Structural Diversity from Anion Order in Heteroanionic Materials

Heteroanionic materials leverage
the advantages offered by two different anions coordinating the same
or different cations to realize unanticipated or enhanced electronic,
optical, and magnetic responses. Beyond chemical variations offered
by the anions, the ability to control the anion order present within
a single transition metal polyhedron via anion-sublattice engineering
offers a potentially transformative strategy in tuning material properties.
The set of design rules for realizing and controlling anion order,
however, are incomplete, which is due in part to the limited anion-ordered
diversity in known structures. This aspect makes formulating such
principles from experiment alone challenging. Here, we demonstrate
how computational methods at multiple levels of theory are effective
at investigating the anion site order dependencies in heteroanionic
materials, HAMs, and enable the construction of crystal-chemistry
principles. Our approach relies on a database of anion ordered structure
variants in which we manipulate the lattice degrees of freedom through
the incorporation of structural distortions. Structure–property
relationships and anion-order descriptors are data mined from group
theoretical techniques and density functional theory calculations.
Using our combined computational scheme, we uncover a hybrid improper
mechanism to stabilize polar phases, propose the chemical link between
local and long ranger anion order, and detail the sequence of order–disorder/displacive
transitions observed experimentally in the oxyfluoride Na3MoO3F3. Our method is scalable and transferable
to many heteroanionic chemistries and crystal families, facilitating
the construction of heteroanionic materials design principles.