What Remains Unexplained about the Properties of Halide Perovskites

The notion that halide perovskite crystals (ABX(3), where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.