Hello all,
I would like to know how do the covalency and asymmetry correlate with the measured luminescence lifetime values?
Thank you
Rim
This is a very interesting question, though difficult to answer simply.
Covalency. When the covalency of the bonds in the first coordination sphere of the RE ions increases (note that is rarely exceeds 5-6%), it means more 4f-orbital mixing with surrounding orbitals and has two consequences. Firstly if energy is transferred from the surroundings of the emitting ion (sensitization process), then this transfer will be more efficient, if the implied mechanism is spin exchange (Dexter mechanism), which is often the case for short distances. Second, the 4f orbitals being less pure, it means that the radiative lifetime will decrease.
Asymmetry. What do you mean? Probably less symmetric environment. When the symmetry in the surrounding of the emitting ion is lowered then more transitions become allowed and overall the luminescence intensity will increase. In turn, since only wavefunctions with the same irreducible representation can mix, less symmetrical environment also induce more orbital mixing, and again this can affect the radiative lifetime.
However quantitative correlations between the observed lifetime and covalency/symmetry are difficult to unravel since the lifetime reflects the balance between radiative processes and non-radiative ones and the latter depends on several other parameters such as phonon energy and density for instance.
This is a very interesting question, though difficult to answer simply.
Covalency. When the covalency of the bonds in the first coordination sphere of the RE ions increases (note that is rarely exceeds 5-6%), it means more 4f-orbital mixing with surrounding orbitals and has two consequences. Firstly if energy is transferred from the surroundings of the emitting ion (sensitization process), then this transfer will be more efficient, if the implied mechanism is spin exchange (Dexter mechanism), which is often the case for short distances. Second, the 4f orbitals being less pure, it means that the radiative lifetime will decrease.
Asymmetry. What do you mean? Probably less symmetric environment. When the symmetry in the surrounding of the emitting ion is lowered then more transitions become allowed and overall the luminescence intensity will increase. In turn, since only wavefunctions with the same irreducible representation can mix, less symmetrical environment also induce more orbital mixing, and again this can affect the radiative lifetime.
However quantitative correlations between the observed lifetime and covalency/symmetry are difficult to unravel since the lifetime reflects the balance between radiative processes and non-radiative ones and the latter depends on several other parameters such as phonon energy and density for instance.
Thank you for your detailed answer, also concerning the covalency, its effect lowers the barycenter of the orbital levels. We also note in this context that the term covalency refers to the bonding interaction between the dopant and the ligands, rather than to the ionic or covalent character of the undoped host.
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