Whereas a low vapor pressure in general restricts gas-phase chemistry of rare earths to a quite low number of compounds, it has been realized in the last forty years that solid state chemistry is a very important and in many aspects a much more innovating counterpart of the earlier solution chemistry of rare earths. As
EXCITED STATE PHENOMENA IN VITREOUS MATERIALS 83 far as high reactive solids are concerned, and their unexpected variation from one element to the next, the ion-exchange separation developed by Spedding and the availability of metallic elements have provided an enormous impetus to important research. The purpose of the present chapter is to point out the spectroscopic and other physical properties of isotropic glasses as an interesting alternative to crystalline compounds (including substitutionally doped crystals having colorless lanthanum or yttrium compounds as the major constituent). Some chemists are slightly taken back by the nonstoichiometric nature of nearly all glasses (though most minerals are not much better at this point) and may even consider them as a border-line case between genuine chemistry and agglomerates such as rock, lava, or concrete. However, we have anyhow to recognize t'hat very few stoichiometric, crystalline rare-earth compounds contain discrete molecules (with the exception of organometallic compounds with R - C bonds, and certain neutral complexes of chelating ligands forming R - O and R - N bonds). Nearly all binary RmX n involve bridging X.
Our main purpose is to compare absorption and luminescence spectra of R(III) in glasses with the behavior in crystalline compounds and in aqueous solution. In one way, it is surprising how close many of the similarities are, and also how technologically attractive glasses can be as laser materials compared to crystals. It may be noted that the largest-scale terawatt lasers, such as the SHIVA and NOVA systems in Livermore, CA, are neodymium(Ill) glasses. We summarize most of the major results described in this chapter in seven conclusions:
(a) The quantum yield of luminescence of line emission from a higher to a lower J-level of a given lanthanide can be as high as in comparable crystalline materials. The two most important parameters (characterizing the competing process of nonradiative deexcitation) are a large energy difference AE between the emitting J-level and the closest lower J-level, and a small phonon energy (indicating the highest fundamental frequency of the neighbor groups in the glass). The classical cases of fluorescence (even in aqueous solution) correspond to the highest known AE in gadolinium(III) and the two following AE values in europium(III) and terbium(III). Conventional oxide glasses (borate, silicate, phosphate) only fluoresce strongly in these cases, and sometimes in the case of the slightly smaller AE of neodymium(III) (in the near infrared), samarium(III), and dysprosium(III). Oxide glasses with low force constants and heavier nuclei (such as germanate and tellurite) show luminescence for lower AE. This is even more true for fluoride glasses, showing perceptible fluorescence of many J-levels of neodymium(III), holmium(III), and erbium(III) with AE down to slightly below 2000 cm -1. This propensity for luminescence is as high as in crystalline RxLal_xF 3 and RxYI_xF3.
(b) The luminescence of fluoride glasses at room temperature is nearly as strong (at least within a factor 2) as at liquid helium, alleviating the need for cooling of the laser.
84 R. REISFELD and C.K. JORGENSEN
(c) Technological progress is starting along several new lines, such as direct contact between laser sources and optical fibers (of the same composition, excepting R) for communication, use of hole burning for short-term (typically millisecond) memory, and simultaneously present lanthanides and d-group ions, e.g., neodymium(III) and chromium(III), for glasses intended for flat-plate luminescent concentrators of solar energy.
(d) Energy transfer is
normally
more effective in glasses than in crystals, allowing pumping of lanthanides (with narrow, weak absorption bands) via broad, intense absorption bands of other species present. Other useful aspects of energy transfer occur from the lowest quartet state of manganese(II), storing energy for several milliseconds and transferring it efficiently to luminescent lanthanide J-levels.(e) Chromium(III) occurs in several highly successful laser crystals such as ruby and alexandrite. It is notoriously difficult to obtain a high fluorescence yield of Cr(III) in glasses, though energy transfer to lanthanides can be quite effective.
Glass ceramics containing crystallites much smaller than the wavelength of light are known to give very high quantum yields of Cr(III) and might become important for lanthanide luminescence, also because of the favorable thermal conductivity and increased mechanical strength.
(f) The dispersion of individual sites of lanthanides in glass can be studied by the technique of fluorescence line-narrowing. On the whole, the properties of glasses are only weakly influenced by the "ligand field" structure of each J-level.
Several coordination numbers (say, 7, 8, 9 , . . . ) seem to occur in most glasses.
(g) Large-scale experimentation with potential new laser materials can to a great extent be replaced by measurement of spectroscopic characteristics on a laboratory scale. Quite unexpectedly, the J u d d - O f e l t treatment with only three material parameters is excellent (for the weighted average of glass sites), describ- ing absorption and radiative emission probabilities. The Judd-Ofelt parameters are closely connected with other aspects of chemical bonding to the neighbor atoms.
Acknowledgements
We would like to thank Mrs. Esther Greenberg for the careful help in preparing the manuscript, and Dr. Marek Eyal and Dr. Anna Kisilev for providing valuable data. We also thank Dr. Nissan Spector for extensive information about calcula- tions of J-levels and Judd-Ofelt matrix elements in intermediate coupling. We are grateful to Professor Minas D. Marcantonatos for interesting discussions, and to Professor Charles Jacoboni for unique samples of fluoride glasses. Finally, we thank the Swiss National Science Foundation for the grant 2.152-0.83 and their previous grants significantly furthering many of the studies described here.
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