$\mathrm{LaCo}{\mathrm{O}}_3$ is a prototypical perovskite oxide, exhibiting simultaneous $\sim 500\text{−}\mathrm{K}$ insulator-metal and spin-state transitions, which have been intensively studied. As is often the case, understanding which of the various coupled degrees of freedom primarily drives the insulator-metal transition (IMT) in $\mathrm{LaCo}{\mathrm{O}}_3$ has proven challenging. We show here that heteroepitaxial strain in thin-film $\mathrm{LaCo}{\mathrm{O}}_3$ can be used to entirely decouple the high-temperature spin-state and insulator-metal transitions, providing important insight. Specifically, the IMT is found to occur independent of the strain-tuned spin state, establishing that it is the IMT in bulk $\mathrm{LaCo}{\mathrm{O}}_3$ that fundamentally drives the high-temperature spin-state transition, not the inverse. The strain dependence of the IMT temperature in thin-film $\mathrm{LaCo}{\mathrm{O}}_3$ is then shown to be remarkably consistent with the trend versus cell volume across the bulk $\mathrm{RCo}{\mathrm{O}}_3$ series ($\mathrm{R}=\mathrm{rare}$-earth). Density-functional theory results on bulk $\mathrm{RCo}{\mathrm{O}}_3$ and biaxially strained $\mathrm{LaCo}{\mathrm{O}}_3$ support interpretation of these results in terms of trends in electronic bandwidths and orbital degeneracy.

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• Metal-insulator transition
• Spin state transition
• Transport phenomena
• Complex oxides