Activation energies of hopping of vanadium 3<i>d</i>¹ small polaron in 0.7V₂O₅-(0.3-x)P2O₅-x(As₂O₃/Sb₂O₃)<i> </i>(x=0, 0.05) glasses by EPR spectroscopy and DC electrical conductivity measurements

Abstract

The three glasses in the 0.7V₂O₅-(0.3-x)P2O₅-x(As₂O₃/Sb₂O₃)<i> </i>(x=0, 0.05) system, G1 (x=O), G2 (x<i> =</i>0.05:As₂O₃) and G3 (x<i>=</i>0.05<i>: </i>Sb₂O₃) have been prepared by melting the appropriate amounts of the batches in silica crucibles in the temperature ~1120 K and quenching in air. The glassy phase has been ascertained by powder X-ray diffraction of the samples which shows a broad peak around 25^o in 20 vanishing at higher diffraction angles typical of glassy phase. EPR studies of the glasses in the range 10-473 K show that the lineshapes are isotropic in nature. The observed values of the <i>g </i>matrices are similar in all the glasses in the range 10-473 K which shows that the octahedra containing the paramagnetic sites are similar in all the glasses. The V⁴⁺(3<i>d</i>¹) small polaron activation energies of hopping are lower in the case of G2(1.16× l0^-4 eV) and G3 (1.27× l0^-4 eV) as compared with that of G1 (5.27×l0^-4 eV) due to the formation of As-V or Sb-V <i>d</i>_II-<i>d</i>_IIbonds, which is absent in the case of the binary glass G1. The Log (Δv (Hz)) versus 10³<i>/T </i>(K^-1) plots of the glasses in the range 10-473 K are characterized by three regimes, viz. ω₀τ_c˃˃1 , ω₀τ_c =1 and ω₀τ_c ˂˂1 where ω₀ is the resonance frequency in rad/s and τ_c is the correlation time of the polaron in the glassy matrices. The calculated values of the V⁴⁺(3<i>d</i>¹) small polaron hopping activation energies due to viscosity effect on the tumbling of the paramagnetic sites in the regime ω₀τ_c ˂˂1 is found to be ~100 times higher than the activation energy of hopping (<i>E</i>_EPR)<i> </i>due to life time broadening of the EPR lineshapes in the regime ω₀τ_c˃˃1, which again are significantly different from the activation energies obtained for the glasses G1-G3, as 0.358<i> , </i>0.619 and 0.509 eV, respectively from the DC electrical conductivity in the range ~300-473 K. It

shows that the mechanism DC electrical conductivity in these glasses is different from the mechanism of EPR line narrowing.