Letter to the Editor Tb 3 f±d absorption as indicator of the eect of covalency on the Judd±Ofelt X 2 parameter in glasses H. Ebendor-Heidepriem * , D. Ehrt Friedrich Schiller University of Jena, Otto±Schott-Institut, Fraunhoferstr. 6, D-07743 Jena, Germany Received 12 August 1998; received in revised form 24 February 1999 Abstract The two eects, covalency and asymmetry, on the Judd±Ofelt X 2 parameter have been separated by use of Tb 3 4f±5d absorption peak wavelength as a measure of covalency. According to the Judd±Ofelt theory, X 2 increases with increasing reciprocal energy dierence between the 4f N and 4f N1 5d 1 con®gurations. In fact, a linear correlation be- tween X 2 of Eu 3 and the peak wavelength of the Tb 3 4f 8 ® 4f 7 5d 1 absorption band has been found for the ¯uoride, ¯uoride phosphate and phosphate glasses investigated. The dierences in the peak wavelengths are caused by dierences in the polarizability of the ligands, leading to dierences in the covalency of the rare earth sites. This behavior is re- sponsible for the sensitivity of X 2 to the covalency of the bond between rare earth ions and their ligands. The deviation of the ultraphosphate glass investigated from the correlation between X 2 and the Tb 3 4f±5d peak wavelength is at- tributed to a higher degree of asymmetry of the local crystal ®elds around the rare earth ions. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 42.25.Bs; 42.70.Ce; 78.20.Ci; 78.66.Jg 1. Introduction Glasses doped with rare earth ions (RE ions) are widely used as lasers and ampli®ers. The ra- diative properties of the RE ions are described by the Judd±Ofelt intensity parameters, X t . There- fore, the dependence of these parameters on the host glass composition is of special interest. Much work has been done to reveal the relationships between the local structure and bonding of glasses around the RE ions and the spectroscopic prop- erties of these ions [1±25]. However, it is dicult to obtain a unique relationship [26]. It has been well established that the X t param- eters, especially the structure-sensitive X 2 param- eter, depend on the covalency and asymmetry of the RE sites [1]. The X 2 parameter is often attrib- uted to the covalency of the RE sites in the glasses [1±13]. In some cases, the degree of covalency is determined from the nephelauxetic shift of sensi- tive electronic transitions of the RE ions [2±5,14± 17]. A qualitative correlation between the nephelauxetic shift of Nd 3 or Eu 3 4f±4f transi- tions and X 2 parameters or X 2 sensitive ¯uores- cence intensity ratios have been observed [2±4]. In other cases, the covalency of the RE sites is esti- mated from glass basicity or distance between RE www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 248 (1999) 247±252 * Corresponding author: Tel: +49-3641 948 511; fax: +49-3641 948 502; e-mail: c7ebhe@rz.uni-jena.de 0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 2 4 3 - 4