XAS STUDY OF LANTHANUM COORDINATION ENVIRONMENTS IN GLASSES OF THE SYSTEM K2O-SIO2-LA2O3

The La L1 and L3 XANES and L3 EXAFS have been investigated for the series of glasses 10K2O-50SiO2-xLa2O3 (x = 1, 5, 10) and (10 - x)K2O-40SiO2-(x/3)La2O3 (x = 7.5, 5, 2.5) and model compounds La2O3, LaAlO3, LaPO4, La2NiO4, La2CuO4 and La(OH)3. An edge resonance at approximately 25 eV above the L1 edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s --> nd forbidden transition increases with La2O3 concentration, indicating a reduction in the 'average' site symmetry of the first coordination shell of La. Mapping chi(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete chi(k) space as a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 angstrom (+/- 0.03 angstrom) for La-O. The coordination numbers (N less-than-or-equal-to 7 +/- 1.5) were derived by mapping and comparison to the published structures for other La compounds. Chi(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.