We develop a first-principles framework for finite-temperature structural optimization that incorporates vibrational contributions to the free energy through anharmonic phonon theory. We extend and further improve the efficiency of the recent approach, enabling its application to systems in which the size of the primitive cell changes across structural phase transitions. Applying this framework to ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, we establish its pressure-temperature phase diagram and find that the slope of the phase boundary between the high-symmetry and low-symmetry phases is negative, with a magnitude of approximately $-60\mathrm{K}/\mathrm{GPa}$. The present results provide a theoretical foundation for discussing how changes in crystal symmetry influence the emergence of superconductivity.

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• Anharmonic lattice dynamics
• Phase diagrams
• High-temperature superconductors
• Nickelates