Lanthanide Photoluminescence A Tool for Analyzing Local Heterogeneity in Ferroelectrics and Scope for High Performance Optical Thermometry

Abstract

Rare-earth doped phosphors are fascinating both from scientific and technological standpoints because of the great scope it offers for spectral tuning, enabling a large variety of applications: from simple road markers to lighting, telecommunication, biomedical application, etc. Though the intra-4f electronic transitions are parity forbidden, photoluminescence (PL) emissions from rare-earth ions is possible due to coupling with the vibrational state, mixing with wavefunctions of opposite parity, ligand orbitals, or charge transfer state of the host matrix. Consequently, the host matrix plays an important role in determining the PL emission behavior of a given rare-earth ion. Any change in the structural features of the host, induced by chemical modification, temperature, pressure, electric field, is expected to bring about a change in the PL spectrum, providing the basis of spectral tuning. However, the shielding of the 4f levels by the 5d 6s electrons nearly fixes the energy of the Stark bands of a rare-earth ion, irrespective of the ligand environment around it. In this context, spectral tuning merely involves a change in the relative intensities of the Stark bands appearing in different wavelength regions of the PL emission spectrum. The symmetry occupied by the lanthanide ion determines the extent to which the degeneracies of the 2J+1 levels are lifted for the different 2S+1Lj spectroscopic level. The lowering of the local symmetry not only affects the selection rules of intra band transition but also increases the number of Stark manifolds observed in the PL spectrum. The Eu3+ PL emission possesses a special feature called hypersensitive transition i.e. the 5D0 and#8594; 7F2 transition of Eu3+ ion is very sensitive to small changes in the local environment and/or the degree of structural heterogeneity. Eu3+ PL has, therefore, been used as a tool for investigating local symmetry in complex structures and in the quantification of the extent of crystallization during thermal annealing of structural glasses...