Evolution from a heavy-fermion metal to an antiferromagnetic insulator in the A-site ordered perovskite PbCu₃Ru_4-xTi_xO₁₂

Materials tunable between heavy-fermion metals and antiferromagnetic insulators near quantum criticality are promising candidates for exploring unconventional superconductivity, yet such a behavior is rare in transition metal oxides. Here, we report the high-pressure synthesis and characterization of two Pb-based A-site ordered perovskites, PbCu₃Ru₄O₁₂ and PbCu₃Ti₄O₁₂, which are characterized as a heavy-fermion metal and an antiferromagnetic insulator, respectively. Systematic B-site substitution in PbCu₃Ru_4−xTi_xO₁₂ (x = 0−4) reveals a continuous evolution of electronic and magnetic properties, with intermediate compositions exhibiting divergent low-temperature specific heat, indicative of proximity to a quantum critical point. The experimental observation supported by density functional theory calculations reveal that PbCu₃Ru₄O₁₂ exhibits more enhanced effective mass than CaCu₃Ru₄O₁₂, attributed to its enhanced Fermi-level density of states and narrower bandwidth, driven by the elongated Ru-O bonds and the covalent character of Pb²⁺. These results establish PbCu₃Ru_4−xTi_xO₁₂ as a rare platform to study quantum criticality and strong correlations in transition-metal oxides and demonstrate that combine A-site and B-site tuning provides an effective route to tailor electronic and magnetic properties.