Abstract
Light and rare earths (RE) have a long lived relationship that dates from the discoveries of these elements in the nineteenth century. Since then, the increasing comprehension of their spectroscopic properties conducted to a wide and alluring literature about light absorption and emission by this particular and fascinating group. For more than 50 years, RE-containing solid-state optical materials have undoubtfully been an important subject for the development of more efficient lighting, visualization, communication and health everyday applications. Therefore, this review introduces the spectroscopic properties of rare earth materials and their applications, with a brief discussion of the main mechanisms of light absorption and emission from a 4f elements inorganic physical chemistry perspective. Special attention is devoted to the use of these elements in systems concerning photoprotection and to the fundamentals of visible light generation for lighting and visualization.
Highlights
rare earths (RE) elements play a central role in the so-called green policies worldwide,[13,14] since they are strictly associated with several key points of environmentally sustainable economic growth, such as (i) clean energy generation,[17,18] (ii) more efficient energy storage and conversion,[21,22,23,24] (iii) reduction of pollution indices (catalytic converters for engine exhausts[25,26,27,28] andcatalysts for degradation of harmful waste),[29,30] and (iv) optimization of petroleum resources.[31]
As the nature of the crystal field has a low effect on the final energies of (2S + 1)LJ levels, basic properties regarding 4f configurations can be represented for each trivalent lanthanoid in a practically universal scheme applicable to any chemical environment, which is known as the Dieke diagram (Figure 2)
After more than 60 years, no substitutes with equivalent properties have been found for Eu3+doped compounds, whereas several other RE-activated compounds appeared with a wide variety of unrivalled spectroscopic characteristics
Summary
In the ca. 100 years that followed the discovery of yttrium by Gadolin in 1794, rare earth (RE) elements were considered as mere laboratory curiosities with a limited practical importance.[1,2,3,4,5,6] throughout the twentieth century, the improvement of separation procedures enabled the obtainment of larger amounts of RE with increased purity, which lead to the development of countless high technology applications.[3,4,7,8,9,10] In the last years, this group of seventeen chemically similar elements (Sc,Y, and the lanthanoids, La-Lu)[11,12] has definitively left the bottom of periodic tables and chemistry textbooks to figure both in the most up-to-date research subjects and in the geopolitical scenario around the world. Special attention is devoted to the use of these elements in systems concerning photoprotection, against sunlight and UV radiation, and to the fundamentals of the application of RE materials in the generation of visible light for lighting and visualization
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