  Metal halide perovskites (MHPs) combine extraordinary optoelectronic
properties with chemical and mechanical properties not found in their
semiconductor counterparts. For instance, they exhibit optoelectronic
properties on par with single-crystalline gallium arsenide yet exhibit
near-zero formation energies. The small lattice energy of MHPs means they
undergo a rich diversity of polymorphism near standard conditions similar to
organic materials. MHPs also demonstrate ionic transport as high as
state-of-the-art battery electrodes. The most widespread applications for metal
halide perovskites (e.g. photovoltaics and solid-state lighting) typically view
low formation energies, polymorphism, and high ion transport as a nuisance that
should be eliminated. Here, we put these properties into perspective by
comparing them to other technologically relevant semiconductors in order to
highlight how unique this combination of properties is for semiconductors and
to illustrate ways to leverage these properties in emerging applications.
