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Hybrid organic–inorganic halide perovskites with the prototype material of CH₃NH₃PbI₃ have recently attracted intense interest as low-cost and high-performance photovoltaic absorbers. Despite the high power conversion efficiency exceeding 20% achieved by their solar cells, two key issuesthe poor device stabilities associated with their intrinsic material instability and the toxicity due to water-soluble Pb²⁺need to be resolved before large-scale commercialization. Here, we address these issues by exploiting the strategy of cation-transmutation to design stable inorganic Pb-free halide perovskites for solar cells. The idea is to convert two divalent Pb²⁺ ions into one monovalent M⁺ and one trivalent M³⁺ ions, forming a rich class of quaternary halides in double-perovskite structure. We find through first-principles calculations this class of materials have good phase stability against decomposition and wide-range …