Evolution from a heavy-fermion metal to an antiferromagnetic insulator in the ????-site ordered perovskite PbCu3⁢Ru4−????⁢Ti????⁢O12

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, ${\mathrm{PbCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ and ${\mathrm{PbCu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$, which are characterized as a heavy-fermion metal and an antiferromagnetic insulator, respectively. Systematic $B$-site substitution in ${\mathrm{PbCu}}_{3}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{12}$ ($x=0\ensuremath{-}4$) 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 ${\mathrm{PbCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ exhibits more enhanced effective mass than ${\mathrm{CaCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$, attributed to its enhanced Fermi-level density of states and narrower bandwidth, driven by the elongated Ru-O bonds and the covalent character of ${\mathrm{Pb}}^{2+}$. These results establish ${\mathrm{PbCu}}_{3}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{12}$ 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.