Ni₄Nb_1.8W_0.1Ti_0.1O₉: Stabilization of the I-Type Ni₄Nb₂O₉ Structure and First Magnetic Study of This Corundum-like Compound

Abstract

For the first time, we report on the structural and magnetic properties of a polycrystalline sample of Ni 4 Nb 2 O 9 from I-type (Fdd2), obtained by the partial cosubstitution of Nb 5+ by Ti 4+ and W 6+ . The crystal structure is investigated by combining synchrotron X-ray, neutron, and electron diffraction at room temperature. This I-type structure is derived from the corundum-like Ni 4 Nb 2 O 9 II-type (Pbcn) and is noncentrosymmetric and polar. The Ni-lattice is composed of the stacking of distorted honeycomb layers with double zigzag ribbons 60°d isoriented from each other in two successive double layers. The connection between layers is ensured by the sharing of octahedra faces building (Nb,W,Ti) 2 O 9 , Ni 2 O 9 , and Ni 3 O 12 units. This study shows how the disruption of the Nb 2 O 6 units by smaller d 0 cations impacts the structure, significantly modifying the Ni network and, thus, the magnetic properties. The latter were studied by dc-and ac-magnetic susceptibility, and the magnetic structure was solved by neutron powder diffraction. A ferrimagnetic behavior occurs below 68 K, followed by a re-entrant spin-glass-like behavior below ≈50 K.