Perovskite-Type CuNbO3 Exhibiting Unusual Noncollinear Ferrielectric to Collinear Ferroelectric Dipole Order Transition

We report a perovskite oxide with a new type of polar structure, CuNbO3, thanks to the use of high-pressure and high-temperature synthesis. The perovskite-type CuNbO3 was previously obtained under high-pressure and high-temperature conditions, but its crystal structure has not been solved yet. Our structural analysis reveals that CuNbO3 perovskite crystallizes in a polar monoclinic space group (Pc) with a root 2 a(pc) x root 2 a(pc ) x 2a(pc) unit cell (a(pc) being the pseudo-cubic lattice parameter), which VT, is one maximal nonisomorphic subgroup of the polar rhombohedral space group, R3c. This compound exhibits a remarkable "noncollinear ferrielectric" structure due to parallel displacements of Cu+ and antiparallel displacements of Nb5+ along different axes, representing a new type of polar phase in the perovskite structure. We also observe that the noncollinear ferrielectric Pc s tructure transforms around 470 K into the collinear ferroelectric R3c structure that features parallel displacements of Cu+ and Nb5+ in the same direction. The present work extends the accessible composition range of the perovskite niobate series and demonstrates the role of A-O covalency in determining their crystal structures.