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

The Raman spectrum of NiAl(2)O(4) 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 (A(1g)+E(g)+3T(2g)) in a cubic Fd3m spinel. This multiplicity is attributed to the almost fully inverted cation distribution in NiAl(2)O(4), with inversion parameter x≈0.9. The multiplicity of the high-frequency A(1g) band, in particular, is attributed to the different possible configurations of Ni(2+) and Al(3+) cations occupying the three octahedral sites close to a given oxygen ion. A strong downshift of the E(g) mode frequency, as compared to the normal spinel MgAl(2)O(4), 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 NiAl(2)O(4), no significant differences were found between the spectra of samples quenched at different temperatures, from 800 to 1200 °C.