Kavli Affiliate: Kristin A. Persson
| First 5 Authors: Rachel Woods-Robinson, Vladan Stevanović, Stephan Lany, Karen N. Heinselman, Matthew K. Horton
| 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"
ground state 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 high effective temperatures
similar to those achieved using this sputter deposition method, and thus
corroborates experimental results. Additional calculations show that this
destabilization of the wurtsalt polymorph is due to configurational entropic
and enthalpic effects, and that vibrational contributions are negligible.
Specifically, 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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