Chemical Pressure Induced Phase Evolution in Trilayer Square-Planar Nickelates (La_1–xY_x)₄Ni₃O₈

Low-valent nickelates featured by quasi-two-dimensional (quasi-2D) square planar Ni-O layers have drawn extensive attention because of their exotic electronic and magnetic properties, including superconductivity in the infinite layer (Nd,Sr)NiO₂ and quintuple-layer Nd₆Ni₅O₁₂. In our previous studies on trilayer quasi-2D nickelates, we have shown that La₄Ni₃O₈ is a charge- and spin-stripe-ordered insulator below 105 K while Pr₄Ni₃O₈ has a metallic ground state, moreover, we have identified the approximate location of a quantum phase transition at x ≈ 0.45 in (Pr_1–xLa_x)₄Ni₃O₈. However, it is unclear whether the evolution of phase transition is dominant by Pr-O hybridization or by simple chemical pressure effect. To understand the role of size effect in the transition, we carried out the study on (La_1–xY_x)₄Ni₃O₈ solid solution, in which large La³⁺ (r = 103.2 pm) is partially substituted by much smaller rare earth cations, Y³⁺ (r = 90 pm), and no hybridization is involved. Bulk single crystals of (La_1–xY_x)₄Ni₃O₈ (x = 0.1, 0.2 and 0.25) were synthesized via high oxygen pressure (pO₂) floating zone growth followed by topotactic reduction. Characterizations combining crystallography, thermodynamics, electrical transport, magnetic and synchrotron X-ray single crystal diffraction were performed on (La_1–xY_x)₄Ni₃O₈ materials. The results revealed that chemical pressure-stimulated size effect can lead to the evolution from insolating to metallic phase in (La_1–xY_x)₄Ni₃O₈.