# Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TiNiO₃ Nickelate

João Elias Rodrigues,* Angelika Rosa, Gaston Garbarino, Tetsuo Irifune, José Luis Martínez, José Antonio Alonso,* and Olivier Mathon

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Cite This: *Chem. Mater.* 2023, 35, 5079–5090

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Supporting Information

**ABSTRACT:** Rare-earth perovskite nickelates RNiO₃ (R stands for Y, rare earths, Tl or Bi) have attracted wide attention in the past few decades because of their unique electronic properties that emerge from the insulator–metal transition (IMT). This transition can be tuned by chemical substitutions or by external variables, like pressure and temperature, but the mechanism of this transition still remains debated. While most previous studies focused on nickelates with partially filled 4f cations on the R site, here we studied the mechanism of the pressure-induced phase transition on a rare Tl³⁺ nickelate, which comprises fully filled 4f orbitals. We probed the pressure-induced structural and electronic changes taking place at the bulk, medium, and local atomic level at room temperature by using synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) as probing techniques, respectively. High-resolution XRD data under ambient conditions show evidence for the monoclinic phase of TiNiO₃, as characterized by the charge ordering between Ni³⁻ and Ni³⁺ in the insulator regime. High-pressure XRD data elucidated an anomaly in the evolution of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P2₁/n → Pbnm.

High-pressure EXAFS data at the Ni K-edge provide a solid proof for a short-order charge disproportionation under near ambient conditions. The Ni–O pair-bond compression curves unveiled an isotropic overlapping Ni t₂g⁶e⁻¹ → O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distance variance (Debye–Waller exponents) lodged a hardening at this pressure point, suggesting constrained atomic displacements in the average range of ±0.04(5) Å. We propose that this reduced motional freedom could be linked to an electron–phonon coupling, which is the driving force behind the mechanisms of the IMT. We demonstrated that the XANES technique can be applied to ascertain the pressure-induced transition, by tracking the anomalous behavior of the position and width (full width at half-maximum) of the XANES features.

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## 1. INTRODUCTION

Since the pioneering work of Demazeau et al. in the early 1970s on the synthesis and structural properties of RNiO₃ nickelates,¹ many interesting physical and chemical properties of these compounds have been reported. The key findings concern the structure–property relationships, which are prerequisites for explaining the rich phase diagram of these compounds.² Indeed, nickelate-based perovskites with formula RNiO₃ (R = Y, rare earths, Tl or Bi) are an important class of materials for both industry and fundamental sciences because of their exceptional and broad range of electronic and magnetic features.²,³ Apart from their elevated Seebeck coefficient [ISI ~ 300 µV·K⁻¹ at room temperature]⁴ and their unusual magnetic and electronic interfacial properties in heterostructures,⁵ their most interesting feature is the peculiar insulator–metal transition (IMT). This transition can be tuned through pressure, temperature, or chemical substitution at the R cation site, which opens a wide range of possibilities for industrial applications and fundamental investigation. Nickelates have been used to increase the efficiency of electrodes in fuel cells and electromagnetic sensors, the robustness of biosensors, and

nonvolatile memory devices. Despite its importance, the mechanism of the IMT remained debated over the last 30 years, being far from any consensus until now.¹,⁶⁻⁸ This knowledge would be, however, required to obtain a fundamental understanding of the IMT and for fine-tuning properties for specific applications.

Previous reports have shown that the IMT in nickelates can be parameterized by the super-exchange bond-angle (Θ), which is defined between two Ni cations and a central oxygen anion to form the Ni-bridge Ni–O–Ni.¹,⁹⁻¹¹ The bond-angle Θ is a measure of the orbital overlapping among Ni and oxygens and also the competition among the R and Ni cations with the oxygen anions on the R-bridge R–O–Ni. Highly electro-negative R cations will reduce the Ni–O covalency and,

Received: March 16, 2023

Revised: June 1, 2023

Published: June 20, 2023

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therefore, decrease the bond-angle Θ. This bond-angle (Θ) and the electron charge distribution among cations are fully dependent on the R cation size, which is the most crucial parameter for their electronic properties. For instance, the nickelates comprising large R cations and low electronegativity result in large bond-angle, such as the rhombohedral LaNiO₃ [r<sub>La</sub> = 1.36 Å;¹² Θ = 165.2(1)°], exhibiting a metallic behavior at room temperature. For small and medium R cation size nickelates, their low-temperature insulator state is associated with the monoclinic P2₁/n crystal structure, where a charge disproportionation along Ni sites is observed, while its metallic state is featured by an orthorhombic unit cell with the *Pb*nm space group in which the Ni charges are homogeneously distributed.¹³⁻¹⁸ Indeed, the small R cation nickelates (also referred to as highly distorted) with low bond-angle Θ, such as LuNiO₃ [r<sub>Lu</sub> = 0.977 Å;¹² Θ = 144.4(1)°], become metallic only at high temperature when Θ increases (for LuNiO₃, the IMT takes place at 600 K).⁴,¹⁹

A major obstacle that hinders gaining a better understanding on the transition mechanism collides with the synthesis of high-purity compounds with different R cations. Nickelate-based perovskites with small R cations (R = Y, Lu, Tl) are metastable and cannot be easily obtained by conventional solid-state synthesis methods.¹⁹,²⁰ Besides, nickel in these structures is incorporated in its trivalent valence state, a rare and highly unstable oxidation state with a low-spin configuration (t₂g⁶e₉¹). Recently, synthesis approaches based on high-pressure methods have allowed producing pure and highly crystallized nickelates in sufficient quantities for the characterization of their structural, electronic, and transport properties.²¹ TlNiO₃, belonging to the group of the small R cation nickelates, represents a very interesting candidate to probe the mechanism of the IMT. In contrast to the rare-earth cations, Tl³⁺ in TlNiO₃ has fully filled 4f orbitals, which is a unique electronic configuration of an A-site cation in nickelates. Tl exhibits a superior electronegativity compared to R = Y, La ... Lu, which drives the formation of strong covalent bonds to oxygen. Given the competing Tl−O and Ni−O interactions, there is a noticeable weakening of the Ni−O covalency, as manifested in the much higher unit cell volume under ambient conditions (e.g., 213.3 vs 206.4 Å³ for TlNiO₃ and LuNiO₃, respectively) and much lower magnetic ordering temperature T_N = 105¹⁷,²⁴ vs 125 K for LuNiO₃.¹⁰

However, to date, nothing is known about the pressure dependence of the structural and electronic properties of TlNiO₃, mainly because of the experimental difficulties to stabilize Tl³⁺ in a pure nickelate phase. Instead, previous high-pressure X-ray diffraction studies were reported for some RNiO₃ perovskites: Medarde et al. revealed the structural transition *Pb*nm → *R*3c at 4 GPa in NdNiO₃.²² Amboage et al. detected the pressure-induced metallization at 2.5 GPa in SmNiO₃ and an orthorhombic (*Pb*nm) to rhombohedral (*R*3c) transition at 26 GPa.²³ García-Muñoz et al. reported a structural transition in YNiO₃ at 14 GPa, which was consistent with the charge ordering suppression along Ni sites of the monoclinic (P2₁/n) phase.²⁴ At the same time, X-ray absorption (XAS) shed light on both local and electronic mechanisms behind the aforementioned pressure-induced transitions. For example, Ramos et al. combined XAS and ab initio calculations to reveal that YNiO₃ has a structural transition at 14 GPa that is also an IMT as driven by a bandwidth mechanism.²⁵ Massa et al. detected a valence discontinuity at 3.1 GPa in the Ni K-edge whiteline of

SmNiO₃, which was assigned to the transition of 2Ni³⁺ → Ni³⁻ + Ni³⁺δ at the onset of the bandgap opening.²⁶ Evidently, the role of the rare-earth element is completely entangled due to the 4f orbital distribution, which is not the case in TlNiO₃ and this possibly enables a better understanding of the role of the Ni local site for the electronic properties in RNiO₃.

In this work, we synthesized high-purity TlNiO₃ to study the occurrence and mechanism of the pressure-induced structural transition in this compound. We have combined X-ray diffraction (XRD) and X-ray absorption (XAS) probe techniques with high-pressure devices to follow the bulk structural properties (XRD), medium electronic and structural properties [X-ray absorption near edge structure (XANES)], and local atomic structure [extended X-ray absorption fine structure (EXAFS)] around the Ni cations up to 40 GPa at room temperature. The results of this work were used to gain a better understanding of the structural transition systematics in highly distorted nickelates with small R cations.

## 2. EXPERIMENTAL METHODS

### 2.1. High-Pressure Synthesis.
The TlNiO₃ nickelate was synthesized in a Belt-type apparatus under high pressure of 7.5 GPa at 650 °C for 2–3 min. The raw precursors Tl₂O₃ (Merck, 99%) and Ni(OH)₂ (Sigma-Aldrich, 99%) were mixed in appropriate proportions and used as starting materials. KClO₃ (30 wt %) was added, with a total reactants mass of 0.4 g, because it acts as an in situ oxidizing agent during the nickelate synthesis, by following the ratio Tl₂O₃:Ni(OH)₂:KClO₃ = 1:2:1. All the reactants can be manipulated in a room atmosphere. The mixture was encapsulated in a gold tube with a diameter of 3 mm, and placed in a graphite tube (internal heater) with an inner diameter of 5 mm. Sodium chloride powder filled the space between the heater and the gold capsule, forming an electrically insulating barrier and improving pressure homogeneity. After the quenching, the remaining quantities of potassium chloride were removed by washing and filtering the final product. More details on the high-pressure synthesis procedure for nickelates can be found elsewhere.²⁷

### 2.2. Structural Characterization under Ambient Conditions.
To identify the structural properties of the synthetic TlNiO₃ powder and to distinguish between the monoclinic and orthorhombic phases, we performed high-angular resolution X-ray diffraction at the beamline ID22 of the EBS-ESRF (high-resolution powder diffraction beamline). For this purpose, we filled a capillary of 0.5 mm diameter with TlNiO₃ powder. A diffractometer with a multi-analyzer stage (with 13 crystals) was used with an incident energy of 35 keV (λ = 0.35418 Å).²⁸ The X-ray powder diffraction patterns were refined by the Rietveld method using the Fullprof software.²⁹ The refinements included the following parameters: zero-point error, background coefficients, scale factor, asymmetry factors, lattice parameters, atomic fractional coordinates, and isotropic thermal displacements (B<sub>iso</sub>). The B<sub>iso</sub> parameters were refined as independent for Tl, Ni1, and Ni2, but an overall B<sub>iso</sub> was considered for the sites O1, O2, and O3.

### 2.3. High-Pressure Generation.
Two experimental runs under high pressure were performed using XRD and XAS as probe techniques (run1, run2), respectively. In both runs, the membrane-driven diamond anvil cell (mDAC of LeToullec-type) was employed to generate a high-pressure field. The mDACs were equipped with two full single-crystalline diamonds of 300 µm culet size for XRD measurements. For high-pressure XAS measurements at the Ni K-edge, the nano-polycrystalline diamonds (NPDs) were considered that comprised one full and one partially perforated diamonds having culet sizes of 300 µm. Those NPDs yield a continuous diffraction background and, then, allow acquiring glitch-free high-quality EXAFS data.³⁰,³¹ Compacted powder pellets of the starting material with an average thickness of ~10 µm together with a ruby sphere (as the pressure gauge) were loaded with either helium (run1) or neon (run2) as pressure-transmitting medium (PTM). Both PTMs ensured
<figure><img src="image_8.png" /><figcaption>**Figure 1.** Rietveld refinement for high-resolution synchrotron X-ray diffraction of the TiNiO₃ nickelate under room conditions. The empty circles denote the experimental data, and the solid red line represents the calculated profile. The blue line is the difference pattern between experimental and calculated data, and the green vertical bars are the Bragg reflections. The inset confirms the quality of the refinement for high diffraction angles in the 2θ = 26–30° range.</figcaption></figure>

<table><caption>**Table 1.** Structural Parameters of the TiNiO₃ Nickelate under Room Conditions Obtained in the Rietveld Refinement from the High-Resolution Synchrotron X-ray Diffraction Data</caption><thead><tr><th>atom</th><th>Wyckoff site</th><th>x</th><th>y</th><th>z</th><th>B<sub>iso</sub> (Å<sup>2</sup>)</th></tr></thead><tbody><tr><td>Tl1</td><td>4e</td><td>0.98675(4)</td><td>0.04847(3)</td><td>0.25297(8)</td><td>0.382(5)</td></tr><tr><td>Ni1</td><td>2d</td><td>0.50</td><td>0.50</td><td>0.50</td><td>0.42(6)</td></tr><tr><td>Ni2</td><td>2c</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.20(5)</td></tr><tr><td>O1</td><td>4e</td><td>0.1050(5)</td><td>0.4529(7)</td><td>0.252(2)</td><td>0.57(4)</td></tr><tr><td>O2</td><td>4e</td><td>0.694(2)</td><td>0.304(2)</td><td>0.043(1)</td><td>0.57(4)</td></tr><tr><td>O3</td><td>4e</td><td>0.201(2)</td><td>0.209(2)</td><td>0.932(1)</td><td>0.57(4)</td></tr><tr><th colspan="2">unit cell parameters</th><th colspan="2">reliability factors</th><th colspan="2">average bonds and angle</th></tr><tr><td>a (Å)</td><td>5.25785(2)</td><td>R<sub>p</sub> (%)</td><td>6.64</td><td>⟨Ni1−O⟩ (Å)</td><td>1.973(4)</td></tr><tr><td>b (Å)</td><td>5.37013(2)</td><td>R<sub>exp</sub> (%)</td><td>2.99</td><td>⟨Ni2−O⟩ (Å)</td><td>1.969(4)</td></tr><tr><td>c (Å)</td><td>7.56570(3)</td><td>R<sub>wp</sub> (%)</td><td>8.94</td><td>⟨Ni1−O−Ni2⟩ (°)</td><td>145.7(5)</td></tr><tr><td>β (°)</td><td>89.988(1)</td><td>χ<sup>2</sup></td><td>8.95</td><td></td><td></td></tr><tr><td>V (Å<sup>3</sup>)</td><td>213.620(1)</td><td>R<sub>Bragg</sub> (%)</td><td>2.28</td><td></td><td></td></tr><tr><td>ρ (g·cm<sup>−3</sup>)</td><td>9.672</td><td>R<sub>f</sub> (%)</td><td>1.59</td><td></td><td></td></tr></tbody></table>

quasi-hydrostatic conditions up to the highest probed pressure.32 In all the experiments, the pressure was determined using an online luminescence apparatus and the shift of the R₁ line of ruby.30

**2.4. XRD Experiments and Analysis.** High-pressure synchrotron powder X-ray diffraction data in run1 were acquired at the beamline ID15B (High-Pressure Diffraction beamline, EBS-ESRF), with an incident beam of λ = 0.4112 Å that was focused down to 10 × 10 µm² [full width at half-maximum (FWHM)] spot-size using beryllium refractive lenses. A mar555 image-plate detector was placed behind the sample for acquiring 2D-XRD images. The angle dispersive XRD images were acquired in continuous ω-scan (−5° ≤ ω ≤ +5°) with an exposure time of 2 s. The XRD patterns at the lowest pressure points (<9 GPa) were refined by the Rietveld method using the Fullprof software like the high-resolution XRD data. However, as the pressure increased, the parameters in the refinement output began to be unreliable, for instance with negative isotropic thermal displacements. For this reason, all the HP dataset was refined again by the Le Bail method29 to provide only the overall lattice parameters as a function of pressure.

**2.5. XAS Experiments and Analysis.** Transmission-mode XAS data in run2 at Ni K-edge (8.333 keV) were acquired at the beamline BM23 (X-ray Absorption beamline, EBS-ESRF),33 being equipped with a fixed-exit double-crystal Si(111) monochromator (with an energy resolution: ΔE/E ≈ 1.37 × 10⁻⁴). Ionization chambers were

employed and filled with appropriate gas mixtures to measure the beam intensity before and after the sample. The monochromator energy (the Bragg angle) was calibrated using Ni foil (Goodfellow, 99.95%) prior to and frequently during the experiment. In addition, the position of the monochromator glitches was verified for all the spectra. For high-pressure XAS experiments in the mDAC (run2), the monochromatic beam was focused down to a spot size of 3 × 3 µm² (FWHM) using two Pt-coated mirrors in the Kirkpatrick−Baez geometry.34,35 Higher order harmonics were rejected by setting these mirrors to an incidence angle of 5.75 mrad. The EXAFS data were analyzed using the software Artemis, which is based on FEFF's multiple-scattering path expansion.36,37 Two Fourier-transform (FT) windows in both *k* and *R* spaces were set to values of Δ*k* = 9.2 Å⁻¹ and Δ*R* = 3.3 Å, which enable a free parameter space of 25 points. Only single scattering paths up to the third shell were considered, as we will discuss later. Fitted parameters included the average path distance and the path distance variance (or Debye−Waller exponent) for each path. The coordination number (CN) was kept fixed as extracted from our refined X-ray diffraction data. The amplitude reduction factor (S₀²) was kept constant to the value obtained from a fit of the reference Ni foil (S₀² ≈ 0.71698). In total of 14 parameters were used in the fittings including the background parameters.
## 3. RESULTS

### 3.1. Probing the Charge Disproportionation Using HR-XRD.
Prior to the high-pressure experiments, we characterized the structure of TlNiO₃ at room conditions using high-resolution synchrotron X-ray diffraction (HR-XRD) Figure 1 shows the XRD pattern together with its Rietveld refinement. The room temperature phase was indexed according to the monoclinic space group P2₁/n, which has one Tl site (Tl1), two nonequivalent Wyckoff sites for Ni ions (Ni1, Ni2) for accounting the charge disproportionation, and three nonequivalent oxygens (O1, O2, O3). The monoclinic phase is expected from the phase diagram of RNiO₃ nickelates; however, the angle β of TlNiO₃ is very close to 90° by Δβ = 0.012(1)° meaning that such an unit cell has a strong orthorhombic (Pbnm) character (see in Figure S1 of the Supporting Information). For the sake of comparison, TlNiO₃ has a monoclinic unit cell at room condition with β = 89.988(1)°, while YNiO₃ has β = 90.081(1)° and LuNiO₃ with β = 90.163(1)° at the same conditions.<sup>14,15</sup> Table 1 lists the atomic positions, the lattice parameters, the reliability factors, and the main interatomic distances and angles obtained for TlNiO₃. We noted that the octahedra [Ni1O₆] (Ni³⁻) and [Ni2O₆] (Ni³⁺) follow the 1:1 arrangement in the refined structure, which is a feature of an ordered perovskite structure due to the charge disproportionation.<sup>20</sup> This observation suggests that the charge ordering (2Ni³⁺ → Ni³⁻ + Ni³⁺) indeed occurs in TlNiO₃. The average bond-angle 〈Ni1–O–Ni2〉 in TlNiO₃ was evaluated to be 145.7(5)°, agreeing with the bond-angle of 144.4(1)° for LuNiO₃, which is also a small R nickelate; however, the average distances of 〈Ni1–O〉 = 1.973(4) Å and 〈Ni2–O〉 = 1.969(4) Å obtained from the refinement are very similar (ΔR<sub>1,2</sub> = 0.004 Å), showing that the charge disproportionation in TlNiO₃ may not extent at long-range regimes. Instead, one would expect well distinguishable average distances for Ni1,2–O bonds, such as 〈Ni1–O〉 = 1.995(1) Å and 〈Ni2–O〉 = 1.917(1) Å (ΔR<sub>1,2</sub> = 0.078 Å) for LuNiO₃ that has a defined long-range charge ordering. The extent of the charge ordering in TlNiO₃ will be better addressed using local-environment sensitive techniques, viz., X-ray absorption spectroscopy as discussed later.

### 3.2. Pressure-Induced Phase Transition in TlNiO₃ Using XRD.
In the next step, we monitored the structural behavior of TlNiO₃ up to 32 GPa using X-ray diffraction. Because the high-resolution XRD data demonstrated that TlNiO₃ has a monoclinic phase at room conditions with a strong orthorhombic character, we applied the orthorhombic structural model for the Le Bail refinement of all the pressure points (see the Experimental Methods). Indeed, the limited angular resolution of the high-pressure XRD instrumentation and possible preferred orientation of the TlNiO₃ sample at the lowest pressure points inhibited a precise refinement of small oxygen position variations and the β angle, which are required to describe the monoclinic phase. Therefore, only the overall structural changes including the pressure evolution of the unit cell parameters *a*, *b*, and *c* were tracked using this structural model. Uncertainties introduced in these parameters are discussed in the Supporting Information, see Tables S1 and S2. Le Bail refinements at 0.35 and 31.1 GPa are displayed in Figure 2a,b, respectively. For the highest pressure point, we also tested the rhombohedral unit cell (R3c) as an input model, similar to that reported for SmNiO₃;<sup>23</sup> however, the number of

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<figure><img src="image_10.png" /><figcaption>**Figure 2.** Le Bail refinements in the TlNiO₃ nickelate: (a) at 0.35 and (b) 31.1 GPa using the orthorhombic unit cell belonging to the Pbnm space group. The empty circles denote the experimental data, and the solid red line represents the calculated profile. The blue line is the difference pattern between experimental and calculated data, and the green vertical bars are the Bragg reflections.</figcaption></figure>

Bragg reflections did not match with that for the rhombohedral model, and therefore discarding the occurrence of this phase.

The pressure evolution of the lattice parameters using the orthorhombic unit cell model is displayed in Figure 3a. Clear discontinuities in the pressure evolution of all three unit cell parameters *a*, *b*, and *c* are evident at 10.8 GPa. These discontinuities are more prominently represented by the “tetragonality” factors *c*/*a* and *c*/*b*, as shown in Figure 3b. At 25.4 GPa, additional but smaller slope changes are observed in these factors. Figure 4 shows some selected synchrotron X-ray diffraction patterns at different pressures in two 2θ ranges: 8.6–9.4° (Figure 4a and 15.2–16.4° (Figure 4b. We have sectored the pressure range into three sub-regions: 0–10.8 (I), 10.8–25.4 (II), and above 25.4 GPa (III). Visually, it is difficult to identify new diffraction peaks appearing or disappearing as a result of a phase transition, except a slight peak width increase as pressure increases. The resistance to the compression in TlNiO₃ was evaluated by fitting the third-order Birch–Murnaghan (BM) isothermal equation of state (EOS),³⁸ to the data displayed in Figure 4c to extract the bulk modulus (B₀) and its first derivative (B₀' = ∂B₀/∂P). The fitting was performed at the entire pressure range available and provided values for B₀ = 159(1) GPa, B₀' = 5.8(1), and V₀ = 213.80(3) Å³. These values are close to those previously reported for SmNiO₃ and YNiO₃ with B₀ of 167(4) and 169(3) GPa, respectively.<sup>23,24</sup>

The P–V curve did not show anomalies at 10.8 and 25.4 GPa. An alternative method to unveil small compressional
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<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a) Pressure-dependence of the lattice parameters (*a*, *b*, and *c*) and (d) ratios (*c/a* and *c/b*) for the TiNiO$_{3}$ nickelate as extracted from the Le Bail refinement of the high-pressure XRD data. We sectored the pressure range into three sub-regions: 0–10.8 (I), 10.8–25.4 (II), and above 25.4 GPa (III).</figcaption></figure>

anomalies is considering the $f_E - F_E$ approach as shown in Figure 4c. $f_E$ is the Eulerian strain, given by $f_E = [(V/V_0)^{-2/3} - 1]/2$, and $F_E$ is the normalized pressure (in units of P), as obtained from $F_E = P/[3f_E(1 + 2f_E)^{5/2}]$.³⁹,⁴⁰ For an anomaly-free compression, one may expect no variation in $F_E$ as the Eulerian strain increases. In the $f_E - F_E$ plot, three different slopes are recognized, coinciding with the pressure sub-regions I, II, and III. The variation in the compressibility may be evidence of a structural phase transition induced under pressure, see Table S3. In our case, we have seen two anomalies as a function of pressure at 10.8 and 25.4 GPa. An interesting trend is that the parameter *c* becomes more compressible when pressure increases, which is not the case along the parameter *a* (see Table S3). In the literature,²⁴ it is reported that the YNiO$_{3}$ nickelate undergoes the structural transition $P2_1/n \rightarrow Pbnm$ at 14 GPa, followed by a drop in the optical bandgap and a step-jump of the lattice parameter *c*, which is also featured by compressibility variations. Indeed, these anomalies along the lattice parameters are a common signature of the charge rearrangement to stabilize the metallic state in small *R* cation nickelates.¹⁹,²³,²⁵ Then, the anomaly at 10.8 GPa in Figure 3c is a strong evidence for the structural phase transition $P2_1/n \rightarrow Pbnm$. Besides, by similarity, we may hypothesize that this transition is accompanied by the IMT as for YNiO$_{3}$ and TiNiO$_{3}$ belonging to the same nickelate group with small *R* cation size; however, the final confirmation of this assumption needs additional measurements by means of infrared spectroscopy and/or resistivity. The second anomaly at 25.4 GPa is probably not linked to a particular structural phase transition. One hypothesis would be a rhombohedral phase, as occurring in SmNiO$_{3}$ at 26 GPa; however, this phase was discarded by our Le Bail analysis.²³ A second hypothesis comprises an electron–lattice interaction, which is driven by an orbital coupling. In TiNiO$_{3}$, both mechanisms can be

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Selected X-ray diffraction patterns under pressure in two $2\theta$ ranges: (a) 8.6–9.4° and (b) 15.2–16.4°. (c) Unit cell volume as a function of pressure and the third-order Birch–Murnaghan isothermal equation of state fitting for TiNiO$_{3}$. The inset of *c* denotes the pressure evolution of the normalized pressure $F_E$ against the Eulerian strain $f_E$, which should follow a linear trend. Deviations from this linear trend are highlighted by vertical dashed lines. (d) Pressure dependence of the lattice parameter *c*, as obtained from the Le Bail refinement of the synchrotron X-ray diffraction (SXRD) and X-ray absorption spectroscopy (EXAFS). In panel 4d, the open symbol means the pressure point taken under pressure release.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 5.** EXAFS data of the TlNiO₃ nickelate at 2.4 GPa: (a) *k*²-weighted oscillation χ(*k*), Fourier transform magnitude of (b) *k*²χ(*k*) and (c) real part of χ(*R*), and (d) sketches of the single scattering paths used to best fit the EXAFS data. Experimental points are denoted by open symbols, while solid lines are the best fit obtained using an input structural model outlined in Table 2. The right panel represents the single scattering paths to best fit the EXAFS oscillations.</figcaption></figure>

manifested along variations observed in the compressibility coefficients (see Table S3).

**3.3. Local Atomic Structure under Pressure Using EXAFS.** We acquired high-pressure X-ray absorption spectroscopy data to probe the electronic and local structural evolution of TlNiO₃ with pressure in detail. Figure 5a exhibits the experimental and fitted *k*²-weighted χ(*k*) oscillations for the TlNiO₃ nickelate at 2.4 GPa. We have considered five scattering paths Γⱼ to describe the EXAFS oscillations in the *k* range 1.8–11 Å⁻¹ (see Figure 5d). The modulus and the real part of the FT oscillations χ(*R*) of TlNiO₃ are drawn in Figure 5b,c, respectively. Table 2 lists the fitted parameters (Rᵣ, σᵢᵢ²) for each scattering path at 2.4 GPa. One may not expect variations of the CN around the Ni absorber; therefore, the Nᵣ parameter was kept fixed. The modulus of the FT |χ(*R*)|

<table><caption>**Table 2.** Structural Data Refined from EXAFS under Room Conditions, where Five Single Scattering Paths Were Used To Best Fit the Spectrum<sup>a</sup></caption><thead><tr><th rowspan="2">attribution</th><th rowspan="2">path</th><th colspan="3">EXAFS at 2.4 GPa</th><th>ND at RT</th></tr><tr><th>R<sub>Γ</sub> (Å)</th><th>N<sub>Γ</sub><sup>20,27</sup></th><th>σ<sub>Γ</sub><sup>2</sup> (×10<sup>3</sup> Å<sup>2</sup>)</th><th>d (Å)</th></tr></thead><tbody><tr><td>Γ<sub>1</sub></td><td>Ni–O<sub>1a</sub></td><td>1.967(4)</td><td>4</td><td>6.2(3)</td><td>1.971(5)</td></tr><tr><td>Γ<sub>2</sub></td><td>Ni–O<sub>2a</sub></td><td>1.932(4)</td><td>2</td><td>6.2(3)</td><td>1.937(10)</td></tr><tr><td>Γ<sub>3</sub></td><td>Ni–Tl<sub>3</sub></td><td>3.045(9)</td><td>2</td><td>5.9(9)</td><td>3.074(1)</td></tr><tr><td>Γ<sub>4</sub></td><td>Ni–Tl<sub>4</sub></td><td>3.159(9)</td><td>2</td><td>5.9(9)</td><td>3.188(1)</td></tr><tr><td>Γ<sub>5</sub></td><td>Ni–Ni</td><td>3.825(8)</td><td>6</td><td>9.9(8)</td><td>3.781(1)</td></tr></tbody></table>

<sup>a</sup>Rᵣ is the average path distance, Nᵣ the average coordination number kept fixed according to our X-ray diffraction and the neutron powder diffraction data (ND),<sup>20,27</sup> and σᵢᵢ² the Debye–Waller exponent.

exhibits a first peak at 1.95 Å, followed by a second one at 2.98 Å and a last feature at 3.58 Å. As a starting point before EXAFS modeling, we considered the scattering paths calculated from the monoclinic phase (P2₁/n); however, two nickel atoms (Ni1, Ni2) at different sites within this monoclinic phase did not enter directly as variables of our fittings, since the usable range in *k* for EXAFS analysis was Δ*k* = 9.2 Å⁻¹ and, thus, limiting the number of independent variables extracted from the fittings. Instead, the first oxygen shell surrounding the Ni absorber was modeled taking two different average bonds (the pair-distance Γ₁ and Γ₂) to describe the two average octahedral units [Ni1O₆] and [Ni2O₆] with charge disproportionation Ni³⁻ and Ni³⁺, respectively. This reliable model of Ni–O distances with two sets of averaged bond distances, which will be referred to as Ni–O₁a and Ni–O₂a (“a” means “average”), was built by (1) averaging the 8-long Ni–O distances with *d*₁ ≥ 1.955 Å and (2) averaging the remaining Ni–O paths with *d*₂ < 1.95 Å, which are the 4-short Ni2–O₁₃ (see the sketch of the local environment of [Ni1,2O₆] units in Figure S3). The same model was recently applied for EXAFS modeling in PrNiO₃.<sup>41</sup> The correct weighting of path contributions during fitting was set to the CN of 4 and 2 for Ni–O₁a and Ni–O₂a, respectively.

The reliability of the EXAFS model demonstrated that two Ni–O distances are clearly seen from the fitting in Table 2, i.e., Ni–O₁a [R₁ = 1.967(4) Å] and Ni–O₂a [R₂ = 1.932(4) Å], which proves that two octahedral [Ni1,2O₆] units possess different average distances, with ΔR₁₂ = 0.035 Å due to the charge ordering between Ni³⁻ and Ni³⁺. However, this ordering is claimed to take place only at short-range and being well-localized around the Ni absorbers. It means that although the long-range charge disproportionation was almost dis-
<figure><img src="image_15.png" /><figcaption>**Figure 6.** Pressure-dependent Ni K-edge EXAFS raw data and fits: (a) $k^3$-weighted EXAFS oscillations $\chi(k)$ at selected pressures. (b) Modulus and real part of the Fourier transform EXAFS oscillations $|\chi(R)|$ and $\text{Re}[\chi(R)]$ in R space. The open symbols are the experimental data, while solid lines are the best fit adjusted. Vertical dotted lines are guides for the eye.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 7.** Pressure dependence of the path distances and their respective parallel MSRDs as derived from the EXAFS fitting: (a) pair-bonds Ni–O and (b) parallel MSRDs of paths $\Gamma_1$ and $\Gamma_2$; (c) cationic scattering path Ni–Tl and (d) parallel MSRDs of paths $\Gamma_3$ and $\Gamma_4$; and (e) cationic scattering path Ni–Ni and (f) its parallel MSRDs. The open symbols represent points taken under pressure release.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Figure 8. Pressure-dependent XANES at Ni K-edge of the TlNiO$_{3}$ nickelate: (a) Stacked XANES spectra as a function of pressure in the range 0–40 GPa. The pressure evolution of the peak shift (ΔA) and full width at half-maximum (FWHM) for the features (b) A$_{1}$, (c) A$_{2}$, (d) A$_{3}$, and (e) A$_{4}$, respectively. Spheres represent the shifts and open squares of the FWHM.</figcaption></figure>

regarded from HR-XRD, such a process still occurs locally in the TlNiO$_{3}$ nickelate according to EXAFS data.

The peak of the FT modulus at around ~3 Å can be associated with the cationic Ni–Tl pair units, with two paths Γ$_{3}$ and Γ$_{4}$. The cationic Ni–Ni pair unit accounts for the feature at ~3.6 Å. Because our EXAFS input model is an approximate solution for describing the short-range structure of the TlNiO$_{3}$ nickelate in the entire pressure range,⁴¹ we expect that associated errors with this assumption will be manifested through the Debye–Waller exponent [σ$_{‖}$² or ⟨u$_{‖}$²⟩] of all the scattering paths.

Based on the above EXAFS model, we probed the short-range structure (the path distance and path distance variance or Debye–Waller factor) of TlNiO$_{3}$ perovskite at room temperature and in a pressure range of 0–40 GPa. The raw EXAFS spectra are stacked in Figure 6: (a) k³χ(k) oscillations and (b) the moduli of χ(R) and their real parts Re[χ(R)]. A progressive increase of the amplitude FT oscillations |χ(R)| is observed as the pressure increases. This fact is regarded as the vibrational contributions to the Debye–Waller exponent, meaning that the high-pressure environment starts to constrain the atomic motions along the parallel direction to the path (i.e., the parallel MSRD ‖, mean-square relative displacement). A monotonic reduction of the pair distances R$_{Γ}$ is evidenced by the shift to smaller R values when pressure increases. For the first shell (Γ$_{1}$ and Γ$_{2}$), this contraction is 6.7–6.8% at 41 GPa from the room condition values.

The pressure dependence of the EXAFS structural parameters is exhibited in Figure 7. In panel Figure 7a, the pressure evolution of Ni–O bond distances is presented: it shows two slight anomalies at around 10.8 and 25.4 GPa in good agreement with the high-pressure XRD data. Based on these observations, the pressure interval was also divided into three sub-regions I, II, and III, in which the Ni–O bond distance variations with pressure display differing slopes. The parallel MSRDs (σ$_{‖}$²) of the pair-bond Ni–O (constrained to be the same for both Γ$_{1}$ and Γ$_{2}$) are shown in panel Figure 7b. It first monotonically decreases from ambient pressure to 10.8 GPa and then stabilizes up to the highest pressures. In panel Figure 7c, the pressure evolution of the cationic Ni–Tl pair unit is shown, which has a similar trend like the Ni–O pair

bond. The sub-region II has a smaller compression slope, which may denote that the octahedral network suffers a geometrical rearrangement instead of a volume reduction. For this path scattering, the Debye–Waller exponent in panel Figure 7d (constrained to be the same for both Γ$_{3}$ and Γ$_{4}$) also exhibits an anomalous trend at 10.8 GPa. The cationic Ni–Ni pair unit displays three different compression slopes within the sub-regions I, II, and III (see Figure 7e). This pressure evolution of the Ni–Ni cation pair distance is interesting because the Ni–Ni length is half of the lattice parameter c (R$_{5}$ = ½c), which enables a direct comparison between XRD and XAS, as shown in Figure 4d. As expected, the absolute values of c from EXAFS are higher than those estimated from diffraction, in view of the identity⁴²,⁴³

$$ \langle r \rangle \approx R_c + \frac{\langle u_{\perp}^2 \rangle}{2R_c} \quad (1) $$

where R$_{c}$ is the crystallographic distance (from diffraction), ⟨u$_{⊥}$²⟩ is the perpendicular MSRD, and ⟨r⟩ is the mean distance from EXAFS.

In crystalline materials, the MSRD is a temperature- and volume-dependent quantity. In turn, the volume compression is described by an appropriate EOS and, therefore, the MSRD decreases monotonically when pressure increases, see Table S4.⁴⁴,⁴⁵ At higher pressures, the atomic motions start to be more constrained as the volume shrinks, and any structural transition would be monitored by the Debye–Waller exponent. In Figure 7b, the drop of the Debye–Waller exponent at 10.8 GPa of the parallel MSRDs of pair-bonds Ni–O corroborates the above statement. A hybridization between Ni 3d and O 2p orbitals is expected to follow the IMT in nickelates, which promotes the bandgap closing and could explain the drop of the Debye–Waller exponent from sub-region I to II.⁴¹,⁴⁶ At region II, the most likely hypothesis is that a collective interaction takes place because of the strong hybridization of the single electron Ni e$_{g}$ and O 2p that leads to a bandwidth enhancement in the metallic state. Once the metallic state is stabilized, the crystal structure can be compressed again at 25.4 GPa.
**3.4. Electronic Properties under Pressure Using XANES.** Figure 8a exhibits the Ni K-edge XANES of TiNiO₃ at selected pressures at the interval 0–40 GPa. We can enumerate four well-defined XANES features occurring at 8334.6 (A₁, pre-peak), 8350.1 (A₂, whiteline), 8356.4 (A₃, first XANES peak), and 8364.7 eV (A₄, second XANES peak) for room conditions. To better visualize pressure-induced changes, we have performed a spectral decomposition that considered four Lorentzian profiles (Aⱼ) and an error function for describing the edge step. Figure S5 has two examples of these decompositions for pressures of 4.9 and 39.4 GPa, showing that the best fittings are reliable for accounting the position, FWHM, and height of each feature Aⱼ. The pre-peak was fitted with only one Lorentzian profile for the entire pressure range without signals of splitting; we did not discard pre-peak splitting as a result of the charge disproportionation, but it would require better resolution down to ΔE/E ~ 10⁻⁵ to properly solve the peak splitting.⁷ The spectral decomposition was applied for each pressure point, and the pressure evolution of the position and FWHM of all the features Aⱼ are shown in Figure 8b–e, respectively. The extracted peak energy positions were subtracted from the value extracted at ambient pressure to get the relative energy position shifts [ΔA(P) = A(P) – A(P₀), P₀ = 1 bar].

The feature A₁ is almost unchanged with a maximum shift of ~0.5 eV at 40 GPa, while the whiteline A₂ has a slight height variation (see in Figure S6) but with monotonic shifts up to ~1 eV; however, changes occurred at 10.8 GPa for both shift and FWHM. Pronounced anomalies near this pressure point can be seen in the FWHM of A₂ and A₃. One may notice that the FWHM of A₂, A₃, and A₄ became almost constant above 10.8 GPa, similar to that observed for the Debye–Waller factor of the Ni–O bonds. All features moved to higher energies due to the bond contraction, such that A₃ and A₄ displayed the highest shifts of ~3 eV but with slight anomalous drops at 10.8 GPa. The pressure dependence of the height of the features Aⱼ is shown in Figure S6. One may also notice a progressive increase in the feature A₃ height, which starts to be pronounced at pressures above 10.8 GPa. It is important to highlight that changes or anomalous trends at 25.4 GPa are less pronounced when compared to the pressure point at 10.8 GPa.

# 4. DISCUSSION

## 4.1. Structural Transition by Bulk Parameters.

Concerning TiNiO₃, only a few studies were devoted for describing its properties and structural phase transitions. Up to now, two magnetic ordered phases were reported at low temperatures at T_N = 104 K and T_N* = 202 K.⁸ The charge disproportionation at the Ni sites was characterized by neutron powder diffraction and Mössbauer data.²⁰ However, the IMT as induced by temperature has not yet been reported and is only predicted to occur at ~600 K.¹¹ The same can be said for the pressure-induced structural transition in the TiNiO₃ nickelate, which was the main open question addressed by this work.

To stabilize both Tl and Ni in the average valence state of 3+, we have employed the high-pressure synthesis method at the pressure of 7.5 GPa. HR-XRD revealed an impurity-free final product and a monoclinic unit cell (P2₁/n) at room condition with β = 89.988(1)° but with a strong orthorhombic character. Indeed, the further away from 90°, the unit cell will be more monoclinic and less orthorhombic. When subjected to hydrostatic pressures, we observed a pronounced anomaly

along the lattice parameters at 10.8 GPa. Indeed, the nickelates have a strong hybridization between Ni 3d electrons and O 2p ligands and, then, this critical point in pressure may be associated with the metallization process taking place in TiNiO₃. This conclusion is supported by the fact that we do have a monoclinic phase of TiNiO₃ at room condition and the anomalies in lattice parameters are evidence for a second-order structural transition in RNiO₃,²³,²⁴ in this case to an orthorhombic metallic phase Pbnm. Therefore, we may say that the critical pressure of the IMT in TiNiO₃ could be around 10.8 GPa, which is lower than that reported for YNiO₃ (at 14 GPa)²⁴ and higher than the value for EuNiO₃ (at 6 GPa).⁴⁷ Indeed, we have observed indirect evidence for the IMT in TiNiO₃, meaning that futures direct measurements are required such as infrared and/or resistivity measurements. If the IMT is later confirmed, our results present a clear dependence between the R cation size and the critical pressure of the IMT in RNiO₃ nickelates.

## 4.2. Structural Transition by Short- and Medium-Range Parameters.
At a local level around the Ni absorbers, the EXAFS technique was essential to show that the structural transition in TiNiO₃ has indeed a local origin that comprises the short-range charge disproportionation between Ni³⁻⁻⁶⁻O₁ᵃ [R₁ = 1.967(4) Å] and Ni³⁺⁻⁶⁻O₁ᵃ [R₂ = 1.932(4) Å]. This charge ordering is, however, maintained locally above the critical pressure, since a distinguishable bond length difference (ΔR₁,₂) is still observed at the entire probed pressure range (Figure 7a). The Ni–O compression curves showed changes along the linear Murnaghan isothermal EOS, in excellent agreement with the results from XRD. In the direction of the paths Γ₁ and Γ₂, the orbital overlapping Ni t₂g⁶e₉¹ → O 2p may occur isotropically since the isothermal linear bulk modulus for both paths are virtually equal (see in Table S4). The Debye–Waller exponents of the paths Γ₁ and Γ₂ depicted a hardening above 10.8 GPa, meaning that from this pressure point forward the atomic motions are constrained to small displacements of ±0.04(5) Å around the equilibrium position. This motion constraint may induce electronic coupling and more efficient orbital overlapping that is the driven force for the IMT starting at the local level down to the octahedral [Ni1,2O₆] unit. This effect could be directly linked to the electron–phonon coupling: both orbital overlapping and electron–phonon coupling (via the phonon hardening) work in parallel to yield the metallic state. This observation on the TiNiO₃ compound could imply that a similar behavior is operational in other nickelates as well as in different compounds with IMT.²⁶,⁴⁸,⁴⁹ The monitoring of the bond compression Ni–O at the level of [Ni1,2O₆] units by a short-range order technique is essential to unveil more detail on the transitions in nickelates and their mechanisms.

The XANES technique unveiled a valence state of +2.6(4) for Ni, denoting a partial charge transfer to oxygen anions possibly due to the vacancies formed during pressure release after the synthesis. The XANES features of TiNiO₃, as extracted from the spectral decomposition, showed clear changes that may be used to follow the pressure-induced structural transition in this compound and, potentially, in similar nickelates. Notably, the FWHM of both whiteline A₂ and feature A₃ depicted a pronounced drop taking place at 10.8 GPa, being almost constant for higher pressures. Such drops may be ascribed to the charge transfer Ni³⁻⁻⁶ + Ni³⁺⁻⁶ → 2Ni³⁺, which is the main key for stabilizing the metallic state in the orthorhombic phase.²⁵ This work is the first high-pressure
investigation concerning the crystal structure and local
properties in the TlNiO₃ nickelate, which provides important
new information on the small R cation size nickelates for
complementing our understanding of their phase diagrams and
electronic transitions.

**4.3. Transition Systematics in Nickelates by Bulk Modulus.** Figure 9 represents a dependence of the bulk

<figure><img src="image_18.png" /><figcaption>**Figure 9.** Bulk modulus ($B_0$) of RNiO$_3$ nickelates as a function of the BVS tolerance factor. The high-pressure compression data were extracted from refs 23, 24, and 46. Colored lines are a guide for the eye.</figcaption></figure>

modulus ($B_0$) for nickelates with the R cation size. We plot the
reported $B_0$ values of several RNiO$_3$ compounds ($R$ = Lu, Y,
Gd, Eu, Sm, Pr, Nd, La) as a function of the BVS tolerance
factor (i.e., $t \propto d_R - o/d_{Ni} - o$; BVS stands for the bond-valence
sum) at room temperature. This factor describes the distortion
degree of the perovskite lattice.⁵⁰ Those values in the range
0.9–0.94 concern the orthorhombic distortions, while 0.87–
0.9 is related to the monoclinic distortion. One may clearly see
two behaviors for $B_0$ depending on the region of interest. The
main feature of both regions is the stabilization or not of the
rhombohedral phase at higher pressures. For instance, the
medium R cation size nickelates, like PrNiO$_3$, EuNiO$_3$, and
SmNiO$_3$, depicted Pbnm → R3c transition under pressure,
which was not observed for the small R cation size YNiO$_3$ and
TlNiO$_3$ that maintain the Pbnm phase for the highest pressure
point probed. The new data point provided for Tl allows us to
better constrain this systematic for small R cation size, shown
by the red shaded region in Figure 9. Based on available
literature data, we find a linear relationship between $B_0$ and
tolerance factor *t*. Using this systematic, we would expect that
(1) bulk moduli for Yb- and Er-based nickelates would be close
to ~200 and 175 GPa (calculated BVS tolerance factors: 0.873
and 0.881), respectively; (2) these nickelates would not
undergo a transition to the rhombohedral phase with
increasing pressure; however, further experimental studies are
required to confirm such a linear relation.

**5. CONCLUSIONS**

The high-purity TlNiO₃ nickelate has been produced using
high-pressure methods. Synchrotron HR-XRD revealed a
stabilization of the monoclinic phase with a strong
orthorhombic character. The EXAFS technique confirmed
instead the short-range charge disproportionation between
Ni³⁻ and Ni³⁺ under room conditions. This important finding
means that X-ray absorption can be employed to obtain
information on charge ordering at short-range in materials,

even if this phenomenon is not evident at long-range scales. Under high-pressure conditions, XRD data evidenced an anomaly of the lattice parameters at 10.8 GPa, which we assigned to the structural transition P2₁/n → Pbnm. From HP-EXAFS at the Ni K-edge, the Ni–O pair-bond compression curves displayed an isotropic overlapping Ni t₂g⁶e₉¹ → O 2p, with an anomaly occurring at 10.8 GPa. This anomaly is followed by a hardening of the bond length variance of the Ni–O bonds, denoting that from this pressure point forward the atomic vibrations are constrained to small displacements of ±0.04(5) Å. We argued that the description of the metallic state in nickelates requires consideration of the orbital overlapping and electron–phonon coupling. We have demonstrated that the XANES technique can be applied to ascertain the structural transitions and possible pressure-induced IMT in nickelates, by following the position and width (FWHM) trends of the XANES features.

<figure><img src="image_19.png" /></figure>

ASSOCIATED CONTENT

<figure><img src="image_20.png" /></figure>

**Supporting Information**

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.3c00593.

(1) 3D visualization of the TlNiO₃ nickelate in two different crystal structures; (2) Rietveld refinement plots for two crystal structures probed for TlNiO₃ at 0.35 GPa; (3) linear Murnaghan isothermal EOS as obtained from XRD and XAS data; (4) sketches of the local structure of the TlNiO₃ nickelate in the monoclinic crystal structures to build the EXAFS model; (5) Ni K-edge experimental XANES spectra at room conditions to evaluate the Ni valence within TlNiO₃; spectral decomposition of the TlNiO₃ XANES spectra at 4.9 and 39.9 GPa; and the pressure evolution of the height for the XANES features A₁, A₂, A₃, and A₄ (PDF)

<figure><img src="image_21.png" /></figure>

AUTHOR INFORMATION

Corresponding Authors

João Elias Rodrigues – *European Synchrotron Radiation Facility (ESRF), Grenoble 38000, France*;
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 Rosa – *European Synchrotron Radiation Facility (ESRF), Grenoble 38000, France*

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

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

José Luis Martínez – Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain

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

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemmater.3c00593

Notes

The authors declare no competing financial interest.
## ACKNOWLEDGMENTS

The authors wish to express their gratitude to the European Synchrotron (EBS-ESRF) for making all the facilities available for the synchrotron X-ray diffraction in ID15B (in-house research) and ID22 (proposal HC-4990), and X-ray absorption spectroscopy in BM23 (in-house research) experiments. J.A.A. and J.L.M. thank funding from the Spanish Ministry for Science and Innovation (MCIN/AEI/10.13039/501100011033) with Grant number PID2021-122477OB-100. J.E.R. acknowledges Jeroen Jacobs (for the assistance at the ESRF's High-Pressure laboratory) and Catherine Dejoie (for the HR-XRD measurements).

## REFERENCES

(1) Demazeau, G.; Marbeuf, A.; Pouchard, M.; Hagenmuller, P. Sur Une Série de Composés Oxygènes Du Nickel Trivalent Derivés de La Perovskite. *J. Solid State Chem.* **1971**, *3*, 582–589.

(2) Catalan, G. Progress in Perovskite Nickelate Research. *Phase Transitions* **2008**, *81*, 729–749.

(3) Green, R. J.; Müller-caspary, K.; Lobato, I.; Li, L.; Aert, V.; Verbeeck, J.; Huijben, M.; Grisolia, M. N.; Rouco, V.; El Hage, R.; Villegas, J. E.; Mercy, A.; Bibes, M.; Ghosez, P.; Sawatzky, G. A.; Liao, Z.; Gauquelin, N.; Green, R. J.; Müller-caspary, K.; Lobato, I.; Li, L. Correction for Liao et al., Metal–Insulator-Transition Engineering by Modulation Tilt-Control in Perovskite Nickelates for Room Temperature Optical Switching. *Proc. Natl. Acad. Sci. U. S. A.* **2018**, *115*, E10284–E10284.

(4) Cheng, J. G.; Zhou, J. S.; Goodenough, J. B.; Alonso, J. A.; Martínez-Lope, M. J. Pressure Dependence of Metal-Insulator Transition in Perovskites $RNiO_3$ (R=Eu, Y, Lu). *Phys. Rev. B: Condens. Matter Mater. Phys.* **2010**, *82*, No. 085107.

(5) Grisolia, M. N.; Varignon, J.; Sanchez-Santolino, G.; Arora, A.; Valencia, S.; Varela, M.; Abrudan, R.; Weschke, E.; Schierle, E.; Rault, J. E.; Rueff, J. P.; Barthélémy, A.; Santamaria, J.; Bibes, M. Hybridization-Controlled Charge Transfer and Induced Magnetism at Correlated Oxide Interfaces. *Nat. Phys.* **2016**, *12*, 484–492.

(6) Lacorre, P.; Torrance, J. B.; Pannetier, J.; Nazzal, A. I.; Wang, P. W.; Huang, T. C. Synthesis, Crystal Structure, and Properties of Metallic $PrNiO_3$: Comparison with Metallic $NdNiO_3$ and Semi-conducting $SmNiO_3$. *J. Solid State Chem.* **1991**, *91*, 225–237.

(7) Medarde, M.; Dallera, C.; Grioni, M.; Delley, B.; Vernay, F.; Mesot, J.; Sikora, M.; Alonso, J. A.; Martínez-Lope, M. J. Charge Disproportionation in $RNiO_3$ Perovskites (R=rare Earth) from High-Resolution X-Ray Absorption Spectroscopy. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2009**, *80*, No. 245105.

(8) Korosec, L.; Pikulski, M.; Shiroka, T.; Medarde, M.; Luetkens, H.; Alonso, J. A.; Ott, H. R.; Mesot, J. New Magnetic Phase in the Nickelate Perovskite $TiNiO_3$. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2017**, *95*, No. 060411.

(9) Goodenough, J. B. Theory of the Role of Covalence in the Perovskite-Type Manganites $[La,M(II)]MnO_3$. *Phys. Rev.* **1955**, *100*, 564–573.

(10) Goodenough, J. B. An Interpretation of the Magnetic Properties of the Perovskite-Type Mixed Crystals $La_{1-x}Sr_xCoO_{3-\lambda}$. *J. Phys. Chem. Solids* **1958**, *6*, 287–297.

(11) 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 $PrNiO_3$: Illustrating the Potential of Symmetry-Adapted Distortion Mode Analysis for the Study of Phase Transitions. *Phys. Rev. B* **2019**, *100*, No. 205137.

(12) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. *Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr.* **1976**, *32*, 751–767.

(13) 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 $RNiO_3$ Perovskites: Simultaneous Metal-Insulator and Structural Transition in $YNiO_3$. *Phys. Rev. Lett.* **1999**, *82*, 3871–3874.

(14) Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; Aranda, M. A. G.; Fernández-Díaz, M. T. Metal-Insulator Transitions, Structural and Microstructural Evolution of $RNiO_3$ (R = Sm, Eu, Gd, Dy, Ho, Y) Perovskites: Evidence for Room-Temperature Charge Disproportionation in Monoclinic $HoNiO_3$ and $YNiO_3$. *J. Am. Chem. Soc.* **1999**, *121*, 4754–4762.

(15) Alonso, J.; Martínez-Lope, M.; Casais, M.; García-Muñoz, J.; 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: Condens. Matter Mater. Phys.* **2000**, *61*, 1756–1763.

(16) Fernández-Díaz, M. T.; Alonso, J. A.; Martínez-Lope, M. J.; Casais, M. T.; García-Muñoz, J. L. Magnetic Structure of the $HoNiO_3$ Perovskite. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2001**, *64*, 1444171–1444175.

(17) Muñoz, A.; Alonso, J. A.; Martínez-Lope, M. J.; Fernández-Díaz, M. T. On the Magnetic Structure of $DyNiO_3$. *J. Solid State Chem.* **2009**, *182*, 1982–1989.

(18) 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 $NdNiO_3$. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2009**, *79*, No. 134432.

(19) Serrano-Sánchez, F.; Fernández-Díaz, M. T.; Martínez, J. L.; Alonso, J. A. On the Magnetic Structure and Magnetic Behaviour of the Most Distorted Member of the Series of $RNiO_3$ Perovskites (R = Lu). *Dalton Trans.* **2022**, *51*, 2278–2286.

(20) Kim, S. J.; Martinez-Lope, M. J.; Fernandez-Diaz, M. T.; Alonso, J. A.; Presniakov, I.; Demazeau, G. Evidence of Ni(III) Disproportionation in the $TiNiO_3$ Perovskite Lattice through Neutron Powder Diffraction and Mössbauer Spectroscopy. *Chem. Mater.* **2002**, *14*, 4926–4932.

(21) Gainza, J.; Serrano-Sánchez, F.; Rodrigues, J. E. F. S.; Nemes, N. M.; Martínez, J. L.; Alonso, J. A. Metastable Materials Accessed under Moderate Pressure Conditions ($P \le 3.5$ GPa) in a Piston-Cylinder Press. *Materials* **2021**, *14*, 1946.

(22) 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 $NdNiO_3$. *Phys. B* **1997**, *234–236*, 15–17.

(23) Amboage, M.; Hanfland, M.; Alonso, J. A.; Martínez-Lope, M. J. High Pressure Structural Study of $SmNiO_3$. *J. Phys. Condens. Matter* **2005**, *17*, S783.

(24) 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 $YNiO_3$. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2004**, *69*, No. 094106.

(25) 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 $YNiO_3$. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2012**, *85*, No. 045102.

(26) Massa, N. E.; Ramos, A. Y.; Tolentino, H. C. N.; Sousa-Neto, N. M.; Fonseca, J.; Alonso, J. A. Temperature and High-Pressure Dependent X-Ray Absorption of $SmNiO_3$ at the Ni K and Sm L₃ Edges. *Mater. Res. Express* **2015**, *2*, No. 126301.

(27) Kim, S.-J.; Demazeau, G.; Alonso, J. A.; Choy, J.-H. High Pressure Synthesis and Crystal Structure of a New Ni(III) Perovskite: $TiNiO_3$. *J. Mater. Chem.* **2001**, *11*, 487–492.

(28) 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*, 1721–1733.

(29) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. *Phys. B: Condens. Matter* **1993**, *192*, 55–69.
(30) 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.* **2019**, *40*, 65–81.

(31) Irifune, T.; Kurio, A.; Sakamoto, S.; Inoue, T.; Sumiya, H. Ultrahard Polycrystalline Diamond from Graphite. *Nature* **2003**, *421*, 599–600.

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

(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) Kirkpatrick, P.; Baez, A. V. Formation of Optical Images by X-Rays. *J. Opt. Soc. Am.* **1948**, *38*, 766.

(35) Dabin, Y.; Rostaing, G.; Hignette, O.; Rommeveaux, A.; Freund, A. K. The Present State of Kirkpatrick-Baez Mirror Systems at the ESRF. In *Proceedings X-Ray Mirrors, Crystals, and Multilayers II*; Freund, A. K.; Macrander, A. T.; Ishikawa, T.; Wood, J. L., Eds.; 2002; p 235.

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

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

(38) Birch, F. Finite Elastic Strain of Cubic Crystals. *Phys. Rev.* **1947**, *71*, 809–824.

(39) Jeanloz, R.; Hazen, R. M. Finite-Strain Analysis of Relative Compressibilities: Application to the High-Pressure Wadsleyite Phase as an Illustration. *Am. Mineral.* **1991**, *76*, 1765–1768.

(40) Rosa, A. D.; Garbarino, G.; Briggs, R.; Svitlyk, V.; Morard, G.; Bouhifd, M. A.; Jacobs, J.; Irifune, T.; Mathon, O.; Pascarelli, S. Effect of the Fcc-Hcp Martensitic Transition on the Equation of State of Solid Krypton up to 140 GPa. *Phys. Rev. B* **2018**, *97*, No. 094115.

(41) 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*, 462–471.

(42) Fornasini, P.; Monti, F.; Sanson, A. On the Cumulant Analysis of EXAFS in Crystalline Solids. *J. Synchrotron Radiat.* **2001**, *8*, 1214–1220.

(43) Fornasini, P.; Grisenti, R. On EXAFS Debye–Waller Factor and Recent Advances. *J. Synchrotron Radiat.* **2015**, *22*, 1242–1257.

(44) Hieu, H. K.; Duc, N. B.; Hung, N. Van; Hanhe, P. T. M.; Haif, T. T. Pressure Effects on the EXAFS Debye Waller Factor of Iron. *J. Synchrotron Radiat.* **2020**, *27*, 1372–1375.

(45) Fornasini, P.; Grisenti, R.; Irifune, T.; Shinmei, T.; Mathon, O.; Pascarelli, S.; Rosa, A. D. Bond Compressibility and Bond Grüneisen Parameters of CdTe. *J. Phys.: Condens. Matter* **2018**, *30*, 245402.

(46) 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*, 3–6.

(47) Kobayashi, H.; Ikeda, S.; Yoda, Y.; Hirao, N.; Ohishi, Y.; Alonso, J. A.; Martinez-Lope, M. J.; Lengsdorf, R.; Khomskii, D. I.; Abd-Elmeguid, M. M. Pressure-Induced Unusual Metallic State in EuNiO₃. *Phys. Rev. B: Condens. Matter Mater. Phys.* **2015**, *91*, No. 195148.

(48) Farber, D. L.; Krisch, M.; Antonangeli, D.; Beraud, A.; Badro, J.; Occelli, F.; Orlikowski, D. Lattice Dynamics of Molybdenum at High Pressure. *Phys. Rev. Lett.* **2006**, *96*, No. 115502.

(49) Otto, M. R.; Pöhls, J. H.; René de Cotret, L. P.; Stern, M. J.; Sutton, M.; Siwick, B. J. Mechanisms of Electron-Phonon Coupling Unraveled in Momentum and Time: The Case of Soft Phonons in TiSe2. *Sci. Adv.* **2021**, *7*, 1–8.

(50) Lufaso, M. W.; Woodward, P. M. Using Bond Valences to Model the Structures of Ternary and Quaternary Oxides. In *Bond Valences*; 2013; pp 59–90.

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NOTE ADDED AFTER ASAP PUBLICATION

This paper published ASAP on June 20, 2023, with an error in Figure 7. The corrected paper was repost on June 28, 2023.