The interplay between electronic modification and lattice distortion is critical for understanding emergent phenomena in correlated oxide heterostructures. For the conventional $\mathrm{LaAl}{\mathrm{O}}_3//\mathrm{SrTi}{\mathrm{O}}_3$ interface fabricated on the $\mathrm{SrTi}{\mathrm{O}}_3$ substrates, a 3% lattice mismatch dominates the interfacial lattice distortion, thereby hindering the exploration of the coupling between $\mathrm{LaAl}{\mathrm{O}}_3$ crystal structure and $\mathrm{SrTi}{\mathrm{O}}_3$ transport property. Here, when the epitaxial strain is reduced to 2% by growing the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ bilayers on the $\left(\mathrm{La},\mathrm{Sr}\right)\left(\mathrm{Al},\mathrm{Ta}\right){\mathrm{O}}_3$ substrates, the $\mathrm{LaAl}{\mathrm{O}}_3$ layers exhibit diffraction features consistent with the coexistence of two structural phases, labeled as $P_1$ and $P_2$. This structural inhomogeneity could be attributed to the competing interlayer interactions between $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$. While the charge transfer that compensates the polar discontinuity results in the formation of oxygen vacancy in $\mathrm{LaAl}{\mathrm{O}}_3$ to expand its lattice, the interfacial redox reaction drives the oxygen migration from $\mathrm{SrTi}{\mathrm{O}}_3$ to $\mathrm{LaAl}{\mathrm{O}}_3$ with $\mathrm{LaAl}{\mathrm{O}}_3$ lattice shrinkage. Increasing the $\mathrm{LaAl}{\mathrm{O}}_3$ layer thickness reduces the lattice difference between $P_1$ and $P_2$ phases, suppressing the potential fluctuations near the conduction band minimum of $\mathrm{SrTi}{\mathrm{O}}_3$ and improving interfacial carrier mobility. Our findings provide new insights into the structural evolution at the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface, as well as the interlayer coupling between crystal structure and electrical transport across the correlated oxide heterostructure.

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• Surface & interfacial phenomena
• Heterostructures
• Thin films
• Two-dimensional electron gas
• Resistivity measurements
• Strain engineering