Kavli Affiliate: Kristin A. Persson
| First 5 Authors: Rachel Woods-Robinson, Vladan Stevanović, Stephan Lany, Karen N. Heinselman, Kristin A. Persson
| Summary:
In materials science, it is often assumed that ground state crystal
structures predicted by density functional theory are the easiest polymorphs to
synthesize. Ternary nitride materials, with many possible metastable
polymorphs, provide a rich materials space to study what influences
thermodynamic stability and polymorph synthesizability. For example, ZnZrN2 is
theoretically predicted at zero Kelvin to have an unusual layered "wurtsalt"
crystal structure with compelling optoelectronic properties, but it is unknown
whether this structure can be realized experimentally under practical synthesis
conditions. Here, we use combinatorial sputtering to synthesize hundreds of
ZnxZr1-xNy thin film samples, and find metastable rocksalt-derived or
boron-nitride-derived structures rather than the predicted wurtsalt structure.
Using a statistical polymorph sampler approach, it is demonstrated that
although rocksalt is the least stable polymorph at zero Kelvin, it becomes the
most stable polymorph at effective temperatures > ~1150 K, corroborating
experimental results since sputtering yields high effective temperatures.
Additional calculations show that this temperature-induced change in phase
stability 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. This
understanding of the role of disorder tolerance in the synthesis of competing
polymorphs can enable more accurate predictions of synthesizable crystal
structures and their achievable material properties.
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