Tantalum pentoxide $\left({\mathrm{Ta}}_2{\mathrm{O}}_5\right)$ is a polymorphic wide-band-gap semiconductor with outstanding dielectric properties and widespread use in optical and electronic technologies. Its rich structural diversity, arising from multiple polymorphs accessible under different synthesis conditions, has made ${\mathrm{Ta}}_2{\mathrm{O}}_5$ a long-standing subject of interest. However, a unified understanding of the thermodynamic stability and phase transitions of its polymorphs across pressure-temperature (P-T) space has remained elusive. Here, using first-principles calculations, we map the thermodynamic landscape of ${\mathrm{Ta}}_2{\mathrm{O}}_5$ and establish a comprehensive P-T phase diagram together with a phase-stability hierarchy. We find that γ-${\mathrm{Ta}}_2{\mathrm{O}}_5$ and $\mathrm{B}\text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$ dominate the phase diagram over a broad range of P-T conditions: $\gamma \text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$ is stabilized at low pressures, while $\mathrm{B}\text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$ becomes thermodynamically favored at higher pressures up to $\sim 60$ GPa, beyond which Y-${\mathrm{Ta}}_2{\mathrm{O}}_5$ emerges as the most stable phase. Crucially, the zero-point energy (ZPE), one aspect of nuclear quantum effects (NQEs), plays a significant role in determining relative phase stability, contributing substantially to the Gibbs free energy and altering phase boundaries. A reentrant phase transition between γ- and $\mathrm{B}\text{−}{\mathrm{Ta}}_2{\mathrm{O}}_5$ is predicted near $\sim 2$ GPa, revealing unexpected complexity in the phase behavior of this oxide. More generally, we identify a characteristic temperature $\left(T_0\right)$, at which zero-point and thermal phonon contributions to the free energy become comparable, and show that $T_0$ is approximately one-third of the Debye temperature. This relationship provides a simple, physically transparent criterion for assessing the importance of NQEs in phase stability, with implications extending beyond ${\mathrm{Ta}}_2{\mathrm{O}}_5$ to a broad class of complex oxides.

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• Phase diagrams
• Phonons
• Wide band gap systems
• Density functional calculations