  Stability is one of the key issues in mixed-halide perovskite alloys which
are promising in emergent optoelectronics. Previous density-functional-theory
(DFT) and machine learning studies indicate that the formation-energy convex
hulls of these materials are very shallow, and stable alloy compositions are
rare. In this work, we revisit this problem using DFT with special focus on the
effects of configuration and vibration entropies. Allowed by the $20$-atomic
models for the $\text{CsPb}(\text{I}_{1-x}^{}\text{Br}_x^{})_3^{}$ and
$\text{CsPb}(\text{Br}_{1-x}^{}\text{Cl}_x^{})_3^{}$ series, the partition
functions and therewith thermodynamic state functions are calculated by
traversing all possible mixed-halide configurations. We can thus evaluate the
temperature- and system-dependent configuration entropy, which largely corrects
the conventional approach based on the ideal solution model. Finally,
temperature-composition phase diagrams that include $\alpha$, $\beta$, $\gamma$
and $\delta$ phases of both alloys are constructed based on the free energy
data, for which the contribution of phonon vibrations is included.
