Raman spectroscopic study of cation disorder in poly- and single crystals of the nickel aluminate spinel

The Raman spectrum of NiAl2O4 inverse spinel has been studied in quenched polycrystalline pellets produced by solid-state reaction and in single crystals grown by the floating zone method. The lattice parameters and inversion degrees were determined by x-ray diffraction. Polarization measurements in single crystals allow mode symmetry assignment. Then, a correlation is established between the bands observed in polycrystalline samples and those of single crystals. Both kinds of sample present more bands than the five expected (A1g+Eg+3T2g) in a cubic Fd3m spinel. This multiplicity is attributed to the almost fully inverted cation distribution in NiAl2O4, with inversion parameter x≈0.9. The multiplicity of the high-frequency A1g band, in particular, is attributed to the different possible configurations of Ni2+ and Al3+ cations occupying the three octahedral sites close to a given oxygen ion. A strong downshift of the Eg mode frequency, as compared to the normal spinel MgAl2O4, is attributed to the longer bonding distance between oxygen and octahedral cations in inverse II–III spinels. Due to the small range of variation of x upon thermal treatment in NiAl2O4, no significant differences were found between the spectra of samples quenched at different temperatures, from 800 to 1200 °C.