Reconstructive Transitions from Rotations of Rigid Heteroanionic Polyhedra

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.