EPR study of RLi 2O.V 2O 5, RNa 2O.V 2O 5, RCaO.V 2O 5, and RBaO.V 2O 5 modified vanadate glass systems

Abstract

Experimental EPR spectra in several modified vanadate glass systems reveal hyperfine structure (hfs) lines whose widths vary with the molar ratio of modifier to vanadium pentoxide, R. In the RNa 2O.V 2O 5 system, for example, hfs lines show no resolution at low R values (near 0.1); by contrast, these lines exhibit dramatic narrowing as R approaches 0.5. In the model proposed here, this narrowing is due to an increase in hopping time for small polarons associated with V 4+ ions in these systems. Increases in polaron hopping times are accompanied by increases in electron spin-spin relaxation times T 2's, and, an associated narrowing of EPR linewidths. Experiments confirm that spectral widths are limited by electron T 2's due to the fact that EPR linewidths do not vary with temperature down to 4.2 K. Resolved spectra in RNa 2O.V 2O 5 at R = 0.5 reveal a hyperfine coupling parameter of 0.0177 ± 0.0008 T, corresponding to an upper-limit polaron hopping frequency of 487 ± 20 MHz. By similar analyses, the systems of RCaO.V 2O 5, RBaO.V 2O 5, and RLi 2O.V 2O 5 exhibit comparable polaron hopping frequencies limits of 480 ± 20 MHz, 469 ± 20 MHz, and 468 ± 20 MHz, respectively, when R is near 1.0. In addition to the relaxation effects discussed here, results of modeling of resolved spectra to obtain hyperfine coupling constants A || and A ┴, and g ┴ values g || and g ┴ are presented and discussed.