  The chemical stability of methylammonium lead iodide (\ce{MAPbI3}) under
humid conditions remains the primary challenge facing halide perovskite solar
cells. We investigate defect processes in the water-intercalated iodide
perovskite (\ce{MAPbI3}\_\ce{H2O}) and monohydrated phase
(\ce{MAPbI3}$\cdot$\ce{H2O}) within a first-principles thermodynamic framework.
We consider the formation energies of isolated and aggregated vacancy defects
with different charge states under I-rich and I-poor conditions. It is found
that a \ce{PbI2} (partial Schottky) vacancy complex can be formed readily,
while the \ce{MAI} vacancy complex is difficult to form in the hydrous
compounds. Vacancies in the hydrous phases create deep charge transition
levels, indicating the degradation of halide perovskite upon exposure to
moisture. Electronic structure analysis supports a novel mechanism of
water-mediated vacancy-pair formation.
