Pressure effect on the Raman and photoluminescence spectra of Eu³⁺-doped Na₂Ti₆O₁₃nanorods

Eu3+-doped Na2Ti6O13(Na2Ti6O13:Eu) nanorods with diameters of 30 nm and lengths 400 nm were synthesized by hydrothermal and heat treatment methods. Raman spectra at ambient conditions indicated a pure monoclinic phase (space group C2/m) of the nanorods. The relations between structural and optical properties of Na2Ti6O13:Eu nanorods under high pressures were obtained by photoluminescence and Raman spectra. Two structural transition points at 1.39 and 15.48 GPa were observed when the samples were pressurized. The first transition point was attributed to the crystalline structural distortion. The later transition point was the result of pressure-induced amorphization, and the high-density amorphous (HDA) phase formed after 15.48 GPa was structurally related to the monoclinic baddeleyite structured TiO2(P21/c). However, the site symmetry of the local environment around the Eu3+ions in Na2Ti6O13increased with the rising pressure. These above results indicate the occurrence of short-range order for the local asymmetry around the Eu3+ions and long-range disorder for the crystalline structure of Na2Ti6O13:Eu nanorods by applying pressure. After releasing the pressure from 22.74 GPa, the HDA phase is transformed to low-density amorphous form, which is attributed to be structurally related to the -PbO2-type TiO2.