Speaker
Description
The A$_4$M$_2$O$_9$ family (A = divalent cations; M = Nb/Ta), which typically adopts a trigonal P$\overline{3}c$1 structure derived from corundum, exhibits magnetoelectric (ME) properties in antiferromagnetic compounds such as Co$_4$Nb$_2$O$_9$. Ni$_4$Nb$_2$O$_9$ stands out as an exception, crystallizing in a different but close structure described in the orthorhombic $Pbcn$ space group that is ferrimagnetic and does not show ME activity. Here, we demonstrate how Ti$^{4+}$/W$^{6+}$ co-substitution on Nb$^{5+}$ sites stabilizes a superstructure derived from $Pbcn$ and described in the polar I-type $Fdd2$ space group for Ni$_4$Nb$_{1.8}$W$_{0.1}$Ti$_{0.1}$O$_{9}$. This structural study that combines synchrotron X-ray and neutron diffraction data with electron microscopy reveal a strongly distorted corundum-like framework. Magnetic characterizations were performed by DC and AC-susceptibility measurements, showing a ferrimagnetic transition at 68 K, followed by a re-entrant spin-glass-like state below $\approx$ 50 K. The magnetic structure is established by neutron diffraction, evidencing a collinear ferrimagnetic structure.
Acknowledgements
We thank CELLS-ALBA for synchrotron access, CANAM/NPI CAS Řež for neutron diffraction facilities (MEYS LM2023041), and the Nord-Pas de Calais Regional Council/ERDF for supporting the Lille TEM facility.
