Transformation from Metallic to Insulating Stripe-Ordered Phase in Trilayer Square-Planar Nickelates

Low-valent nickelates are intriguing for their structural and electronic features that are similar to superconducting cuprates, including superconductivity. Among them, the crystallographic isomorphs Pr₄Ni₃O₈ and La₄Ni₃O₈ with trilayer square-planar structural configuration have been demonstrated to possess the singularly different ground states: Pr₄Ni₃O₈ is metallic while La₄Ni₃O₈ exhibits insulating stripe-ordered phase. To explore and understand the evolution of the ground states from metallic Pr₄Ni₃O₈ to stripe-ordered La₄Ni₃O₈ in the R₄Ni₃O₈ family, we have successfully grown a series of isovalent-substituted single crystals (Pr_1-xLa_x)₄Ni₃O₈ (x = 0.1, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.9) using high oxygen pressure (pO₂) floating zone furnace. We performed a comprehensive study combining thermodynamics, electrical transport, magnetization as well as high-resolution synchrotron x-ray powder diffraction and synchrotron x-ray single crystal diffraction on this series of crystals. The results revealed that this series of nickelates undergo a systematic transformation from correlated metal to stripe-ordered insulator near x = 0.4, potentially hinting at the existence of a quantum critical point.