X-ray absorption spectroscopic study of the binary semiconducting glass PbV2O6

The 1:1 PbO:V2O5 semiconducting glass prepared by quenching an equimolar mixture of PbO and V2O5 melt (650°C) at liquid-nitrogen temperature has been investigated by XAS (X-ray absorption spectroscopy) at 25 and -180°C. The spherical wave approximation was used, applying parameters extracted from the crystallographically known analogue PbV2O6 (form I) at the same temperatures. At 25°C the local geometry at VV(i.e. V-O bonds < 2.3 Å) may be described as a trigonally distorted square pyramid, with three V-O bonds of 1.71 Å, and two longer ones of 1.99 and 2.24 Å (±0.02 Å). 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 PbV2O 6. This result confirms previous suggestions from an X-ray diffraction study. At -180°C, the local structure changes to a more strictly tetrahedral environment, with two V-O = 1.64 Å and two V-O = 1.78 Å, in agreement with an increase in intensity in the '3d-1s' pre-edge feature in the XANES spectrum. In both cases, further, long V-O bonds are also present. The results are compared with previous spectroscopic investigations (EPR and IR) and neutron and X-ray diffraction studies. The crystalline PbV 2O6 (form I), used as a model compound, also shows significant changes in bond lengths (although not in geometry, which remains elongated octahedral) on cooling to -180°C.