It was noted that the pure material consistently exhibited 2 strong thermoluminescence glow peaks near to 90 and 128K. A model was proposed in which these peaks arose from the annealing of halogen defect sites; similarly to the H and Vk centers of alkali halides. Relaxation and decay of these defects in the pure lattice then resulted in a broad-band intrinsic luminescence. The addition of rare-earth impurity ions had 2 effects. Firstly, the broad-band emission was replaced by a narrow-band line emission, determined by the trivalent rare-earth dopants. Secondly, it preferentially governed the formation of the halogen defect sites at impurity lattice sites. Such sites appeared to increase in thermal stability since the glow peak increased from 128K, in intrinsic material, up to 141K via the sequence of rare-earth dopants from La to Er. The temperature movement was directly related to changes in the ionic size of the rare-earth ions; when allowance had been made for differences in the effective coordination number of the impurity ions. The data suggested that 2 alternative lattice sites could be occupied. The model emphasized that the intense thermoluminescence signals arose from internal charge rearrangements and the annealing of defect complexes, rather than via the more conventional model of separated charge traps and recombination centers. At higher temperatures, there existed a complex array of glow peaks which depended not only upon the dopant concentration but were also specific to each type of rare earth.

Low-Temperature Thermoluminescence Spectra of Rare-Earth Doped Lanthanum Fluoride. B.Yang, P.D.Townsend, A.P.Rowlands: Physical Review B, 1998, 57[1], 178-88