title: Reconstructive Transitions from Rotations of Rigid Heteroanionic Polyhedra.
abstract: Phase transitions are ubiquitous in structurally complex transition metal compounds composed of homoanionic polyhedra, including nitrides, oxides, and fluorides. The symmetry breaking that occurs across polymorphic transitions is often achieved by small atomic displacements, rendering these displacive transitions reversible. In contrast, elemental crystals, alloys, and simple minerals will exhibit reconstructive "bond-breaking" transitions. Here we show that a reconstructive transition occurs in the heteroanionic compound KNaNbOF5, owing to reorientations of the [NbOF5](2-) units that trigger a reconfiguration of the cation lattice. Using a combination of synchrotron-based measurements, empirical dynamic simulations, and ab initio calculations, we report structure changes across the transition and formulate an atomistic minimum energy transition path to explain its irreversible nature. Our results indicate that multianionic compounds are likely to host reconstructive transitions that are frequently difficult to study and functionalize in simpler compounds. We anticipate that our insight into the forces that drive the transition will also lead to novel methods to control the assembly of structures in the solid state.
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Michael
authors.initials: M
authors.lastname: Holland
authors.affiliation: Department of Materials Science and Engineering, Drexel University , Philadelphia, Pennsylvania 19104, United States.
authors.forename: Nenian
authors.initials: N
authors.lastname: Charles
authors.affiliation: Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: James M
authors.initials: JM
authors.lastname: Rondinelli
authors.affiliation: Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.|Department of Materials Science and Engineering, Northwestern University , Evanston, Illinois 60208, United States.
authors.forename: Kenneth R
authors.initials: KR
authors.lastname: Poeppelmeier