Crystal structure and magnetic properties of complex oxides Mg4-xNixNb2O9, 0≤x≤4

In the Mg4-xNixNb2O9 (0≤x≤4) system two ranges of solid solution have been found. One of the solid solutions has a corundum-related structure type (space group P3-barc1); the second one adopts the II-Ni4Nb2O9 structure type (space group Pbcn). The unit cell constants and atomic positions have been determined and refined using neutron powder diffraction data. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) from MgNi3Nb2O9 crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg3NiNb2O9, measured in the temperature range T=2-300 K, shows no indications of magnetic ordering at low temperatures, while for MgNi3Nb2O9 there is a magnetic ordering at temperatures below 45.5 K. - Graphical abstract: HREM image showing planar defects in MgNi3Nb2O9 and their schematic representation