JOURNAL OF SOLID STATE CHEMISTRY 71, 396-402 (1987) An EXAFS Study of the Luminescent Bi3+ Center in LaPO,,-Bi F. B. M. VAN ZON,* D. C. KONINGSBERGER,* E. W. J. L. OOMEN,t AND G. BLASSEt *Laboratory for Inorganic Chemistry and Catafysis, Eindhoven University of Technology, P.O. Box 513,560O MB Eindhoven, The Netherlands; and tPhysica1 Laboratory, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Received January 26, 1987 In order to determine the oxygen coordination of the Bi3+ ion in LaPO,-Bi, extended X-ray absorption fine structure (EXAFS) spectra were measured of BiPO, and LaPO.,-Bi. Analysis of the EXAFS data shows that the Bi3+ ion in LaP04-Bi occupies the La’+ site, but that the oxygen coordination of the Bi3+ ion is distorted relative to that of La’+ in undoped LaP04. The oxygen coordination shows a resemblance to that of Bi’+ in BiPOI. The importance of these results for the Bi3+ luminescence is discussed. 0 1987 Academic Press, hc. 1. Introduction The luminescence of Bi3+ in solid mixed- metal oxides is well known and its charac- teristics have been shown to vary strongly from lattice to lattice (1, 2). The variation of the Stokes shift with more than a factor of 10 is especially impressive and intriguing (3, 4). It has been proposed that this varia- tion can be related to the 6s2 electronic configuration of the Bi3+ ion which tends to induce a very asymmetrical coordination (4). Small Stokes shifts were observed for Bi3+ in very constrained surroundings in which case it is assumed that there is no space for changes in coordination relative to the unsubstituted situation. This is espe- cially observed if the Bi3+ ion is substituted for smaller ions (e.g., Sc3+, Lu3+, Y3+) on six-coordinated sites (I, 2). which is substituted for larger ions on less constrained sites, e.g., Bi3+ in LaP04 (with an oxygen coordination of eight or nine for La3+ (3, 5-8)), or Bi3+ in bismuth com- pounds like Bi3Ge40t2 (I, 2, 4, 9) and Bi2 Ge309 (I, 2, 4, 10). In the latter compounds X-ray diffraction studies have shown that Bi3’ is asymmetrically coordinated. In fact, asymmetrically coordinated Bi3+ ions al- ways exhibit broadband emission with a large Stokes shift. Therefore, it has been assumed that the Bi3+ ion is also asymmet- rically coordinated in LaP04, or more gen- erally, that broadband emission with a large Stokes shift indicates that the Bi3+ ion has a coordination which has been adjusted to the 6s2 configuration. Large Stokes shifts are observed for Bi3+ 0022-4596187 $3.00 Copyright 8 1987 by Academic Press, Inc. All rights of reproduction in any form reserved. 396 However, there is no direct proof for this assumption. Actually, it is rather difficult to determine directly the position of the oxy- gen ions surrounding a small amount of Bi3+ ions in a host lattice. EXAFS (extended