Patrick Woodward1
Ohio State University1
In this talk I will discuss three case studies involving the interplay between local bonding preferences and long range crystallographic order. I start with oxide-nitride and oxide-fluoride perovskite derivatives containing early transition metal ions, like BaTaO<sub>2</sub>N and K<sub>3</sub>MoO<sub>3</sub>F<sub>3</sub>. In these compounds, pi-bonding interactions with empty d-orbitals on the transition metal ion provide a strong driving force for local trans ordering of anions. In these examples we see that the presence of local ordering is a necessary but not sufficient condition for long range anion ordering. The next vignette explores the crystal chemistry of compounds containing thiocyanate and halide anions. In these compounds the bonding preferences of the polyatomic thiocyanate ion drive the local ordering of anions and the long-range topology of the structure. In the final chapter, I discuss how the chemical inequivalence of anion sites in hybrid layered halide perovskites (Ruddlesden-Popper phases) plays a key role in driving orientational ordering of the organic cations and octahedral tilting of the inorganic layers.