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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

Rare Earth Elements and Materials 11

Thereverseprocess,thatis,thetransitionofelectronsfrom
Typical rare
n in 4 f n earth ion S L J an excited state to a lower-energy state corresponds to en-
ergy release via either radiative (light-emitting) or non-
0 La 3+ , Ce 4+ 0 0 0 radiative (heat-releasing) pathways or both. The electro-
1 Ce 3+ , Pr 4+ 1 3 3
2 2 magnetic radiation emitted due to 4 f transitions of the rare
2 Pr 3+ 1 5 4 earth ions is usually in the visible range, but can also be in
3 Nd 3+ 3 6 9
2 2 other spectral regions, such as the ultraviolet or infrared.
4 Pm 3+ 2 6 4 The trivalent cations of the rare earths have photolumi-
5 Sm 3+ 5 5 5
2 2 nescent properties that are favorable for several kinds of
6 Eu 3+ 3 3 0 applications. However, it is difficult to generate this lumi-
7 Gd 3+ , Eu 2+ , Tb 4+ 7 0 7
2 2 nescence by direct excitation of the lanthanide ion because
8 Tb 3+ 3 3 6 of the forbidden nature of the f – f transition, as stated
9 Dy 3+ 5 5 15
2 2 earlier. This difficulty can be overcome by some indirect
10 Ho 3+ 2 6 8 means of excitation of the ion to an upper level. For exam-
11 Er 3+ 3 6 15
2 2 ple, the rare earth ions can be incorporated into a matrix
12 Tm 3+ 1 5 6 or host of oxide or glass-forming materials. A carefully
13 Yb 3+ 1 2 3 7 2 chosen impurity species, commercially called sensitizer,
14 Lu 3+ , Yb 2+ 0 0 0 absorbs ultraviolet radiation and the energy is transferred
to the emitter (activator) through the crystalline host lat-
n
Divalentandtrivalentionsarelistedforeach4 f asallrare tice. The excited rare earth ion then decays to the ground
earth elements contain either trivalent or divalent ions, the state which will involve emission of light quanta corre-
remainder of the valence electrons, 6s and 5d, having been sponding to the energy differences with the ground level.
The mechanism of such matrix-assisted energy transfer
donated to the sea or gas of “free” electrons which conduct
and subsequent luminescence is schematically shown in
the electrical current (called the conduction electrons).
Fig. 5.
An ordered lattice facilitates this energy transfer but
C. Spectroscopic and Magnetic
may simply serve to allow transfer to an inadvertent impu-
Properties of Rare Earth Ions
rity ion that provides a mechanism for deactivation with-
1. Transition in Rare Earth Ions: out emission. The degree of order of the lattice has to be
Absorption Spectroscopy optimized, as too does the concentration of the chosen
emitter. Among the most efficient of commercial phos-
The excitation of an atom, ion, or molecule from its ground
phors are those based on the red emission of Eu(III), the
electronic level to higher lying level (excited state) may
green emission of Tb(III), and the blue emission of Eu(II).
be effected by the absorption of light. When the incident
Another approach to sensitize rare earth luminescence
light energy is exactly equal to the difference between the
is to prepare rare earth complexes with chelating ligands
ground state and an excited state, a quantum of light will
such as EDTA and the like. If the ligands contain organic
be absorbed. For rare earth elements, however, the ma-
functional groups (called chromophores) that are capable
jority of the electronic transitions involve a redistribution
of absorbing light energy, highly luminescent rare earth
of electrons within the 4 f orbitals, which by the spectro-
complexes can be obtained. The chromophore acts like
scopicSelectionRulesareforbidden.Thisleadstothelong
some sort of “antenna.” The energy absorbed by such
excited state lifetimes in the micro- to millisecond range
chromophores can be transferred to a nearby lanthanide
and the low extinction coefficients indicated by the pale
colors of rare earth-containing compounds. Moreover, the ion, which is then able to emit its characteristic lumines-
cence. The chelating ligands provide a protecting envi-
electrons in the 4 f orbitals are shielded by filled 5s and
ronment in such a way that deactivation of the excited
5p shells. As a result of this shielding, the influence of the
state (or so-called quenching of luminescence) can be mit-
host lattice on transitions within the 4 f shell is insignifi-
igated. Various lanthanide complexes containing organic
cant. In other words, optical spectra of rare earth materials
chromophores are known to show efficient photolumines-
are virtually independent of environment. Indeed, similar
cence. The basic architecture of these systems is depicted
sharp line-like spectra are observed in gaseous, solution,
in Fig. 6.
and solid states.
2. Luminescence of the Rare Earths 3. Magnetic Properties and Related Topics
Absorption of photon energy occurs when electrons from All rare earth elements except La, Yb, and Lu have
a lower-energy state are promoted to a higher-energy state. nonzero values of S and L. As both orbital and spin