 700 K) to 161.8(1)° (at 900 K). The latter value
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Integrated SXRD patterns of PrNiO₃ in the 2θ range 13 to 17° and their evolution with pressure at room temperature (a). Volume fraction evolution (in percentage wt %) of the orthorhombic (*Pbnm*) and rhombohedral (*R*3c) phase as a function of pressure (b). Integrated diffraction pattern of PrNiO₃ at 17.6 GPa (red open circles), the Le Bail refinement (black line), and the fit residuals (blue line) (c). The color codes blue, black, and orange denote the orthorhombic, the coexistence range (O+R), and rhombohedral phases, respectively.</figcaption></figure>

for Φ is close to the one reported previously for rhombohedral PrNiO₃ at 873 K [of 161.9(2)°].²⁴

### 3.1.2. Medium- and Local-Length Scales (XANES).
The temperature evolution of the normalized Ni K-edge XANES spectra from 10 to 973 K is shown in Figure 3a. We observed only slight changes along the XANES features in the investigated temperature range. For better comparison, we labeled individual features, as follows: A₁ (pre-edge: 8.333 keV), A₂ (first XANES peak: 8.350 keV), A₃ (shoulder on the first XANES peak: 8.359 keV), and A₄ (second XANES peak: 8.367 keV). To evaluate the temperature evolution of these features, we fitted four Lorentzian peak profiles to each XANES spectrum and an error function as a baseline (see Figure S4 in the Supporting Information). This fitting model reproduces the measured spectra. The extracted peak energy positions were subtracted from the value at 10 K to obtain the relative energy position shifts [ΔA(T) = A(T) - A(T₀), T₀ = 10 K]. In Figure 3b–e, the temperature evolutions of ΔA values are shown while the temperature evolutions of individual peak features such as their FWHM and height are summarized in Figure S5. The pre-edge energy position (A₁) remains constant between 10 and 130 K, but it decreases beyond 130 K across the P2₁/n → Pbnm transition by ΔA₁ ~ -0.2 eV. Between 130 and 200 K, the pre-edge energy position oscillates before increasing again by ~0.1 eV. Above 200 K, the pre-edge energy position depicts a regular reduction reaching again a value of ΔA₁ ~ -0.2 eV at 973 K. The temperature evolution of the first XANES peak energy position (A₂) differs to the one of the pre-edge A₁. First, it increases slightly with increasing temperature by ΔA₂ ~ 0.05 eV up to 130 K, and the P2₁/n → Pbnm transition before it decreased monotonically by ΔA₂ ~ -0.25 eV up to 900 K. Above 900 K, we observed a slight increase to a ΔA₂ ~ -0.1 eV.

The energy position of the shoulder on the first XANES peak (A₃) moves progressively to lower energy values upon

heating from 10 to 973 K, with an absolute energy shift of ΔA₃ ~ -0.8 eV, and it exhibits the strongest variation with temperature. Like A₃ but less pronounced, the energy position of the second XANES peak (A₄) shifts rather progressively to lower values with increasing temperature, reaching an absolute shift value of ΔA₄ ~ -0.6 eV at 973 K.

### 3.2. Pressure-Induced Transitions.
3.2.1. Long- and Medium-Length Scales (SXRD).
Figure 4a displays the pressure evolution of selected integrated SXRD patterns up to 18 GPa; the raw diffraction data can be found in Figure S2 of the Supporting Information. The diffraction patterns at the lowest investigated pressure point (0.5 GPa) could be indexed with a pure orthorhombic phase. With increasing pressure, the unit-cell of this phase contracts progressively, as attested by the continuous shift of the diffraction peaks to higher 2θ values [blue patterns in Figure 4a]. Clear changes of diffraction peak shapes are observed at 5.8 GPa [black pattern in Figure 4a]. Notably, two separate diffraction peaks merge into a single peak at ~13.8° (2θ), a new broad peak at around 14.3° (2θ) appears, and the doublet at ~16° (2θ) starts to split into three individual ones. With further pressure increments, these changes evolve progressively [grey patterns in Figure 4a]. We interpret these findings as a sluggish phase change from Pbnm to R3c starting at 5.8 GPa rather than a sharp structural transition. This onset pressure and the persistent phase coexistence agree with the observations of Zhou et al.²⁵ Between 12.2 and 13.5 GPa, the diffraction patterns changed more drastically; all the observed peaks broadened and significantly lost intensity [orange patterns in Figure 4a]. We associated the drastic changes at 12.2 and 13.5 GPa with completion of the phase transition and the formation of a pure rhombohedral phase in PrNiO₃.

To quantify the phase fractions and their evolution with pressure, we performed Le Bail refinements of individual integrated diffraction patterns. As presented in Figure 4b, the
<figure><img src="image_14.png" /><figcaption>**Figure 5.** Normalized Ni K-edge XANES spectra of PrNiO₃ as a function of pressure at room temperature (a) and extracted energy shifts of spectral features ΔA (shift calculated relative to the values at ambient pressure) (b–e). The dashed vertical lines in panels (b–e) delineate the onset and completion pressures of the Pbnm → R3c transitions, as derived from the observed discontinuities in the diffraction data [see Figure 4b]. The color codes blue, black, and orange denote the orthorhombic, the coexistence range (O+R), and rhombohedral phases, respectively. Error bars of shifts ΔA were estimated from the peak fitting procedure (see in Figure S4).</figcaption></figure>

volume fraction evolution with pressure underlines our interpretations above and brackets the coexistence domain in the pressure interval between ~5.8 and 12.2 GPa. As can be seen in Figure 4c, the diffracted pattern at 17.6 GPa can be unequivocally indexed with a pure rhombohedral phase, which assesses the completion of the phase transition at this pressure.

### 3.2.2. Medium- and Local-Length Scales (XANES).
The pressure evolution of normalized Ni K-edge XANES spectra of PrNiO₃ up to 21 GPa is shown in Figure 5a. We fitted the normalized XANES spectra in a manner similar to that for the high-temperature data, using four Lorentzian peaks and a baseline function. The noise level of the pre-edge signal increased significantly compared to high-temperature data, because of the strong absorption signal from the diamond anvils in this energy range (E < 10 keV).³⁵ However, the spectra still allowed resolving the pre-edge peak position variations using a single Lorentzian peak fit to the feature A₁ [Figure 5b]. The extracted peak energy positions were subtracted from the value obtained at ambient pressure to get the relative energy position shifts [ΔA(P) = A(P) - A(P₀), P₀ = 1 bar]. Apart from ΔA₁ (pre-edge energy position) that remains almost constant up to 12.5 GPa, all spectral features present a progressive shift to higher energies with pressure from 0 to 21 GPa. For instance, the first XANES peak shifts by ~0.5 eV (ΔA₂) from 0.5 to 12.5 GPa, the shoulder on the first XANES peak shifts by ~1.5 eV (ΔA₃), and the second XANES peak position increases by ~1.8 eV (ΔA₄). The pressure evolutions of individual peak features, as the FWHM and height, are listed in Figure S6. The progressive shift is in accordance with the SXRD data, which similarly suggested a smooth change in the diffraction peak shapes. At 21 GPa, i.e., far above the completion pressure of the Pbnm → R3c transition, the pre-edge energy position increases by ΔA₁ ~ 0.5 eV.

### 3.3. Pressure- and Temperature-Induced Structural Transitions.
We investigated the pressure dependence of the antiferromagnetic ordering temperature (T_N) and compared it

with the reported data on the pressure evolution of the insulator-metal transition (T<sub>IM</sub>). The AFM ordering in nickelates due to the nickel sublattice at ambient pressure is commonly studied by neutron diffraction,⁵,¹⁷,⁴³,⁴⁵ which requires a fairly large amount of sample to ensure statistically valid data acquisition, and this is even more crucial for high-pressure experiments. The use of vibrating-coil SQUID magnetometer for characterizing the AFM ordering under pressure may circumvent such a disadvantage but with an additional difficulty from the weak Ni sublattice signal, which may be eventually masked by the Pr paramagnetic response. Even so, we performed direct magnetic susceptibility measurements at 0.1 T in a nonmagnetic CuBe cylindrical pressure cell up to 1.5 GPa and using a very narrow temperature stepping of 0.3 K. This technique permitted us to define the minimum change in the first derivative of the susceptibility with very high precision, which is presented in Figure S8 of the Supporting Information together with the pressure dependence of T_N and previous literature data of T<sub>IM</sub> reported in refs 6,46. The progressive decrease of the AFM transition temperature with pressure can be described by a linear function characterized by a negative slope of -49.2 K·GPa⁻¹. This value lies in between those reported for T<sub>IM</sub> of -42.7 and -56.7 K·GPa⁻¹.¹⁶,⁴⁶ These discrepancies to previous works may be related to the much lower data coverage of these studies that derived the slopes for T<sub>IM</sub> from only four resistivity data points. The overall similar and linear pressure evolutions of T<sub>IM</sub> and T_N indicate a common mechanism for both AFM and IMT transitions that is not altered with pressure or temperature.

## 4. DISCUSSION

In the following, we used our new data set to elaborate the interplay between structural, electronic, and magnetic properties at the local-, medium-, and long-range scales across these transitions and to establish a more complete *P/T* phase diagram of PrNiO₃.
### 4.1. The I(AFM)MT in PrNiO₃. 4.1.1. Absence or Presence of an Intermediate Phase.
Recent literature suggests that the IMT in nickelates has different mechanisms for R = Pr, Nd (medium size) and R = Lu, Tl, Y, Sm (small size).<sup>7,22,23</sup> It remains under debate to what extent structural, vibrational, electronic, and magnetic degrees of freedom influence this transition and how their involvements change for distinct rare earth elements. For small R cation size nickelates, it has been proposed that an intermediate paramagnetic insulator phase occurs during the IMT due to the band gap opening.<sup>41</sup> For medium R size nickelates, most studies have not observed this intermediate phase; instead, it has been proposed that the IMT is accompanied by the occurrence of an insulator AFM state in the low-temperature phase featured by the orbital ordering suppression.<sup>14</sup> Electron doping may, however, induce an intermediate metallic phase with AFM ordering in medium R cation size nickelates, as it is the case of Ce-doped NdNiO₃.<sup>10</sup> For PrNiO₃, Gawryluk et al. proposed a coexistence interval of the monoclinic and orthorhombic phases between 115 and 130 K, such that both AFM insulator and paramagnetic metallic phases coexist in a narrow temperature interval.<sup>5</sup>

To verify the existence or absence of an intermediate phase, we performed two phase refinements of the diffraction data for temperatures close to this interval (109 and 132 K). These refinements provided a very low fit quality compared with the fits using only one phase. Although the phase coexistence may be difficult to identify due to multifold peak overlap, we considered that data quality and refinement procedure would allow identifying the presence of a minor phase. Therefore, we discarded the coexistence of two phases between 109 to 132 K and proposed a sharp transition occurring at ~130 K. However, we also noticed that the pseudo cubic lattice parameters and volumes as well as the monoclinic $β$ angle exhibit an already orthorhombic-like character at around 109 K [Figure 2a–e] and, thus, below the suggested transition temperature. We argue, therefore, that the monoclinic phase in the vicinity of the I(AFM)MT transition and at T = 109 K exhibits a highly deformed structure.

### 4.1.2. Comparison of Multilength-Scale Properties across the Transition.
To discuss the concomitant evolution of structural properties at different-length scales and their interplay, we compare in Figure 6a–d the parameters of PrNiO₃ acquired in the temperature range 10–210 K across the I(AFM)MT transition from SXRD, XANES (this work), and EXAFS (Rodrigues et al.²³). The comparison enables a complete characterization from short- to long-range orders, which may help to distinguish among different mechanism scenarios for the transitions in PrNiO₃. As displayed in Figure 6, cooling below ~130 K leads to distinct changes of the EXAFS parameters probing the local structure [EXAFS in Figure 6a] and bulk structure [SXRD in Figure 6d] while parameters related to the medium-range structure [XANES and angle $Φ$ in Figure 6b,c] vary progressively. In particular, the Debye–Waller exponent ($σ²$) from EXAFS, describing the Ni–O bond distance variance, exhibits a departure from the Einstein model at the I(AFM)MT below 130 K. This finding was assigned to a strong spin–phonon coupling in the low-temperature phase of PrNiO₃.²³ In contrast to the EXAFS data, the XANES features, describing the medium-range structure and electronic features, evolve rather smoothly and exhibit small variations under cooling. For example, the $A₁$ feature exhibits only slight and progressive changes at ~125–130 K [Figure 6b], similar to the estimated super exchange angle $Φ$

<figure><img src="image_15.png" /><figcaption>**Figure 6.** Comparison among temperature dependencies of different physical parameters obtained from distinct experimental techniques for covering the short (EXAFS), medium (XANES), and long (SXRD) range orders. These parameters are the Debye–Waller exponent ($σ²$) for Ni–O bond in ref 23 (a), the XANES features $A₁$ (b, f) and $A₃$ (e), the super exchange angle $Φ$ (c, g), and the pseudo cubic volume $V_p$ (d, h).</figcaption></figure>

that displays a progressive drop under cooling and across the I(AFM)MT transition starting from ~130 K [Figure 6c]. The gradual increase of $Φ$ during this structural transition promotes the increase of the degree of orbital overlap between Ni 3d orbitals and O 2p orbitals.⁴⁷ At the bulk scale, we observed sharp changes that are shifted compared with those from the local and medium-length scales. For instance, upon cooling, the pseudo cubic volume $V_p$ shown in Figure 6d decreases progressively across ~130 K and down to ~110 K and, then, exhibits an increase below this temperature with a volume variation of ~0.2%. This change can be attributed to the electronic localization phenomena in the insulator phase.¹⁷ The comparison clearly demonstrates a local origin of the transition at the Ni–O bond with the potential involvement of the spin–phonon coupling and magnetoelastic phenomena.²³

### 4.1.3. Interplay between Structural and Electronic Properties across the Transitions.
To provide insights on the electronic origin of this transition at local and medium scales, we recalculated the temperature-dependent XANES spectra using full multiple scattering analysis based on the structural data obtained from our Rietveld refinements (see in Figure S7 of the Supporting Information). The full multiple scattering calculations demonstrate a relative decrease of filled states in the conduction band of the monoclinic phase compared to the orthorhombic one. This result suggests that the density of states in the monoclinic structure becomes more localized, concomitant with a loss of orbital hybridization of the Ni–O pair bonds, which stabilizes the insulator state in PrNiO₃. This electronic structural change could also explain the energy position variations of the features and across the orthorhombic non-localized to monoclinic localized electronic configuration [Figure 6b]. Indeed, we have tracked the position variations of features $A₁$ and $A₂$ (in shift $ΔA$) provided by full multiple scattering calculations (ab initio) and
compared with the experimental results [see in Figure 7]. The
shifts around the structural transitions at 130 and 800 K are

<figure><img src="image_16.png" /><figcaption>**Figure 7.** Comparison between experimental and calculated (ab initio) energy shifts of spectral features $A_1$ (a) and $A_2$ (b). The shifts $\Delta A$ are determined relative to the values at 10 K. The dashed vertical lines highlight the temperatures for the $P2_1/n \rightarrow Pbnm$ and $Pbnm \rightarrow R\bar{3}c$ transitions.</figcaption></figure>

quite well reproduced, and the overall trends for both $\Delta A_1$ and $\Delta A_2$ are reproduced to some extent. The energy position ($A_1$) decreased between 10 and 90 K, achieving a minimum around 151 K ($\Delta A_1 \sim -0.2$ eV) and being almost constant from 295 K with $-0.15$ eV. Around 800 K, a jump can be observed and the shift $\Delta A_1 \sim$ spans $-0.1$ eV. The calculated variation for the first XANES peak energy position ($A_2$) was more pronounced at the onset of the transition $P2_1/n \rightarrow Pbnm$ transition with a global shift of 0.73 eV ($\Delta A_2 \sim 0.36$ eV at 90 K vs $\Delta A_2 \sim -0.37$ eV at 151 K), while the transition around 800 K is less evident from the point of view of feature $A_2$. This comparison suggests that the energy shift has a structural nature and reflects progressive orbital overlapping (Ni 3d → O 2p), which is the main feature of the nickelate phase transitions from low to high temperatures.

We also observed conspicuous increases in the energy
position of the $A_3$ and $A_4$ features in Figure 3d,e upon cooling
and across the I(AFM)MT transition. The overall evolution of
these feature exhibits a similar but inverted trend compared to
the one of the super exchange angle ($\Phi$) derived from
diffraction. Therefore, we argue that these features could be
used as markers to track the super exchange angular evolution.
In the case of the I(AFM)MT transition, the decrease in the
super exchange angle suggests a decrease in the orbital overlap
between Ni 3d orbitals and O 2p orbitals, favoring the
insulator phase. Our conclusions and observations are in
agreement with previous works that proposed that this
transition is only mediated by a bandwidth mechanism and
it does not depend on the cation-anion charge transfer.<sup>18,47</sup>
The present local and long-range data indicate the absence of a
coexistence phase interval and demonstrate that the I(AFM)-
MT transition is characterized by clear discontinuities of local
(Debye-Waller exponent: $\sigma^2$) and bulk (pseudo cubic volume:
$V_p$) structural parameters, while medium-scale parameters
reveal rather progressive changes in the structural and
electronic properties.

**Low-Temperature and High-Pressure Evolution of the I(AFM)MT Transitions.** There is a general agreement that the I(AFM)MT transition in PrNiO₃ occurs at *T*<sub>N</sub> = 125–130 K at ambient pressures.<sup>43,45,47,48</sup> Variations along the IMT temperature can be induced by playing with the isotope effect <sup>16</sup>O–<sup>18</sup>O,<sup>49</sup> or by modulating the vibrational modes via lattice mismatch between substrate and deposited material onto thin films.<sup>50</sup> We studied the pressure variation of I(AFM)MT from the magnetic susceptibility data up to 1.5 GPa [see in Figure S8] and found a linear behavior with a slope of *dT*<sub>N</sub>/*dP* = –49.2 K·GPa<sup>–1</sup>. A previous experimental study by Zhou et al.<sup>20</sup> found anomalous trends along the pressure evolution of the bulk resistivity *ρ*(*T*) in orthorhombic PrNiO₃ close to the phase boundary at 0.8–1.3 GPa. This work also reported bulk non-Fermi liquid behavior for pressures exceeding 1.3 GPa around the quantum critical points (QCPs), making PrNiO₃ a “strange” metal under these conditions. Both trends were inferred from the unusual exponents of *T* along *ρ*(*T*) ~ *T*<sup>m</sup> obtained for the PrNiO₃ sample (*n* = 2, Fermi liquid as LaNiO₃; *n* < 2, non-Fermi liquid). The authors predicted a first-order transition, at higher pressures and low temperatures near QCPs, from the bulk non-Fermi liquid (orthorhombic) to the Fermi liquid phase with the rhombohedral lattice. From our data, we cannot distinguish whether the transition from non-Fermi to Fermi liquid exhibits a coexistent domain similar to the pressure-induced O↔R transition at room temperature or if it is a first-order transition, because our structural high-pressure data were performed at ambient temperature instead. However, only high-pressure SXRD data at low temperature and resistivity characterization above 2 GPa could shed light on this topic and probe whether the phase coexistence plays a role in the non-Fermi liquid behavior in PrNiO₃.

**4.2. The O↔R Transition. 4.2.1. High-Temperature-Induced Transition.** The structural O↔R transition is exclusive to the medium R cation size group of nickelates. Only one work reports on the occurrence of this transition in PrNiO₃ at 773 K.<sup>24</sup> This rhombohedral phase is reported at 1193 K for NdNiO₃, as obtained from differential thermal analyses,<sup>21,51</sup> showing the R cation size effects on the transition onset temperature. We used the multiscale information gained in this work as presented in Figure 6e–h to track the evolution of the structural and electronic properties across such a transition. The bulk structure obtained from SXRD [Figure 6h] depicts a sharp volume contraction of ~2% across the O↔R transition. This contraction is similar but 10 times greater compared to the volume contraction observed across the I(AFM)MT (Figure 6d). The medium-range parameters [XANES, $A_3$ and $A_1$ features in Figure 6e,f] decrease progressively across the O↔R transition, which is very similar but more pronounced to their behavior across the monoclinic to orthorhombic I(AFM)MT. Also, the superexchange angle $\Phi$ follows the same behavior across the O↔R and monoclinic to orthorhombic I(AFM)MT transition, yielding in both cases increasing values with increasing temperatures of $\Delta\Phi$ ~ 3.2 and 1° [Figure 6c,g], respectively. We also observed again an inverse correlation of the temperature evolutions of angle $\Phi$ and the $A_3$ feature, which was found at the I(AFM)MT transition. Across the O↔R transition, the $A_3$ feature shifts by ~0.8 eV at 900 K, which is the largest observed temperature-induced shift among the investigated XANES features. We suggest that, similar to the I(AFM)MT transition, but in a more pronounced way, the sharp increase of the superexchange angle induces a more efficient cation–anion orbital over-
lapping (Ni 3d → O 2p) that might be at the origin of the observed strong structural contraction. In contrast to I(AFM)-MT, the strong structural contraction across the O↔R transition cannot be explained by phonon- or spin-mediated processes, and thus, only electronic and structural changes can be at its origin.

### 4.2.2. High-Pressure-Induced Transition.
Our high-pressure SXRD data clearly indicate progressive evolutions of bulk structural parameters across the O↔R transition over a coexistence interval between 5.8 and 12.2 GPa (Figure 4). This observation is in stark contrast to the sharp transitions observed for these parameters in the temperature-induced I(AFM)MT and the O↔R transition. Both the onset pressure and the phase coexistence agree with the observations of Zhou et al.,²⁵ who performed laboratory high-pressure XRD measurements using epoxy as PTM, which is known to become nonhydrostatic above 1 GPa. In this case, pressure determination may be hampered by uncertainties in the thermal diffusivity near the epoxy glass transition.⁵² The pressure-induced limitation of the diffusion/collision of molecules can state the deviatoric stress along the sample.⁵³ Because we have used the most quasi-hydrostatic pressure medium (He),³⁷ the effect of nonhydrostatic stress on this transition can be ruled out as an origin for the phase coexistence. Like PrNiO₃, the phase coexistence (Pbnm + R₃c) was reported for SmNiO₃ above 21 GPa while the formation of a pure high-pressure rhombohedral structure was found only above 30 GPa.¹⁶ This Sm-based sample lies near the boundaries separating medium and small R cation size nickelates. In PrNiO₃, the coexistence domain appears in a smaller pressure interval of ~7 GPa compared to ~10 GPa (SmNiO₃). Interestingly, this coexistence is expected to occur also in NdNiO₃, but the only work reporting the pressure-induced O↔R transition in this sample observed an onset at 4 GPa without additional detail on coexistence and its extent.⁵⁴

The newly formed PrNiO₃ R₃c phase nucleates as nanosized grains, as derived from the reduced peak intensities and the broad profiles of the rhombohedral phase [Figure 4a]. The R₃c phase remains nanocrystalline upon further pressure increase, and the transition progresses up to the highest investigated pressure. These observation contrasts with those on the temperature-induced O↔R transition for which the newly formed PrNiO₃ R₃c phase exhibited narrow and intense diffraction peaks, suggesting a well-crystallized phase [in Figure 1c and Figure S1]. Upon heating, the O↔R transition also occurs in a rather narrow interval that does not include a phase coexistence domain. We can rule out that the phase coexistence and nanograin size are related to nonhydrostatic stresses. Therefore, the rhombohedral grain growth is believed to be kinetically hindered in the pressure-induced transition at ambient temperature.

Similar to the SXRD data and to the observations at high temperatures, the XANES features (i.e., the medium-scale structure) evolve in a continuous manner across the pressure-induced O↔R transition (Figure 5b–e). To extract the evolution of the super exchange angle Φ, we estimated the lattice parameters of individual phases from Le Bail refinements of the raw high-pressure SXRD data at 0.5 and 17.6 GPa. In accordance with the high-temperature observations, the angle Φ increases across the pressure-induced O↔R transition from 158.1(1)° (Pbnm, at room conditions) to 161.7(3)° (R₃c, at 17.6 GPa). Despite the fact that the pressure-induced O↔R transition exhibits apparent differences

from the high-temperature one, such as the coexistence domain and the nanograin-sized rhombohedral phase, the strong similarities in the evolution of the structural parameters suggest a similar transformation mechanism: drastic increase of the angle Φ and orbital overlapping, which enhances the metallic character of the rhombohedral phase. In addition, XANES results elucidated that the pressure-induced rhombohedral phase has a positive ΔAᵢ shift (Figure 5), while the temperature-induced rhombohedral phase has a net negative shift, for instance in A₁ or A₂ (Figure 3). We can hypothesize that the variation along the shifts when changing the phase is changed has a pure structural origin. Both rhombohedral structures (both HP and HT phases) are quite identical, but the positive shift is expected since the unit-cell parameters are smaller than those at room temperature (compression)⁵⁵ while the negative shift comes from the expanded unit cell when compared to room temperature one (expansion).⁵⁶

## 5. CONCLUDING REMARKS

We have investigated in detail the pressure- and temperature-induced phase transitions of synthetic PrNiO₃ nickelate—P2₁/n → Pbnm [I(AFM)MT] and Pbnm → R₃c (O↔R)—using SXRD, X-ray absorption spectroscopy, and magnetic measurements between 10 and 973 K and up to 21 GPa. The P/T phase diagram determined in this work is summarized in Figure 8. The combination of the employed methods enabled

<figure><img src="image_17.png" /><figcaption>**Figure 8.** Pressure/temperature (P/T) phase diagram for the PrNiO₃ nickelate. Colored filled circles indicate the points obtained from SXRD, open squares denote those extracted from XANES, and the diamond symbol designates the phase boundary derived from the magnetic susceptibility data. The color codes green, blue, black, and orange designate the monoclinic (P2₁/n), orthorhombic (Pbnm), coexistence range (O+R), and rhombohedral (R₃c) phases, respectively. The phase boundaries as thick white lines were extrapolated based on the present results. Additional data are required under simultaneous low- or high-temperature and high-pressure conditions to track the evolution of these phase boundaries.</figcaption></figure>

us to investigate the progress of structural, electronic, and magnetic properties across these transitions with local and long ranges. Overall, we observed strong similarities between the structural mechanisms of the temperature- and pressure-induced O↔R transition, which in turn also shows similarities to I(AFM)MT. For example, in agreement with the displacive nature of the transitions, we noticed rather sharp changes of the bulk crystallographic parameters, expected for the pressure-induced transitions, while parameters describing the medium- and local-range ordering presented progressive variations.⁵,²⁴ In addition, our structural data and full multiple scattering
simulation suggest that the super exchange angle (Ni−O−Ni) drives the band gap narrowing at the IMT and the metal−metal O↔R transition. Our data reveal uncommon behaviors in PrNiO₃, including for example a sharp drop of the pseudo cubic volume (Vₚ) with increasing temperature at the onset of the O↔R transition. This uncommon volume reduction under heating is similar to that observed for the I(AFM)MT transition and could be explained by increased orbital overlapping at the local scale of the Ni−O pair bond. Another distinct feature is the phase coexistence of the orthorhombic and rhombohedral phases at high pressure over a large pressure interval of ~7 GPa. This phase coexistence has been observed for selected nickelates, and the present data suggest that there is a relation between the R cation size and the extension of this phase coexistence. Further investigations are needed, especially at simultaneous low- or high-temperature and high-pressure conditions, to reveal the nature of the anomalies across the I(AFM)MT at high pressure and to better track the phase boundary evolution for the O↔R transition. Those results may also help to understand the evolution of the phase boundaries in Figure 8.

■ ASSOCIATED CONTENT

**Data Availability Statement**

The data sets used and/or analyzed are available from the
corresponding author on reasonable request.

Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.3c02949.

Temperature-dependent raw SXRD patterns of PrNiO₃
perovskite; high-pressure raw SXRD patterns of PrNiO₃
perovskite; structural parameters of the PrNiO₃ with the
monoclinic phase (P2₁/n) in the range 10–109 K;
structural parameters of PrNiO₃ with the orthorhombic
phase (Pbnm) in the range 132–700 K; structural
parameters of PrNiO₃ with the rhombohedral phase
(R3c) in the range 800–900 K; structural parameters of
PrNiO₃ at selected pressure points (0.5, 13.5, and 17.6
GPa); experimental Ni K-edge XANES spectra acquired
at room conditions where PrNiO₃ XANES is plotted
together with standard samples Ni, La(Ni₀.₉Al₀.₁)O₂.₁,
NiO, and LaNiO₃; spectral decomposition of the
XANES spectra in different temperature and pressure
conditions; temperature dependence of the parameters
(shift, FWHM, height) of the XANES features; pressure
dependence of the parameters (shift, FWHM, height) of
the XANES features; simulated XANES spectra at Ni K-
edge of PrNiO₃ nickelate as a function of temperature
using FDMNES code; temperature dependence of the
magnetic susceptibility derivative at two selected
pressures and pressure dependence of the antiferromag-
netic ordering temperature (Tₙ) for PrNiO₃ (PDF)

■ AUTHOR INFORMATION

**Corresponding Authors**

João Elias Rodrigues — European Synchrotron Radiation
Facility (ESRF), Grenoble 38000, France; ● orcid.org/
0000-0002-9220-5809; Email: rodrigues.joaoelias@
gmail.com, jerodrigues@cells.es

José Antonio Alonso — Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain;

● orcid.org/0000-0001-5329-1225; Email: ja.alonso@icmm.csic.es

**Authors**

Angelika D. Rosa – *European Synchrotron Radiation Facility* (ESRF), Grenoble 38000, France

Javier Gainza – Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain; ● orcid.org/0000-0002-1999-3116

Romualdo S. Silva – Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain;
● orcid.org/0000-0002-6749-5293

Emin Mijit – European Synchrotron Radiation Facility (ESRF), Grenoble 38000, France; ● orcid.org/0000-0002-9232-2538

Gaston Garbarino – European Synchrotron Radiation Facility (ESRF), Grenoble 38000, France

Tetsuo Irifune – Geodynamics Research Center (GRC), Ehime University, Matsuyama 790-8577, Japan

T. Shinmei – Geodynamics Research Center (GRC), Ehime University, Matsuyama 790-8577, Japan

Catherine Dejoie – European Synchrotron Radiation Facility (ESRF), Grenoble 38000, France; ● orcid.org/0000-0003-3313-3515

Norbert M. Nemes – Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain;
Departamento Física de Materiales, Universidad Complutense de Madrid, Madrid E-28040, Spain

José Luis Martínez – Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain;
● orcid.org/0000-0001-9046-8237

Mohamed Mezouar – European Synchrotron Radiation
Facility (ESRF), Grenoble 38000, France

Olivier Mathon – European Synchrotron Radiation Facility
(ESRF), Grenoble 38000, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.3c02949

**Author Contributions**

The original idea was conceived by J.E.R., A.D.R., O.M., and J.A.A. The experiments were conducted by J.E.R., A.D.R., J.A.A., J.G., R.S., G.G., E.M., M.M., C.D., J.L.M., N.M.N., and O.M. T.I. and T.S. provided nanopolycrystalline diamonds for the high-pressure X-ray absorption experiments. The data were analyzed and discussed by J.E.R., A.D.R., O.M., J.A.A., and J.L.M. The manuscript was written by J.E.R., A.D.R., and J.A.A. with contributions from all the coauthors.

**Funding**

This research was partially funded by the Spanish Ministry for
Science and Innovation (MCIN/AEI/10.13039/
501100011033) with grant number PID2021-122477OB-I00.

**Notes**

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

All the authors acknowledge the European Synchrotron
(ESRF, Grenoble) for making all the facilities available for
the X-ray diffraction (ID22, HC-4990; ID27, in-house
research) and X-ray absorption experiments (BM23, in-house
research). J.E.R. is grateful to Jeroen Jacob (for all the support
in the ESRF's high-pressure laboratory). J.E.R., J.L.M., and
N.M.N. thank the technical support from Eva Cespedes for the pressure-dependent magnetic measurements.

## ■ REFERENCES

(1) Catalan, G. Progress in Perovskite nickelate Research. *Phase Transitions* **2008**, *81* (7–8), 729–749.

(2) Zhou, J. S.; Goodenough, J. B. Chemical Bonding and Electronic Structure of RNiO₃ (R = rare Earth). *Phys. Rev. B - Condens. Matter Mater. Phys.* **2004**, *69* (15), 20–23.

(3) Prajapati, G. L.; Das, S.; Rana, D. S. Emergence of Quenched Disorder as a Dominant Control for Complex Phase Diagram of Rare-Earth nickelates. *J. Phys.: Condens. Matter* **2021**, *33* (41), No. 415401.

(4) Zhou, J.-S. Structural Distortions in Rare-Earth Transition-Metal Oxide Perovskites under High Pressure. *Phys. Rev. B* **2020**, *101* (22), No. 224104.

(5) Gawryluk, D. J.; Klein, Y. M.; Shang, T.; Sheptyakov, D.; Keller, L.; Casati, N.; Lacorre, P.; Fernández-Díaz, M. T.; Rodríguez-Carvajal, J.; Medarde, M. Distortion Mode Anomalies in Bulk PrNiO₃: Illustrating the Potential of Symmetry-Adapted Distortion Mode Analysis for the Study of Phase Transitions. *Phys. Rev. B* **2019**, *100* (20), No. 205137.

(6) Obradors, X.; Paulius, L. M.; Maple, M. B.; Torrance, J. B.; Nazzal, A. I.; Fontcuberta, J.; Granados, X. Pressure Dependence of the Metal-Insulator Transition in the Charge-Transfer Oxides RNiO₃ (R = Pr, Nd, Nd₀.₇La₀.₃). *Phys. Rev. B* **1993**, *47* (18), 12353–12356.

(7) Alonso, J. A.; García-Muñoz, J. L.; Fernández-Díaz, M. T.; Aranda, M. A. G.; Martínez-Lope, M. J.; Casais, M. T. Charge Disproportionation in RNiO₃. Perovskites: Simultaneous Metal-Insulator and Structural Transition in YNiO₃. *Phys. Rev. Lett.* **1999**, *82* (19), 3871–3874.

(8) Li, D.; Lee, K.; Wang, B. Y.; Osada, M.; Crossley, S.; Lee, H. R.; Cui, Y.; Hikita, Y.; Hwang, H. Y. Superconductivity in an Infinite-Layer nickelate. *Nature* **2019**, *572* (7771), 624–627.

(9) Zeng, S.; Li, C.; Chow, L. E.; Cao, Y.; Zhang, Z.; Tang, C. S.; Yin, X.; Lim, Z. S.; Hu, J.; Yang, P.; Ariando, A. Superconductivity in Infinite-Layer nickelate La₁⁻ₓCaₓNiO₂ Thin Films. *Sci. Adv.* **2022**, *8* (7), 1–6.

(10) Song, Q.; Doyle, S.; Pan, G. A.; El Baggari, I.; Ferenc Segedin, D.; Córdova Carrizales, D.; Nordlander, J.; Tzschaschel, C.; Ehrets, J. R.; Hasan, Z.; El-Sherif, H.; Krishna, J.; Hanson, C.; LaBollita, H.; Bostwick, A.; Jozwiak, C.; Rotenberg, E.; Xu, S.-Y.; Lanzara, A.; N'Diaye, A. T.; Heikes, C. A.; Liu, Y.; Paik, H.; Brooks, C. M.; Pamuk, B.; Heron, J. T.; Shafer, P.; Ratcliff, W. D.; Botana, A. S.; Moreschini, L.; Mundy, J. A. Antiferromagnetic Metal Phase in an Electron-Doped Rare-Earth nickelate. *Nat. Phys.* **2023**, *19* (4), S22–S28.

(11) Puebla, J.; Kim, J.; Kondou, K.; Otani, Y. Spintronic Devices for Energy-Efficient Data Storage and Energy Harvesting. *Commun. Mater.* **2020**, *1* (1), 1–9.

(12) Shamblin, J.; Heres, M.; Zhou, H.; Sangoro, J.; Lang, M.; Neuefeind, J.; Alonso, J. A.; Johnston, S. Experimental Evidence for Bipolaron Condensation as a Mechanism for the Metal-Insulator Transition in Rare-Earth nickelates. *Nat. Commun.* **2018**, *9* (1), 1–7.

(13) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; García-Muñoz, J. L.; Fernández-Díaz, M. T. Room-Temperature Monoclinic Distortion Due to Charge Disproportionation in Perovskites with Small Rare-Earth Cations Y, Er, Tm, Yb, and Lu): A Neutron Diffraction Study. *Phys. Rev. B* **2000**, *61* (3), 1756–1763.

(14) Mazin, I. I.; Khomskii, D. I.; Lengsdorf, R.; Alonso, J. A.; Marshall, W. G.; Ibberson, R. M.; Podlesnyak, A.; Martínez-Lope, M. J.; Abd-Elmeguid, M. M. Charge Ordering as Alternative to Jahn-Teller Distortion. *Phys. Rev. Lett.* **2007**, *98* (17), 1–4.

(15) Varignon, J.; Grisolia, M. N.; Íñiguez, J.; Barthélémy, A.; Bibes, M. Complete Phase Diagram of Rare-Earth nickelates from First-Principles. *npj Quantum Mater.* **2017**, *2* (1), 21.

(16) Amboage, M.; Hanfland, M.; Alonso, J. A.; Martínez-Lope, M. J. High Pressure Structural Study of SmNiO₃. *J. Phys.: Condens. Matter* **2005**, *17* (11), S783–S788.

(17) Lacorre, P.; Torrance, J. B.; Pannetier, J.; Nazzal, A. I.; Wang, P. W.; Huang, T. C. Synthesis, Crystal Structure, and Properties of

Metallic PrNiO₃: Comparison with Metallic NdNiO₃ and Semi-conducting SmNiO₃. *J. Solid State Chem.* **1991**, *91* (2), 225–237.

(18) Ramos, A. Y.; Piamonteze, C.; Tolentino, H. C. N.; Souza-Neto, N. M.; Bunau, O.; Joly, Y.; Grenier, S.; Itié, J. P.; Massa, N. E.; Alonso, J. A.; Martínez-Lope, M. J. Stability of Ni Sites across the Pressure-Induced Insulator-to-Metal Transition in YNiO₃. *Phys. Rev. B - Condens. Matter Mater. Phys.* **2012**, *85* (4), 1–5.

(19) García-Muñoz, J. L.; Amboage, M.; Hanfland, M.; Alonso, J. A.; Martínez-Lope, M. J.; Mortimer, R. Pressure-Induced Melting of Charge-Order in the Self-Doped Mott Insulator YNiO₃. *Phys. Rev. B - Condens. Matter Mater. Phys.* **2004**, *69* (9), 1–6.

(20) Zhou, J.-S.; Goodenough, J. B.; Dabrowski, B. Pressure-Induced Non-Fermi-Liquid Behavior of PrNiO₃. *Phys. Rev. Lett.* **2005**, *94* (22), No. 226602.

(21) García-Muñoz, J. L.; Rodríguez-Carvajal, J.; Lacorre, P.; Torrance, J. B. Neutron-Diffraction Study of RNiO₃ (R = La, Pr, Nd, Sm): Electronically Induced Structural Changes across the Metal-Insulator Transition. *Phys. Rev. B* **1992**, *46* (8), 4414–4425.

(22) García-Muñoz, J. L.; Aranda, M. A. G.; Alonso, J. A.; Martínez-Lope, M. J. Structure and Charge Order in the Antiferromagnetic Band-Insulating Phase of NdNiO₃. *Phys. Rev. B - Condens. Matter Mater. Phys.* **2009**, *79* (13), 1–5.

(23) Rodrigues, J. E.; Rosa, A. D.; López-Sánchez, J.; Sebastiani-Tofano, E.; Nemes, N. M.; Martínez, J. L.; Alonso, J. A.; Mathon, O. EXAFS Evidence for the Spin–Phonon Coupling in the Monoclinic PrNiO₃ nickelate Perovskite. *J. Mater. Chem. C* **2023**, *11* (2), 462–471.

(24) Huang, T. C.; Parrish, W.; Toraya, H.; Lacorre, P.; Torrance, J. B. High-Temperature Crystal Structures of Orthorhombic and Rhombohedral PrNiO₃. *Mater. Res. Bull.* **1990**, *25* (9), 1091–1098.

(25) Zhou, J. S.; Goodenough, J. B.; Dabrowski, B. Structure Anomaly and Electronic Transition in RNiO₃ (R = La, Pr, …, Gd). *Phys. Rev. B - Condens. Matter Mater. Phys.* **2004**, *70* (8), 3–6.

(26) Fitch, A.; Dejoie, C.; Covacci, E.; Confalonieri, G.; Grendal, O.; Claustre, L.; Guillou, P.; Kieffer, J.; de Nolf, W.; Petitdemange, S.; Ruat, M.; Watier, Y. ID22 – the High-Resolution Powder-Diffraction Beamline at ESRF. *J. Synchrotron Radiat.* **2023**, *30* (5), 1003–1012.

(27) Dejoie, C.; Coduri, M.; Petitdemange, S.; Giacobbe, C.; Covacci, E.; Grimaldi, O.; Autran, P.-O.; Mogodi, M. W.; Šišak Jung, D.; Fitch, A. N. Combining a Nine-Crystal Multi-Analyser Stage with a Two-Dimensional Detector for High-Resolution Powder X-Ray Diffraction. *J. Appl. Crystallogr.* **2018**, *51* (6), 1721–1733.

(28) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. *Phys. B Phys. Condens. Matter* **1993**, *192* (1–2), 55–69.

(29) Mezouar, M.; Perrillat, J.-P.; Garbarino, G.; Crichton, W. A.; Bauchau, S.; Witsch, H. Toward Fully Automated High-Pressure Beamlines: Recent Developments at Beamline ID27, ESRF. *Acta Crystallogr. Sect. A Found. Crystallogr.* **2008**, *64* (a1), C610–C610.

(30) Poręba, T.; Comboni, D.; Mezouar, M.; Garbarino, G.; Hanfland, M. Tracking Structural Phase Transitions via Single Crystal X-Ray Diffraction at Extreme Conditions: Advantages of Extremely Brilliant Source. *J. Phys.: Condens. Matter* **2023**, *35* (5), No. 054001.

(31) Prescher, C.; Prakapenka, V. B. Dioptas: A Program for Reduction of Two-Dimensional X-Ray Diffraction Data and Data Exploration. *High Press. Res.* **2015**, *35* (3), 223–230.

(32) Chijioke, A. D.; Nellis, W. J.; Soldatov, A.; Silvera, I. F. The Ruby Pressure Standard to 150 GPa. *J. Appl. Phys.* **2005**, *98* (11), No. 114905.

(33) Mathon, O.; Beteva, A.; Borrel, J.; Bugnazet, D.; Gatla, S.; Hino, R.; Kantor, I.; Mairs, T.; Munoz, M.; Pasternak, S.; Perrin, F.; Pascarelli, S. The Time-Resolved and Extreme Conditions XAS (Texas) Facility at the European Synchrotron Radiation Facility: The General-Purpose EXAFS Bending-Magnet Beamline BM23. *J. Synchrotron Radiat.* **2015**, *22*, 1548–1554.

(34) Ziegler, E.; Aquilanti, G.; Mathon, O.; de Panfilis, S.; van Vaerenbergh, P. EXAFS Measurements at the Micrometer-scale Spatial Resolution Using Achromatic Multilayer-based Focusing Optics. *X-Ray Spectrom.* **2009**, *38* (3), 250–253.
(35) Ishimatsu, N.; Matsumoto, K.; Maruyama, H.; Kawamura, N.; Mizumaki, M.; Sumiya, H.; Irifune, T. Glitch-Free X-Ray Absorption Spectrum under High Pressure Obtained Using Nano-Polycrystalline Diamond Anvils. *J. Synchrotron Radiat.* **2012**, *19* (5), 768–772.

(36) Rosa, A. D.; Mathon, O.; Torchio, R.; Jacobs, J.; Pasternak, S.; Irifune, T.; Pascarelli, S. Nano-Polycrystalline Diamond Anvils: Key Devices for XAS at Extreme Conditions: Their Use, Scientific Impact, Present Status and Future Needs. *High Press. Res.* **2020**, *40* (1), 65–81.

(37) Klotz, S.; Chervin, J.-C.; Munsch, P.; Le Marchand, G. Hydrostatic Limits of 11 Pressure Transmitting Media. *J. Phys. D. Appl. Phys.* **2009**, *42* (7), No. 075413.

(38) Ravel, B.; Newville, M. *ATHENA and ARTEMIS* **2020**, *I*, 1–5.

(39) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. *J. Synchrotron Radiat.* **2005**, *12* (4), 537–541.

(40) Bunău, O.; Joly, Y. Self-Consistent Aspects of X-Ray Absorption Calculations. *J. Phys.: Condens. Matter* **2009**, *21* (34), No. 345501.

(41) Medarde, M.; Dallera, C.; Grioni, M.; Delley, B.; Vernay, F.; Mesot, J.; Sikora, M.; Alonso, J. A.; Martínez-Lope, M. J. Charge Disproportionation in RNiO₃ Perovskites (R = Rare-Earth) from High-Resolution X-Ray Absorption Spectroscopy. *Phys. Rev. B* **2009**, *80* (24), No. 245105.

(42) Woodward, P. M. Octahedral Tilting in Perovskites. *I. Geometrical Considerations*. *Acta Crystallogr. Sect. B Struct. Sci.* **1997**, *53* (1), 32–43.

(43) Medarde, M.; Fernández-Díaz, M. T.; Lacorre, P. Long-Range Charge Order in the Low-Temperature Insulating Phase of PrNiO₃. *Phys. Rev. B - Condens. Matter Mater. Phys.* **2008**, *78* (21), 2–5.

(44) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained from a Systematic Analysis of the Inorganic Crystal Structure Database. *Acta Crystallogr. Sect. B Struct. Sci.* **1985**, *41* (4), 244–247.

(45) Mesot, J.; Medarde, M.; Rosenkranz, S.; Fischer, P.; Lacorre, P.; Gobrecht, K. Neutron Diffraction Study of the Metal Insulator Transition in PrNiO₃. *High Press. Res.* **1995**, *14* (1–3), 35–40.

(46) Cheng, J. G.; Zhou, J. S.; Goodenough, J. B.; Alonso, J. A.; Martinez-Lope, M. J. Pressure Dependence of Metal-Insulator Transition in Perovskites RNiO₃ (R = Eu, Y, Lu). *Phys. Rev. B - Condens. Matter Mater. Phys.* **2010**, *82* (8), 1–7.

(47) Acosta-Alejandro, M.; De León, J. M.; Medarde, M.; Lacorre, P.; Konder, K.; Montano, P. A. Local Lattice Structure Change in PrNiO₃ across the Metal-Insulator Transition: X-Ray Absorption near-Edge Structure Spectroscopy and Ab Initio Calculations. *Phys. Rev. B - Condens. Matter Mater. Phys.* **2008**, *77* (8), 1–5.

(48) Medarde, M.; Fontaine, A.; García-Muñoz, J. L.; Rodríguez-Carvajal, J.; de Santis, M.; Sacchi, M.; Rossi, G.; Lacorre, P. R NiO₃ Perovskites (R = Pr, Nd): Nickel Valence and the Metal-Insulator Transition Investigated by X-Ray-Absorption Spectroscopy. *Phys. Rev. B* **1992**, *46* (23), 14975–14984.

(49) Medarde, M.; Lacorre, P.; Conder, K.; Fauth, F.; Furrer, A. Giant ¹⁶O–¹⁸O Isotope Effect on the Metal-Insulator Transition of RNiO₃ Perovskites (R = Rare Earth). *Phys. Rev. Lett.* **1998**, *80* (11), 2397–2400.

(50) Caviglia, A. D.; Scherwitzl, R.; Popovich, P.; Hu, W.; Bromberger, H.; Singla, R.; Mitrano, M.; Hoffmann, M. C.; Kaiser, S.; Zubko, P.; Gariglio, S.; Triscone, J.-M.; Först, M.; Cavalleri, A. Ultrafast Strain Engineering in Complex Oxide Heterostructures. *Phys. Rev. Lett.* **2012**, *108* (13), No. 136801.

(51) Bassou, A. A.; Machado, P. J.; Gomes, M. M.; Manjunath, B.; Vilarinho, R.; Silva, B.; Oliveira, J.; Almeida, B.; Almeida, A.; Fernandes, J. R.; Fernandes, L. S.; Moreira, J. A.; Tavares, P. B. Thermal Stability and Decomposition Kinetics of NdNiO₃-δ at 1 bar of O₂. *Mater. Today Commun.* **2021**, *28* (December 2020), 102663–9.

(52) Sundqvist, B.; Sandberg, O.; Backstrom, G. The Thermal Properties of an Epoxy Resin at High Pressure and Temperature. *J. Phys. D. Appl. Phys.* **1977**, *10* (10), 1397–1403.

(53) Nakamae, K.; Nishino, T.; AiRu, X.; Takatsuka, K. Pressure Dependence of the Curing Behavior of Epoxy Resin. *Polym. J.* **1991**, *23* (10), 1157–1162.

(54) Medarde, M.; Mesot, J.; Rosenkranz, S.; Lacorre, P.; Marshall, W.; Klotz, S.; Loveday, J. S.; Hamel, G.; Hull, S.; Radaelli, P. Pressure-Induced Orthorhombic-Rhombohedral Phase Transition in NdNiO₃. *Phys. B: Condens. Matter* **1997**, *234–236*, 15–17.

(55) Rodrigues, J. E.; Rosa, A.; Garbarino, G.; Irifune, T.; Martínez, J. L.; Alonso, J. A.; Mathon, O. Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO₃ nickelate. *Chem. Mater.* **2023**, *35* (13), 5079–5090.

(56) Joly, Y.; Bunău, O.; Lorenzo, J. E.; Galéra, R. M.; Grenier, S.; Thompson, B. Self-Consistency, Spin-Orbit and Other Advances in the FDMNES Code to Simulate XANES and RXD Experiments. *J. Phys. Conf. Ser.* **2009**, *190*, No. 012007.