X-RAY-ABSORPTION SPECTROSCOPIC STUDY OF THE BINARY SEMICONDUCTING GLASS PBV2O6

The 1:1 PbO:V2O5 semiconducting glass prepared by quenching an equimol ar mixture of PbO and V2O5 melt (650-degrees-C) at liquid-nitrogen tem perature has been investigated by XAS (X-ray absorption spectroscopy) at 25 and -180-degrees-C. The spherical wave approximation was used, a pplying parameters extracted from the crystallographically known analo gue PbV2O6 (form I) at the same temperatures. At 25-degrees-C the loca l geometry at V(V) (i.e. V-O bonds < 2.3 angstrom) may be described as a trigonally distorted square pyramid, with three V-O bonds of 1.71 a ngstrom, and two longer ones of 1.99 and 2.24 angstrom (+/- 0.02 angst rom). The chain structure in the glass thus appears to be intermediate between those of the metastable crystalline forms II (edge-shared VO5 units) and III (corner-shared VO4 units) of PbV2O6. This result confi rms previous suggestions from an X-ray diffraction study. At -180-degr ees-C, the local structure changes to a more strictly tetrahedral envi ronment, with two V-O = 1.64 angstrom and two V-O = 1.78 angstrom, in agreement with an increase in intensity in the '3d-1s' pre-edge featur e in the XANES spectrum. In both cases, further, long V-O bonds are al so present. The results are compared with previous spectroscopic inves tigations (EPR and IR) and neutron and X-ray diffraction studies. The crystalline PbV2O6 (form I), used as a model compound, also shows sign ificant changes in bond lengths (although not in geometry, which remai ns elongated octahedral) on cooling to -180-degrees-C.