SrTiO3:Cr nanocrystalline powders: size effects and optical properties

The crystal structure, optical absorption, and photoluminescence of chromium impurity centers were studied in nanocrystalline SrTiO3:Cr (0.1 mol%) powders with average particle size within the range 13–100 nm prepared by the Pechini-type polymeric sol–gel method. Only the presence of a cubic perovskite phase of Oh1 symmetry was proved for the powders at room temperature, by means of x-ray diffraction. The lattice constant a = 3.910 Å, larger than that of bulk SrTiO3 crystals (a = 3.905 Å), was found for nanoparticles with the size about 20 nm. The optical absorption edge and the zero-phonon R-line () of luminescence of the octahedral Cr3+ centers shifted to higher energies with decreasing nanoparticle size. These size effects were regarded as intrinsic to SrTiO3. An unusual and large temperature shift of the R-line position very similar to the 'dielectric related' one of the bulk crystals was observed for all powders, evidencing their quantum paraelectric behavior. However, the powders with the average particle size about 13 and 20 nm did not reveal completely reproducible behavior of the R-line position at low temperatures. This instability was considered a possible manifestation of a low-temperature phase transition in small enough SrTiO3 nanoparticles.