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

Abstract

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 V^v(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 PbV₂O₆. 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, furthur, 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 PbV₂O₆(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.