Near-Field Measurements of Metal to Charge Ordered Insulator Phase Transitions in Pr/La Doped Tri-layer Square-Planar Nickelates.

(Pr/La)₄Ni₃O₈ are tri-layered, quasi-2d materials that lie in the nominally overdoped region of the nickelate phase diagram. The Praseodymium nickelate compound is a correlated metal that develops a low temperature charge ordered ground state upon isovalent substitution of Lanthanum^[1]. The full range of substitution of these compounds was examined using cryogenic near-field imaging and spectroscopy. These techniques reveal and characterize the onset, and percolation of insulating and metallic domains across the surface of the bulk single crystals.

The metal to insulator transition maintains a similar character across Lanthanum substitution. Nano-imaging reveals a rich and far broader phase transition than suggested in transport measurements. The high temperature correlated metallic phase appears to be stabilized on defects at temperatures that persist to at least 60K below the bulk insulating transition.

Broadband cryogenic infrared spectroscopy characterizes and reveals the metallic, insulating, and partially metallic boundary phase that separates the metallic and insulating regions. Novel instrumentation and analysis techniques reveal the spatial correlations of the developing phase transition across energy, temperature and Pr/La substitution.

[1] X. Chen et al., Chem. Mat. 34, 10 (2022)