Color Centers and Luminescence in Single Crystals of Lanthanum Trichloride Containing Dipositive Europium

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Abstract
The edge of the fundamental absorption band in pure LaCl3 is at about 2250 A. There is no change in the absorption spectrum of pure LaCl3 in the range 2250 to 25 000 A on exposure at room temperature to 107r of gamma rays from a Co60 source. The crystals do not luminesce after such treatment. In contradistinction, LaCl3 containing small amounts of europium exhibits a blue luminescence and thermoluminescence when exposed to ultraviolet or gamma radiation. The luminescent properties are profoundly influenced by long continued gamma irradiation. In addition, color centers are formed which absorb visible radiation. Chemical as well as paramagnetic resonance experiments show that all of these effects are due to the presence in the LaCl3 lattice of dispositive europium. There are at least three different color centers giving rise to three overlapping absorption bands which decay thermally with different rates. The maxima of the absorption bands are at 8000, 6500, and 5500 A. Studies with polarized light show that the color center transitions are all electric dipole in nature and are highly anisotropic. The most intense band has been tentatively identified with an F‐center‐like s→p transition. Experiments on the thermal decay of the color bands and the glow curve of the crystal indicate that an intimate relationship exists between the color center and the luminescence phenomena. It is suggested that when these crystals are irradiated, dipositive europium loses an electron to become tripositive, the electron being captured by an anion vacancy to form a color center. On thermal freeing of the electron, return to the ground state is accompanied by the emission of visible radiation.

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