  In materials science, it is often assumed that the most thermodynamically
stable crystal structure is the easiest polymorph to synthesize. Ternary
nitride materials, with many possible metastable polymorphs, provide a domain
to test this assumption; for example, ZnZrN$_2$ is predicted to have an unusual
layered "wurtsalt" crystal structure and exciting optoelectronic properties,
but can it be realized experimentally? Here, we synthesize hundreds of
Zn$_x$Zr$_{1-x}$N$_y$ thin film samples, and find metastable rocksalt-derived
or boron-nitride-derived structures rather than the thermodynamically stable
wurtsalt structure. Computations show that this divergence is due to both
entropic and enthalpic stabilization effects. Rocksalt- and
boron-nitride-derived structures become the most stable polymorphs in the
presence of disorder because of higher tolerances to cation cross-substitution
and off-stoichiometry than the wurtsalt structure. By understanding the role of
disorder tolerance in the synthesis of metastable polymorphs, we can enable
more accurate predictions of synthesizable crystal structures and their
achievable material properties.
