Sol-gel technology grown monolithic highly porous aluminum oxide with chromium and rare earth ions and size-resonant terahertz acoustic vibrations of nanocrystalline particles

Monolithic highly porous aluminum oxide synthesized by sol-gel technology is built of crystalline gamma-Al2O3 nanometer sized particles. In the present work highly porous gamma-Al2O3 is doped with 3d (Cr3+) and 4f (Eu3+, Pr3+) ions. Observation of narrow-band optical spectra of these ions in gamma-Al2O3 enabled us to exploit the idea of optical probe ions for the studies of general properties of doped porous insulators. In different spectroscopic experiments the lowest-frequency size-quantized acoustic vibrations of nanocrystalline particles were revealed. A very long decay time of these vibrations was observed. In the experiments with thermal treatment of doped gamma-Al2O3 resulting in the transition from gamma-phase to alpha-Al2O3 (corundum) the possibility of incorporation of rare earth ions into the corundum lattice was demonstrated.