CoVO High-Pressure Polymorphs: To Order or Not to Order?

Materials properties are determined by their compositions and structures. In ABO oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO obtained in both ilmenite-(CoVO -I) and LiNbO -type (CoVO -II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO -II shows a cluster-glass behavior while CoVO -I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO -I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO -II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO -II is also remarkable as the first oxide crystallizing with the LiNbO -type structure where both metals contain free d electrons. CoVO polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites.