EFFECT OF PRESSURE ON THE STRUCTURE OF NiAl_2O_4

摘要

NiAl_2O_4 is known to transform from a normal spinel, (Ni~(2+))[Al~(3+)2]O_4, to an inverse spinel, (Al~(3+))[Ni~(2+)Al~(3+)]O_4 and vice versa. In this process, the larger Ni~(2+) ions which occupy the tetrahedral (8a) sites in normal spinel move to the octahedral (16d) sites in inverse spinel while half of the smaller Al~(3+) ions move in the opposite direction. The extent of this move is measured by the disorder or inversion parameter, / (the fraction of tetrahedral sites occupied by the Al ions, I = 0 for normal spinel and I = 1 for inverse spinel). Previous studies suggest that the lattice constant of spinel can decrease as the disorder parameter increases to better accommodate the Ni ions. In-situ neutron diffraction studies performed by us indicate that this process is also occurring during the reduction of NiAl_2O_4 to Ni and Al_2O_3. It is possible that the compressive residual stresses generated during reduction play a role in the structural evolution of NiAl_2O_4. To systematically investigate the effect of pressure on the structure of NiAl_2O_4, X-ray diffraction studies at the X17 beamline of the National Synchrotron Light Source (NSLS) were performed. The pressure (up to 20 GPa) was applied via a diamond anvil cell and the experiments were conducted using a polychromatic X-ray beam. By comparing the relative intensities of certain spinel reflections that are sensitive to cationic disorder, a trend toward inverse spinel as a function of pressure was observed. The results are presented in comparison to previous studies on this material.