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  • 1
    Electronic Resource
    Electronic Resource
    Nature of small-polaron hopping conduction and the effect of Cr doping on the transport properties of rare-earth manganite La0.5Pb0.5Mn1−xCrxO3 (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 1550-1558 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0≤x≤0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x≥0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T〉θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between Tp and θD/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above Tp. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1378018
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  • 2
    Electronic Resource
    Electronic Resource
    Polaron hopping conduction and thermoelectric power in LaMnO3+δ (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4955-4961 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two different phases of LaMnO3+δ [one showing a metal–insulator transition (MIT), referred to as LaMn–C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn–S followed a nonadiabatic hopping conduction mechanism while LaMn–C and the Pb doped samples viz. La1−xPbxMnO3 (x=0.05–0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(EF), Debye temperature (θD), electron–phonon interaction constant (γP), etc. of LaMn–S showed appreciable differences from those of LaMn–C and La1−xPbxMnO3, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1362411
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  • 3
    Electronic Resource
    Electronic Resource
    Electrical properties of semiconducting (1−x)(90V2O5–10P2O5)+xBaTiO3 glass and glass–ceramics (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 11326-11334 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Homogeneous (1−x)(90V2O5–10P2O5)+x(BaTiO3) glasses (abbreviated as VPBT) are formed with lower concentrations (x) of BaTiO3 (x〈0.3). For x〉0.3 microcrystallization of BaTiO3 appears during glass formation (corresponding samples are termed as glass–ceramics). All of these oxide glasses show semiconducting behavior. Electrical conductivity data measured between 80 and 450 K follow the adiabatic small polaron hopping conduction mechanism for glasses with lower concentrations (x〈0.2), but for higher concentrations of BaTiO3 (x〉0.2) the said conduction mechanism is changed to the nonadiabatic regime. Low temperature (T≤150 K) conductivity data of these glass samples can be well fitted with the variable range hopping model. At higher temperatures, Mott's model of phonon assisted small polaron hopping between nearest neighbors is consistent with the conductivity data. The polaron hopping model of Schnakenberg can predict the conductivity data in the entire temperature range of measurement. However, the percolation model, valid for the similar V2O5–Bi2O3+SrTiO3 glass (denoted by VBBT), is not consistent with the experimental results of VPBT glass samples. Again the frequency dependent ac conductivity data of the VPBT glasses follow overlapping large polaron hopping model, while those of the VBBT glasses satisfy the correlated barrier hopping model. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472923
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  • 4
    Electronic Resource
    Electronic Resource
    Structural and transport properties of (Bi,Pb)4Sr3Ca3Cu4−mFemOx (m=0–0.06) glasses: Precursors for high Tc superconductors (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2954-2961 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Microstructure and transport properties (between 80–450 K) of the Fe doped as-quenched Bi3Pb1Sr3Ca3Cu4−mFemOx (m=0.0, 0.02, 0.04, 0.05, and 0.06) type multicomponent glassy precursors for high-temperature superconductors were first reported in this paper. It has been shown from x-ray diffraction, scanning electron microscopy, thermal analysis, density, and oxygen molar volume measurements that single phase homogeneous glasses are formed. Even with small addition of Fe, a large change of glass transition temperature (Tg) indicates appreciable change of glass network structure of the undoped (Bi,Pb)4Sr3Ca3Cu4Ox glass which is reflected in the properties of the corresponding high-temperature annealed glass termed as glass-ceramic (GC) samples. These GC samples are found to be superconductors. Addition of Fe is considered to break up some of the original Cu–O–Cu bonds forming some new Cu–O–Fe and Fe–O–Fe type bonds. A nonlinearity (increase) in the dc conductivity (σdc) of the as-quenched glass sample is observed around m=0.02. This increase of dc conductivity is ascribed to be due to the additional hopping between Fe2+ and Fe3+ which finally destroys superconductivity for m≥0.06. This is also responsible for the nonlinear variation of activation energy of the Fe doped glass and hence change in the superconducting properties of the glass ceramics. The high temperature (above θD/2, θD being the Debye temperature) σdc data of the glasses were explained by considering small polaron hopping conduction mechanism. The dc conductivity of these glasses are found to follow Greaves' variable range hopping model in the intermediate range of temperature. The polaron hopping model of Schnakenberg and Emin can predict the conductivity data in the high-temperature regime. All the Fe-doped glass ceramics showed superconducting transitions with Tc between 100 and 110 K. The corresponding zero resistance temperature, Tco which is maximum (79 K) for m=0, decreases consistently with increase of Fe-concentration and finally superconducting behavior is destroyed for m≥0.06. This behavior is consistent with-pair breaking mechanism. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475682
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  • 5
    Electronic Resource
    Electronic Resource
    ac conductivity in Bi4Sr3Ca3CuyOx (y=0–5) and Bi4Sr3Ca3−zLizCu4Ox (z=0.1–1.0) semiconducting oxide glasses (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 931-937 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Semiconducting oxides such as Bi4Sr3Ca3CuyOx (y=0–5) and Bi4Sr3Ca3−zLizCu4Ox (z=0.1, 0.5, and 1.0), many of which become superconductors in their glass-ceramic phases, have been studied over the wide temperature (77–450 K) and frequency (100–104 Hz) ranges. The universal power-law behavior [exponent s≤1 in σ(ω)=σ0+Aωs] is found to be valid for most of these glasses. Little deviation from this limiting behavior is also observed for some glass compositions in the low-temperature and high-frequency regions. The correlated barrier hopping model based on the pair approximation is found to be more appropriate for explaining the frequency and temperature-dependent ac conductivity data. The random walk type of model, on the other hand, is found to be valid for a limited range of temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.355328
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  • 6
    Electronic Resource
    Electronic Resource
    High conductivity and nonlinear Seebeck coefficient of Ag2O containing (Bi–Pb–Sr–Ca–Cu–O) glassy precursors for high Tc superconductors (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 7320-7325 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical conductivities (σdc) of the as-quenched Bi3.5Pb0.5Sr3Ca3Cu4Ox+zAg2O (with z=1, 3, 5, and 10 wt %) glassy precursors for high Tc superconductors are found to be much higher (∼10−5−101 Ω−1cm−1) than those of the corresponding Ag2O free Bi3.5Pb0.5Sr3Ca3Cu4Ox (denoted by BPB) precursor glass (∼10−13−10−6 Ω−1cm−1). This unusually high conductivity is attributed to the increase of carrier concentrations caused by the addition of Ag2O (also observed from the Hall effect measurements). The experimentally observed high values of σdc do not follow Mott's variable range hopping model which is in sharp contrast to the behavior of the corresponding pure BPB and many other conventional transition metal oxide glasses having high resistivities. Moreover, the Seebeck coefficients (S) of these glassy precursors show nonlinear variations (from negative at lower temperature to positive at higher temperature) which cannot be clearly explained by phonon drag or electron-phonon interaction. This behavior of S which is also supported from Hall effect measurement is considered to be due to the nonlinear thermal variations of carrier concentrations (both hole and electron) present in the glassy samples. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.365330
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  • 7
    Electronic Resource
    Electronic Resource
    Photoacoustic determination of the optical and thermal properties of isovalent S-substituted polymeric Ge–Se system (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 723-728 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Isovalent sulfur (S) -substituted glassy Ge0.05SxSe0.95−x (0≤x≤0.15) alloys have been prepared by quick quenching from the melt. The concentration (x) dependences of optical absorption, band gap, and thermal diffusivity of these alloys have been investigated at room temperature (300 K) by the photoacoustic spectroscopic (PAS) method in the spectral range of 350–800 nm. The small variation of the optical gap E0 with composition is analyzed on the basis of Kastner's model [in the small Ge limit of M. Lannoo and M. Bensoussan, Phys. Rev. B 16, 3546 (1977)], considering the electronegativity difference between the substituting atoms (Se and S) and bond energy calculation. The observed exponential edge (Urbach edge) can be considered as an internal Franz–Keldysh effect arising from the charged impurity-generated as well as "frozen-in'' optical-phonon-generated fields. This can be described in the framework of the Tauc and Dow–Redfield model. The phonon-assisted indirect transition at the band-tail region is also obtained from the present studies. The concentration-dependent thermal diffusivity, estimated from the PAS studies, showed a minimum around x=0.05 similar to the behavior of the observed optical energy gap. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360817
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  • 8
    Electronic Resource
    Electronic Resource
    Multiphonon hopping conduction in nonconventional chromium-doped Bi3Pb1Sr3Ca3Cu4−nCrnOx (n=0.025–0.2) glasses with nanocrystalline particles and clusters (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 5033-5042 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1317237
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  • 9
    Electronic Resource
    Electronic Resource
    Study of microstructural behavior and nonadiabatic small polaron hopping conduction in BaTiO3 doped lead-vanadate glass and glass-ceramics dispersed with ferroelectric nanocrystals (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 3477-3487 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Multicomponent semiconducting oxide glasses like (80 V2O5–20 PbO)+xBaTiO3 (x=5–30 wt %) dispersed with nanocrystalline BaTiO3 have been prepared by a fast quenching technique. Results of microstructural study and transport properties (between 80 and 450 K) of these glasses and some glass ceramics have been reported in this article. It has been shown from x-ray diffraction, scanning electron microscopic, and other studies that homogeneous glasses are formed with wide concentrations of BaTiO3 (x≤30 wt %). Transmission electron microscopic study, however, indicates the presence of nanocrystalline phases in the glass matrix. Mott's variable range (nonadiabatic) hopping conduction mechanism is found to be valid at the low temperature regime (below θD/4, θD being the Debye temperature) while in the high temperature regime (above θD/2), the polaron hopping models of Schnakenberg as well as Emin and co-workers can quantitatively predict the conductivity data for all these special types of multicomponent glass-nanocrystal composites. The model parameters, obtained from best fit of the experimental data with these models, are consistent with the glass compositions. The dielectric constants of these glasses are found to be very high (comparable to that of bulk BaTiO3) and about two orders of magnitude higher than those of BaTiO3 free (80 V2O5–20 PbO) and other transition metal oxide glasses. This novel character is considered to be due to the embedded nanocrystalline BaTiO3 phases. The glass samples annealed at 300 °C for 9 h in air also showed nano and microcrystalline BaTiO3 phase along with traces of PbV2O6, PbTiO3, TiO2, and Pb2V2O7 phases. Broad anomaly (with a maximum around 390 K) is observed from the thermal variation of dielectric constant data of these partially annealed glass-ceramic samples. This anomaly resembles to the anomaly observed around the Curie temperature (∼393 K) in bulk ferroelectric BaTiO3 crystal. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369705
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  • 10
    Electronic Resource
    Electronic Resource
    Particle size and magnetic field dependent resistivity and thermoelectric power of La0.5Pb0.5MnO3 above and below metal–insulator transition (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 5125-5134 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of particle size on the transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature Tp of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T〉θD/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy WH decreases but polaron radius rp increases with the increase of grain size. In the metallic regime (for T〈Tp), resistivity data fit well with ρ=ρ0+ρ2.5 T2.5 and the transport mechanism is attributed mainly to the magnon-carrier scattering (∼T2.5). In all the samples with different grain sizes, S changes sign below Tp. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T〈Tp) for these samples. Thermopower data in the metallic phase (both for B=0.0 and 1.5 T) can be analyzed by considering a spin-wave fluctuation term (∼T4) in addition to the magnon-scattering term similar to the case of resistivity data. Although the variable range hopping mechanism is supported from the resistivity data (for Tp〉T〉θD/2), it is hard to justify this model from the temperature dependent thermopower data. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1459618
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