Structural properties investigation of different alumina polymorphs (η-, γ-, χ-, θ-, α-Al2O3) using Cr3+ as a luminescent probe

The structural properties of eta-, gamma(B)-, gamma(Pb)-, chi-,theta- and alpha-Al2O3 polymorphs were studied by X-ray diffraction and photoluminescence spectroscopy using Cr3+ ion as a luminescent probe in its natural concentration (<10(-3) wt %). The analysis of photoluminescence (PL) and photoluminescence excitation (PLE) data within the crystal field theory revealed a decrease in the crystal field strength Dq from 1810 to 1740 cm(-1), the Racah parameter B from 606 to 432 cm(-1), and the covalence parameter beta(1) from 1.076 to 1.062 in the series alpha-Al2O3 -> gamma-, eta-Al2O3. This indicates that the fraction of covalent bond increases for the low-temperature gamma-Al2O3 and eta-Al2O3 phases as compared to alpha-Al2O3 powder and ruby single crystal. The obtained values of crystal field strength Dq, Racah parameters B and C, and covalence degree beta(1) also differ from each other among low-temperature phases eta(-), gamma(B)-, gamma(Pb)- and chi-Al2O3. This testifies that each low-temperature Al2O3 polymorph has different ionicity/covalence ratio. The analysis of PL and PLE data showed that the chromium ion in chi-Al2O3 occupies the local structural positions that are similar to those in alpha-Al2O3. This occurs because, according to XRD data, the cubic closest packing of oxygen anions in chi-Al2O3 partially transforms into the hexagonal closest packing, which is typical of alpha-Al2O3.