  Inorganic halide perovskite \ce{Cs(Rb)PbI3} has attracted significant
research interest in the application of light-absorbing material of perovskite
solar cells (PSCs). Although there have been extensive studies on structural
and electronic properties of inorganic halide perovskites, the investigation on
their thermodynamic stability is lack. Thus, we investigate the effect of
substituting Rb for Cs in \ce{CsPbI3} on the chemical decomposition and
thermodynamic stability using first-principles thermodynamics. By calculating
the formation energies of solid solutions \ce{Cs$_{1-x}$Rb$_x$PbI3} from their
ingredients \ce{Cs$_{1-x}$Rb$_x$I} and \ce{PbI2}, we find that the best match
between efficiency and stability can be achieved at the Rb content $x\approx$
0.7. The calculated Helmholtz free energy of solid solutions indicates that
\ce{Cs$_{1-x}$Rb$_x$PbI3} has a good thermodynamic stability at room
temperature due to a good miscibility of \ce{CsPbI3} and \ce{RbPbI3}. Through
lattice-dynamics calculations, we further highlight that \ce{RbPbI3} never
stabilize in cubic phase at any temperature and pressure due to the chemical
decomposition into its ingredients \ce{RbI} and \ce{PbI2}, while \ce{CsPbI3}
can be stabilized in the cubic phase at the temperature range of 0$-$600 K and
the pressure range of 0$-$4 GPa. Our work reasonably explains the experimental
observations, and paves the way for understanding material stability of the
inorganic halide perovskites and designing efficient inorganic halide PSCs.
