Lanthanoid Ion Color
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The lanthanoids (often designated Ln) are the 15 elements with atomic numbers 57 (lanthanum) to 71 (lutetium).
Colors, Electron Configurations, and Energy Levels
Characteristic color of lanthanoid ions
The 4f electrons in the lanthanoids are well shielded beneath an outer electron configuration (5s2 5p6 6s2) and so are little influenced by the surrounding solid matrix, and although crystal-field effects (see “Transition-Metal Ion Colors”) contribute to the fine structure of the electronic spectra of the lanthanoid ions, these do not have a gross effect upon the color. This implies that the most important optical properties attributed to the 4f electrons on any particular lanthanoid ion do not (usually) depend significantly upon the host structure, so that lanthanoid elements find use in phosphors, lasers, and other light-emitting devices, where a host lattice can be chosen with respect to processing conditions without significantly changing the desirable color properties of the ion.
Lanthanoid Free-Ion Energy Levels
The energy levels of a free lanthanoid ion are usually labeled with atomic term symbols derived by Russell-Saunders (LS) coupling (although other coupling schemes are also used in this respect). A term is a set of states which are very similar in energy, and the appropriate term symbol is written as 2S+1L where L is a many-electron quantum number describing the total orbital angular momentum of all of the electrons surrounding the atomic nucleus and S is a many-electron quantum number representing the total electron spin. The superscript (2S + 1) is called the multiplicity of the term and is given a name: 1, singlet; 2, doublet; 3, triplet; 4, quartet; and so on. The total angular momentum quantum number L is given a letter symbol: L = 0, S; L = 1, P; L = 2, D; L = 3, F; and thereafter alphabetically, omitting J. The energies of the terms must be determined by quantum mechanical calculations, except for that of the ground state, which is given by Hund’s second rule: the ground state is the term with the highest multiplicity and, if more than one term of the same multiplicity is present, by that with the highest L value.
Ground state terms and levels of principal lanthanoid ions
3H4 3H5 3H6
4I9/2 4I11/2 4I13/2 4I15/2
5I4 5I5 5I6 5I7 5I8
6H5/2 6H7/2 6H9/2 6H11/2 6H13/2 6H15/2
7F0 7F1 7F2 7F3 7F4 7F5 7F6
7F6 7F5 7F4 7F3 7F2 7F1 7F0
6H15/2 6H13/2 6H11/2 6H9/2 6H7/2 6H5/2
5I8 5I7 5I6 5I5 5I4
4I15/2 4I13/2 4I11/2 4I9/2
3H6 3H5 3H4
In the presence of a magnetic field, the spin-orbit levels are split further due to the Zeeman effect. The same is true of static electric fields, where it is called the Stark effect. In both cases, atoms or ions in a gas or free space will show an average effect because of the motion of the particles. However, in a crystal the atom and ion positions are more or less fixed, and the application of either magnetic or electric fields along certain symmetry directions will, in general, cause different degrees of splitting of the levels than the same fields applied along other symmetry directions. Zeeman splitting has been used to change the optical properties of lanthanoid ions in opto-magnetic materials.
Electron transitions are governed by selection rules that give the probability that the transition will occur. Transitions between energy levels derived purely from f orbitals are forbidden by the Laporte selection rule. However, this rule may break down for ions in compounds. The main reason for this is a degree of mixing between s, p, d, and f orbitals can occur when an ion is not located at a center of symmetry. As s, p, or d to f transitions are allowed, transitions giving rise to color are also allowed, to a degree corresponding to the amount of orbital mixing achieved. In addition, transitions are only allowed between states of the same multiplicity, called spin-allowed transitions. Transitions between states of differing spin can be weakly allowed and in some circumstances can also contribute to observed lanthanoid color. The weak colors exhibited by Ln3+ ions are primarily due to these restrictions, especially when compared to typical crystal-field colors of the 3d transition-metal ions (see “Transition-Metal Ion Colors”).
Some Lanthanoid Absorption Colors
The 4f0 ions La3+and Ce4+ and f14 ions Yb4+ and Lu3+ have no f–f energy levels and are colorless. The colorless ions Gd3+ and Tb4+ have a stable 4f7 configuration, and there are no energy levels in the appropriate energy range to give rise to color. The same is true for the 4f13 ion Yb3+.
Ce3+ and Eu2+
Eu2+, with a configuration 4f7, also has a simple energy level diagram because the energy of the state obtained by transferring an f electron to the outer 5d orbitals is lower than the other 4f energy levels (Fig. 1b). As in the case of Ce3+, transitions from the ground state to the upper energy band are allowed. The energy gap is slightly smaller than in the case of Ce3+, and so the absorption moves slightly deeper into the visible spectrum. Because of this, the color of Eu2+ compounds is described as red-brown.
Note that as the d orbitals interact strongly with the surrounding anions, the exact position of the band depends upon the host crystal. Thus, the colors of Ce3+ and Eu2+ compounds, unusually for lanthanoid ions, vary with host structure.
Pr3+, Tm3+, Nd3+, and Dy3+
Some Applications of Lanthanoid Fluorescence Colors
Trichromatic Fluorescent Lamps
Plasma display panels are made up of a pair of glass plates containing a series of cells each of which acts as a miniature fluorescent lamp as described above. Each lamp is several hundred microns in size, and there are several million such lamps in a display. Each pixel consists of three lamps, giving off red, blue, and green lights. The working gas in the cells is a mixture of helium and xenon. When a high voltage is applied across two electrodes above and below a well, the gas is excited into a state that emits ultraviolet radiation, with principal wavelengths of 147 nm and 172 nm. Each well is coated internally with a red, green, or blue phosphor. The main lanthanoid phosphors used at present are yttrium gadolinium borate doped with europium (Y,Gd)BO3:Eu3+, which gives a red emission, and barium magnesium aluminate doped with europium, BaMgAl14O23:Eu2+, for blue emission (see Fig. 3). The green emission utilizes Mn2+ rather than a lanthanoid ion.
Phosphor Electroluminescent Displays
Electroluminescent displays containing a thin film of a phosphor, called thin-film electroluminescent (TFEL) displays, find use as flat panel color displays and backlighting in products such as instrument panels. The most promising devices use ac supplies in a thin-film electroluminescent (ACTFEL) display. Under the influence of an applied electric field, electrons enter the phosphor at the junction with a surface insulating coating. These are accelerated under the influence of the field until they collide with the luminescent centers in the phosphor, transferring energy in the process. The excited luminescent centers then fall back to the ground state and release energy by light emission.
All upconversion spectra from Er3+ (including those using different mechanisms) are similar, but the relative intensities of the three peaks vary with concentration of the ions and the nature of the host matrix.
Energy transfer, in which input radiation is picked up by a sensitizer and is then passed to the emitter, is the mechanism of operation of host structures containing the co-dopants Yb3+/Er3+, which give a strong green emission, and Yb3+/Tm3+ which gives a blue emission. The ion that absorbs the incoming infrared radiation is the Yb3+ ion, which then transfers energy to Er3+ or Tm3+ centers. Other upconversion mechanisms are also known.
Other ions such as Tb3+ are also candidates for quantum cutting devices, but the mechanisms involved are more complex than that with Pr3+.
- 1.Tilley, R.J.D.: Chapter 7. In: Colour and the Optical Properties of Materials, 2nd edn. Wiley, Chichester (2011)Google Scholar
- 2.Nassau, K.: Chapter 4. In: The Physics and Chemistry of Colour, 2nd edn. Wiley, New York (2001)Google Scholar
- 3.Huang, C.-H. (ed.): Rare Earth Coordination Chemistry. Wiley, Singapore (2010)Google Scholar
- 4.Linganna, K., Jayasankar, C.K.: Luminescence Spectroscopy of the Lanthanides. Scholars Press, Saarbrücken (2013)Google Scholar
- 5.Häninen, P., Härmä, H. (eds.): Lanthanide Luminescence. Springer, Heidelberg (2011)Google Scholar