Luminescence activator ions

Table 4.2. Luminescent activator ions and valence states ordered to electronic configurations (column numbers correspond to the periodic table columns)...

Most minerals fall into the class of insulator phosphors. The characteristics of the luminescence are usually defined by the electronic structure of an activator ion as modified by the crystal field of the host crystal structure. Although some energy transfer takes place between nearby ions, appearing as the phenomena of co-activation, luminescence poisons, and activator pair interactions, the overall luminescence process is localized in a "luminescent center" which is typically 2 to 3 nm in radius. From a perspective of band theory, luminescent centers behave as localized states within the forbidden energy gap. 

Fig. 5.43 Luminescent material, containing activator ions A (ions showing the desired emission) and sensitizing ions S (on which e.g. UV excitation can take place).

Finally that intriguing application yielding the laser. In luminescence the radiative decay of the excited state to the ground state occurs by spontaneous emission, i.e. the emission processes on different activator ions are not correlated. If, by some means, the majority of the luminescent ions are in the excited state (this situation is called population inversion), a single spontaneously emitted photon (quantum of radiation) may stimulate other excited ions to emit. This process is called stimulated emission. It is monochromatic, coherent and non-divergent. Laser action depends on emission by a stimulated process. Actually the word la.ser is an acronym for light amplification by stimulated emission of radiation. This book does not deal with lasers or laser physics. However, we will deal with the material where the stimulated emission is generated if this is useful for our purpose. F.very laser material is after all also a luminescent material. 

Defect-induced luminescent materials are also an important kind phosphor as reviewed by Lin [134]. Some a defect emission, combined with the intrinsic emission, could result in both multicolor and fuU-color emission. For example, under UV excitation a strong broadband emission centered at 400 nm has been found in obtained fluoride nanocrystals prepared by hydrothermal method, which has also been reported by Lin s group [135, 136]. Because no any other activated ions were introduced in the experiments, the emission must be caused by some kind of defect or electronic centre. 

It was proposed that monitoring the infra-red luminescence along the diffusion direction of activator ions in crystals of AgBr would be a sensitive and minimally invasive method for determining the diffusion coefficient, diffusion enthalpy, and temperature dependence of these parameters for various ions. This technique was used to study the diffusion of Nd . In order to establish the concentration range within which the luminescence method was useful, the dependence of the... 

If the activator ion is larger than the host-lattice ion which it replaces, e.g. Eu (ionic radius 0.98 A) or Ce (1.07 A) in a Lu host lattice (0.85 A), the environment of the activator will be compelled to expand in order to make room for the activator. If the activator is raised to the excited state, and if this is accompanied by an increase of the equilibrium distance (anion excitation, Ar > 0), then the environment of the activator will have to expand yet further. Since this expansion costs energy, the lattice will tend to oppose the expansion of the luminescent centre, in other words Ar will be relatively small. 

Red colour is produced in rare earth phosphors by Eu activators. In rare earth silicates the emission intensity of bivalent europium seems to be weak (McAllister, 1969). Shiokawa and Adachi (1979) have activated the rare earth siUcates with divalent europium and found the typical emission colour of Eu " -activated phosphors blue or blue-green. Blasse and Bril (1967a) have studied the luminescence properties of Eu +-activated MRSiQ4 (M = Li, Na) (fig. 54). In all compounds the orange transition Dq Fj had the highest intensity. The fluorescence spectra have been used as a structural probe and the crystallographical position of the activator ion has been predicted from the sharpness and number of the emission peaks. 

Campostrini R., Carturan G., Ferrari M., Montagna M., Pilla 0. Luminescence of Eu + ions durino thermal densification of Si02 gel. J. Mater. Res. 1992 7 745-753 Carlos L.D., Sd Ferreira R.A., De Zea Bermudez V., Ribeiro S.J.L. Full-color phosphors from amine-functionalized crosslinked hybrids lacking metal activator ions. Adv. Funct. Mater. 2001 11 111-115... 

Green W.H., Le K.P., Grey J., Au T.T., Sailor M.J. White phosphors from a silicate-carboxylate sol-gel precursor that lack metal activator ions. Science 1997 276 1826-1828 Harreld J.H., Ebina T., Tsubo N., Stucky G. Manipulation of pore size disUibutions in silica and ormosil gels dried under ambient pressure conditions. J. Non-Cryst. Sohds 2002 298 241-251 Hazenkamp M.F., Blasse G. Rare-earth ions adsorbed onto porous ass luminescence as a characterizing tool. Chem. Mater. 1990 2 105-110... 

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