Evidence for a Pressure-Induced Phase Transition in the Highly Distorted TlNiO₃ Nickelate

Abstract

Rare-earth perovskite nickelates RNiO 3 (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 Tl 3+ 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 TlNiO 3 , as characterized by the charge ordering between Ni 3−δ and Ni 3+δ 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 P2 1 /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 t 2g 6 e g 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.

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

“…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) while the negative shift comes from the expanded unit cell when compared to room temperature one (expansion) …”

Exploring the Interplay between Structure and Electronic Behavior across Pressure-Induced Isostructural and Structural Transitions in Weyl-Type Semimetal NbAs

“…The broken inversion symmetry is crucial for the existence of the Weyl semimetal phase, which in contrast preserves time-reversal symmetry [14,15]. It is known that exotic phase transitions in materials can be induced through the application of pressure, potentially leading to the emergence of new electronic or magnetic quantum phases [16][17][18][19][20][21][22][23][24][25]. In NbAs, theoretical calculations predicted a sequence of pressure-induced structural transitions from tetragonal-to-hexagonal (another topological phase with different Weyl node geometry) and subsequently the transition from monoclinic to cubic as follows: I4 1 md → P6m2 → P2 1 /c → Pm3m [26].…”