Multiphonon hopping conduction in nonconventional chromium-doped Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025–0.2) glasses with nanocrystalline particles and clusters

Abstract

Transport properties of Cr containing multicomponent oxide glasses Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025, 0.05, 0.1, and 0.2) dispersed with nanocrystalline particles (5–20 nm depending on the values of n) have been reported in the temperature range of 250–450 K. Conductivity of this glass-nanocrystal composite system shows little decrease with increasing Cr content. Above θD/2 (θD is the Debye temperature), conductivity data can be analyzed with small polaron hopping models. Interestingly, unlike undoped Bi4Sr3Ca3Cu4Ox (or Bi-4334) glasses [showing nonadiabatic small polaron hopping (SPH) conduction at T>θD/2], the Cr doped glasses supports adiabatic SPH conduction mechanism above θD/2 indicating change of glass network structure due to partial substitution of Cu by Cr. But below this temperature Mott’s or Greaves’ variable range hopping models can be consistently used to fit the experimental conductivity data only with larger (compared to the usual transition metal oxide glasses) values of the density of states at the Fermi level N(EF). The most probable transport mechanism for the entire range of temperature and glass compositions is concluded to be due to multiphonon tunneling of large polarons between the nanoclusters present in the glasses which is also in sharp contrast to the behavior of the undoped (Bi-4334) glass. All the glass samples (except n⩾0.2) are found to become superconductors by annealing at higher temperatures.