XRD based methods of investigation the order - Disorder transformation in the spinel structure - A comparative study (Conference Paper)

摘要

Three synthetic and stoichiometric 2:3 type spinels (Fd3m symmetry): MgAl_2O_4, MgFe_2O_4, NiAl_2O_4 with a different initial structural order in cation sublattice were investigated. Investigations by means of high temperature XRPD method at temperatures ranging from 25°C to 1100°C were carried out. Diffraction patterns at each temperature in isothermal conditions were measured. For each tested spinel changes in several temperature-dependent parameters were measured: oxygen positional parameter u(T), lattice parameter a_0(T), cation site occupancies Occ(T) in tetrahedral and octahedral positions and a cation-anion distance in tetrahedral TO(T) and octahedral MO(T) positions. Temperatures of initial order-disorder transformation were determined on the basis of the course of these dependences. Basing on changes of the lattice parameter a_0(T), the values of thermal expansion coefficient α_1 and α_2 for spinel before and after the beginning of order-disorder transformation, respectively, were calculated. The values of measured temperature-dependent parameters were used to calculate the degree of inversion x in the spinel structure defined as the number of 3+ cations in tetrahedral sites. In each case two methods of calculating the degree of inversion from experimental data were applied. The first method involved observing the changes in sites occupancy in the cation sublattice versus temperature, which resulted in a change of diffraction lines intensities. The second method was based on observing the changes in cation-anion distances in tetra- and octahedral coordination versus temperature. The results obtained by both methods were compared, discussed and advantages and disadvantages of each of them were presented. It was shown that when atomic scattering factors of cations 2+ and 3+ in the spinel structure differ significantly, the most precise method is the one based on changes in sites occupancies versus temperature. The method based on calculation of changes in cation-anion distances is recommended when atomic scattering factors of cations differ slightly but oxygen positional parameter and cationanion distances changes significantly during order-disorder transformation like in normal spinel structure when effective ionic radii of 2+ and 3+ cations differ significantly.