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

Rare-earth perovskitenickelates RNiO3 (R standsfor Y, rare earths, Tl or Bi) have attractedwide attention in the past few decades because of their unique electronicproperties that emerge from the insulator-metal transition(IMT). This transition can be tuned by chemical substitutions or byexternal variables, like pressure and temperature, but the mechanismof this transition still remains debated. While most previous studiesfocused on nickelates with partially filled 4f cationson the R site, here we studied the mechanism of thepressure-induced phase transition on a rare Tl3+ nickelate,which comprises fully filled 4f orbitals. We probedthe pressure-induced structural and electronic changes taking placeat the bulk, medium, and local atomic level at room temperature byusing synchrotron X-ray diffraction (XRD), X-ray absorption near edgestructure (XANES), and extended X-ray absorption fine structure (EXAFS)as probing techniques, respectively. High-resolution XRD data underambient conditions show evidence for the monoclinic phase of TlNiO3, as characterized by the charge ordering between Ni3-& delta; and Ni3+& delta; in the insulator regime. High-pressureXRD data elucidated an anomaly in the evolution of the lattice parametersat 10.8 GPa, which we assigned to the structural transition P2(1)/n & RARR; Pbnm. High-pressure EXAFS data at the Ni K-edge providea solid proof for a short-order charge disproportionation under nearambient conditions. The Ni-O pair-bond compression curves unveiledan isotropic overlapping Ni t (2g) (6) e (g) (1) & RARR; O 2p, with an anomaly at 10.8 GPa. The pressure evolution of the bond-distancevariance (Debye-Waller exponents) lodged a hardening at thispressure point, suggesting constrained atomic displacements in theaverage range of & PLUSMN;0.04(5) & ANGS;. We propose that this reducedmotional freedom could be linked to an electron-phonon coupling,which is the driving force behind the mechanisms of the IMT. We demonstratedthat the XANES technique can be applied to ascertain the pressure-inducedtransition, by tracking the anomalous behavior of the position andwidth (full width at half-maximum) of the XANES features.