Crystal structure and morphology of disordered AlNbO4 from X-ray powder diffraction

The crystal structure of AlNbO4 has been reinvestigated (R = 2.2%) by X-ray powder diffraction and electron diffraction from single crystals to clarify discrepancies in the literature. AlNbO4 has a derivative structure of the ReO3 parent structure caused by crystallographic shear operations leading to columns of 2 x 2 octahedra. The metal sites of the different edge-sharing octahedra are statistically occupied by Al and Nb, either similar to 4:1 or similar to 1:4. No superstructure related to ordering of these metals could be observed. Several atomic parameters reported in the literature have been misprinted, thus leading to significantly wrong intensities calculated for powder diffraction patterns. Studies on the crystal chemistry of AlNbO4 showed no phase width or polymorphism up to 1450 degrees C. The morphology of AlNbO4 has also been studied. At 1400 degrees C, the growth of needles is dominant, while platelets are preferred at 1350 degrees C. This behaviour is of importance for the application of AlNbO4 to improve the mechanical properties of ceramic materials.