Toward Reliable Materials Discovery: Lessons from Halide Double Perovskites

Reliable materials discovery requires a systematic approach that addresses three sequential questions: Should it be made? Can it be made? Was it really made? Using halide double perovskites as a central case study, this Perspective examines how insufficiently rigorous attention to address these questions has contributed to a growing concern regarding reliability in the field. First, many candidate compounds are fundamentally unsuitable for their intended applications─for instance, halide double perovskites represent a generally unfavorable design space for photovoltaic absorbers due to their reduced electronic dimensionality. Second, there are methodological challenges in thermodynamic stability assessment, emphasizing the necessity of a complete competition landscape, the uncertainty inherent in stability criteria, and the appropriate use of relative trends to guide experimental synthesis. Third, a pattern of misidentification in the experimental literature is documented, wherein claimed "new" double perovskites have repeatedly proven to be known competing phases upon rigorous reexamination. The lessons extend beyond perovskites, offering practical guidelines for how the broader materials community can approach the discovery of functional compounds with the necessary rigor.