Lanthanide fluorides have some unique characteristics which may improve optical properties over other functional group like oxides, phosphates and sulphates. The following significant factors can improve the luminescent characteristics of inorganic materials:
Fluorides have large band-gaps and empty conduction bands, and can act as electronic insulators. The wide band-gap makes possible emission from high-energy levels, e.g. Nd, Er and Tm5dto4f83, 84
Low nephelauxetic effect1* 85 ( F− < H2O < NH3 < Cl
−
< [CN]− < Br− < I− ) and moderate crystal field splitting (I−< Br−< S2− < Cl−< < F−< OH−< C2O42
−
< H2O <
NH3< CN−< CO)86
In fluorides, the higher energy of the first 5d levels of lanthanide ions allows the existence of line emissions from 4f levels usually located in the 5d bands. e.g. LaF3,
YF3andα-NaYF4: Pr3+ 87.
1 *
As inorganic complexes become more covalent the electrons are, to some extent spread over the ligands so that the electron-electron repulsion is reduced. This reduction of repulsion as covalency increases is called the nephelauxetic effect.83
Weak crystal field energy of fluorides leads to the reduction of the non-radiative process and lowers the decay time off –ftransitions.
The ligand field effect on 5d versus4f in fluorides is more prominent than in oxides, with the consequence that 5d - 4f transitions are at much shorter wavelengths relative to oxides.
Lower phonon energies in fluorides lead to much lower non-radiative decay rates (the quantum efficiency is much higher than oxides). The lower phonon energies in fluorides are favourable for a high stability. e.g. quenching of the 5d-4f emission of Ce3+in LiYF4 (λ=320 nm) starts only at about 900K 88. The thermal stability of
luminescence depends on the energy of the emitting level, electronic transition and phonon energies.
Long life-times of excited levels ofndNand4fNconfigurations.
Electric dipole transitions are allowed for ∆l=1, 3 and ∆S=0 however, parity- forbidden transitions can occur as a result of mixing with states of opposite parity. Because of the ionicity of the bonds formed by fluorine, the probabilities ofd-dandf-f
transitions are lower than in oxides. This results in lower absorption properties and longer lifetime of excited states. Long life-time is favourable for “up-conversion” processes which are used for energy conversion from infrared to visible range89. Fluorides have much wider transparency range compared to oxides and other halides. Lower polarizability*2of fluoride complexes.
Lower refractive index*3of fluoride favours the optical properties.
The exploration of lanthanide fluorides as luminescent materials has become a great interest. Relationship between the site symmetry of the crystal structure and luminescence has been revealed by Zang et. al 90 with lanthanide fluoride nanocrystals, LaF3: Eu3+. The La3+ ion in
LaF3 was shown to be at a site of C2 symmetry, because both electric and magnetic dipole
transitions were allowed and both the transitions of 5D0→ 7F1and5D0→ 7F2were observed
in LaF3: Eu3+(Figure 1.13).
*2 Polarizability is a measure of the ability of a material to become polarised in the presence of an applied electric field. It may occur in both polar and non-polar material
Figure 1.13:- Excitation and Emission Spectra of LaF3: Eu3+(0.1 mol %) nanocrystals. [Zang et. al.90]
However, the relationship between overall crystallographic structure and luminescence properties has not been fully understood and part of the problem lies in the limitation of exploration of different phases. Therefore, developing a philosophy to prepare new structure types for luminescent materials would be a significant step for understanding the structure– property relationship.
Lanthanide doped inorganic materials such as KYF4and KLuF4, RbGd3F10and NaGdF4have
been studied during the last decade to understand the mechanism of their luminescence. The photoluminescence of KYF4and KLuF4:Eu3+ 91 and Eu3+ luminescence in two polymorphic
forms of RbGd3F10 and RbY3F10 92 have been reported. A strategy for photoluminescence
studies of NaGdF4:Eu3+has been introduced by Youet. al.93. They concluded that the energy
difference between 6GJ and 6PJ of Gd3+ matches very well with that of 7FJ → 5D0 of Eu3+ ;
therefore, excited energy is transferred by cross-relaxation (1) between 6GJ → 6PJand7FJ → 5
D0 , resulting in a red emission from 5D0 to 7FJ. The cascade relaxation from 6PJ to 8S7/2
generates a second photon, which pumps the Eu3+ion up to the higher excited state (2) of5D2
or 5D3. The emission of the second photon is, therefore, composed of a full spectrum of all
5
Figure 1.14:- Energy Level Diagram of the Gd3+→Eu3+System [Youet. al.93]
Energy transfer within lanthanide and rare-earth elements illustrates an improvement of understanding luminescence. Contribution of energy transfer mechanism for the luminescence efficiency has been studied by Weghet. al94 and Liet. al 95. The energy transfer mechanism can be identified according to their excitation and emission spectra (Figure 1.15).
Figure 1.15:- (A)- Emission spectra of LiGdF4:Eu3+ (0.5 mol%) upon excitation in the 6IJof
Gd3+ and 273 nm (violet line) and upon excitation of6GJof Gd3+and 202 nm (red line), both
at 300 K. (B) Excitation spectra of LiGdF4:Eu3+ (0.5 mol%) monitoring the 5D0 → 7F1
emission of Eu3+at 554 nm (violet line) and the5D0→ 7F2emission at 614 nm (red line), both
A doping level of 0.5% Eu3+ in LiGdF4 reveals the highest intensity peaks in excitation
spectrum are 8S7/2→6IJ of Gd3+ and 8S7/2→6GJ of Gd3+ which corresponds to the emission
wavelengths 554 nm and 614 nm respectively. However, both excitation wavelengths at 273 nm and 202 nm result in a dominant Eu3+transition of 5D0 → 7FJ (614 nm) in the emission
spectrum. It would conclude that Gd3+acts as a sensitiser and absorbed energy has transferred from Gd3+to Eu3+. The following diagram (Figure 1.16.)shows it clearly.
Figure 1.16:- Energy level diagram of the Gd3+- Eu3+system, showing energy transfer from Gd3+to Eu3+. [Wegh et. al94]