Small-polaron Motion Research Articles

Effect of Ni2+ substitution on magnetic, optical and electrical properties of SrFe2O4

Citrate gel combustion method is used for the synthesis of Ni2+ substituted strontium ferrite Sr1-xNixFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8). The characterization of synthesized material is done by different techniques. Fourier transform Infra-red (FT-IR) spectra revealed the inverse spinel structure and showed slight shift in spectrum. X-ray diffraction (XRD) patterns demonstrated the orthorhombic phase of spinel formation with trace of secondary phase impurity of SrCO3. Scanning electron microscopy (SEM) images showed agglomeration of particles. The effect of Ni2+ substitution in SrFe2O4 on structural, magnetic, optical and direct current (DC) electrical resistivity properties are investigated. Room temperature vibrating sample magnetometer (VSM) studies indicated increase in trend of saturation magnetization (Ms) and remanence magnetization (Mr) with increase in content of Ni2+ ion. The squareness ratio (SQR) Mr/Ms for SrFe2O4 is found to be 0.5752, while for remaining samples it is found to be less than 0.5 which demonstrated multi domain to single domain transition. Variation of band gap energy with Ni2+ substitution is reported with the help of ultra-violet diffuse reflectance spectra (UV-DRS). Temperature dependent DC electrical resistivity by two probe method measurement obeyed Arrhenius equation and revealed the typical semiconducting behavior of the samples. The change in slope of line observed in resistivity due to ferrimagnetic to paramagnetic transition in all samples attributed strong exchange interaction between the outer and inner electrons. Activation energy (Ea) for ferri and paramagnetic region is calculated. For ferrimagnetic region the Ea is found to increase with increasing Ni2+ content while, in paramagnetic region for Ni2+ substituted samples the Ea is found to be greater than 0.2 eV which inferred the hopping motion of small polarons are responsible for conductivity. The complex impedance spectra revealed the information about conduction mechanism.

Electrical transport properties of the six-layer hexagonal SrMn1−xVxO3−δ (x = 0.02)

In contrast to 6H-SrMnO3 which is usually synthesized using a high-pressure and high-temperature technique, SrMn1−xVxO3 oxides can be prepared via a solution combustion synthesis route with final annealing in air. Doping of SrMnO3 with vanadium allows the formation of the SrMn1−xVxO3 oxide (x = 0.02) which crystallizes in a six-layer hexagonal structure (space group P63/mmc, Z = 6). The specific electrical conductivity (σ) and Seebeck coefficient (S) are measured in the temperature (Т) range 350–1220 K in air. The metallic-like change in S with T is explained employing the model of the motion of small polarons considering the disproportionation of Mn3+ ions to Mn2+ and Mn4+ and the spin direction of t2g-electrons manganese. The enthalpy of disproportionation reaction (130 meV) in 6H-SrMn0.98V0.02O3–δ is higher than in other perovskite-type manganites.

Observation of large thermoelectric power in charge ordered La1−xLixMnO3 (x = 0.25) manganite system

In the present investigation detailed structural, electrical, magnetic and thermoelectric measurements on bulk samples of Li-doped La1−xLixMnO3+δ (0.05 ≤ x ≤ 0.3) manganites have been done. The samples are synthesized by wet-chemical mixing route and XRD analysis using Rietveld refinement revealed single phase behaviour upto Li-concentration x = 0.25. All the samples in the present series show ferromagnetic behaviour while metallic behaviour is shown by the samples upto Li concentration x = 0.2. Beyond x = 0.2 the sample (i.e., La0.75Li0.25MnO3) show insulating behaviour for the entire temperature range accompanied by charge-order transition around T = 225 K. The low temperature resistivity data are best fitted by ρ(T) = ρ0 + ρ4.5T 4.5 + C/sinh2(hυs/2kBT), where C is a constant. Such behaviour might be an indication of the small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode that is strongly coupled to the carriers. Thermoelectric power (TEP) measurements reveal interesting results showing a colossal value of −340 μV/K around 25 K for the CO sample in the present series. The probable mechanism responsible for the observed large TEP has been discussed here.

Electrical properties of (1−x) BFO – (x) PZT multiferroics synthesized by sol-gel method: Transition from relaxor to non-relaxor

Crystalline solutions of (1−x) BiFeO3 – (x) Pb(Zr0.52Ti0.48)O3 within the composition range 0.50 ≤ x ≤ 1.00 have been made by the sol-gel route. Solid solution formation and structural transformations have been confirmed by diffraction studies. Electrical behaviour of solid solution compositions have been studied at a few selected frequencies in the temperature range 35 °C–600 °C. Overall, three dielectric anomalies have been observed for compositions 0.50 ≤ x ≤ 0.70 whose positions have been found to shift to low temperature side with increasing x and reduced to one broad anomaly at ∼385 °C for x = 1.0. A switching from relaxor behaviour for 0.50 ≤ x ≤ 0.90 to non relaxor behaviour for x = 1.0 has been observed. All the samples have shown low dielectric loss near room temperature. Temperature dependent conductivity variation indicates an Arrhenius type thermally activated process via localized hopping of electrons in the low temperature region. Small polaron and oxygen vacancy motion dominate at intermediate and high temperature range respectively.

Coexistence of site- and bond-centered electron localization in the high-pressure phase ofLuFe2O4

Magnetic-electronic hyperfine interaction parameters of spectral components are obtained from in situ $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy pressure studies of the mixed-valence $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ multiferroic, up to $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and on recovered high-pressure phase samples. Temperature-dependent M\ossbauer spectra of the low-pressure phase show that $\mathrm{F}{\mathrm{e}}^{2+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$ sites are discernible, consistent with known site-centered charge order in the triangular (frustrated) Fe sublattice network. Magnetic spectra of the high-pressure phase, stabilized in a rectangular Fe sublattice network at $Pg8\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, exhibit fingerprints of iron in an intermediate valence state only. Temperature-dependent resistivity pressure studies evidence thermally activated small polaron motion in the high-pressure phase. These experimental signatures, complemented by ab initio calculations of electronic structure, are considered evidence of asymmetric dimer formation ${\mathrm{Fe}}^{(2+\mathrm{\ensuremath{\Delta}}+)}\ensuremath{\Leftrightarrow}{\mathrm{Fe}}^{(3\ensuremath{-}\mathrm{\ensuremath{\Delta}})+}$, where the minority-spin electron deconfinement coefficient is $\mathrm{\ensuremath{\Delta}}=0.3\ensuremath{-}0.4$. Bragg satellites discerned in electron diffraction patterns of the metastable high-pressure phase possibly stem from this admixture of site- and bond-centered localization (intermediate-state charge order) in a magnetic background. This breaks inversion symmetry and potentially renders $\mathrm{LuF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ in its high-pressure phase as a new charge order instigated (electronic) ferroelectric.

Local structural distortions, orbital ordering, and ferromagnetism in underdopedLa1−xSrxMnO3

In order to elucidate the origin of the ferromagnetism found in underdoped $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, we have grown a series of single crystals with a fine step of doping and characterized them with a structural study and measurements of magnetization, resistivity, thermoelectric power, and thermal conductivity. The paramagnetic phase of the underdoped $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ is a small-polaron conductor. However, the Weiss constant extracted from the paramagnetic susceptibility shows a ferromagnetic coupling and tracks the Curie temperature. Given that the double exchange interaction cannot be delivered by the hopping motion of small polarons, we have made a systematic analysis of the structural changes with $x$ and studied whether the superexchange interaction can account for the ferromagnetic coupling found in the paramagnetic phase. Intrinsic local site distortions in $\mathrm{LaMn}{\mathrm{O}}_{3}$ change as $x$ increases in $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$. The influence of the local structural distortion and the cooperative orbital ordering on lattice parameters have been demonstrated by a comparison between the measured change of lattice parameters versus hole doping and a simulation with the software SPuDs, in which rigid octahedra are assumed. Changes with $x$ of the bond length splitting in $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra induces an orbital mixing of $(3{x}^{2}\ensuremath{-}{r}^{2})+({y}^{2}\ensuremath{-}{z}^{2})\phantom{\rule{0.28em}{0ex}}\mathrm{or}\phantom{\rule{0.28em}{0ex}}(3{y}^{2}\ensuremath{-}{r}^{2})+({z}^{2}\ensuremath{-}{x}^{2})$, which progressively converts two-dimensional ferromagnetism in the parent perovskite $\mathrm{LaMn}{\mathrm{O}}_{3}$ into three-dimensional ferromagnetism. The correlation between the particular lattice distortions and the cooperative orbital ordering and orbital mixing can in turn be used to test the superexchange model by measuring the change of transition temperatures under uniaxial stress. Nearly identical uniaxial stress effects found in crystals of the mixed-valent $\mathrm{L}{\mathrm{a}}_{0.875}\mathrm{S}{\mathrm{r}}_{.125}\mathrm{Mn}{\mathrm{O}}_{3}$ and single-valent $\mathrm{LaM}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}{\mathrm{O}}_{3}$ confirm unambiguously that superexchange interactions play the dominant role in the ferromagnetic coupling of underdoped $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$.

Electronic Conduction Mechanisms in BaTiO3-Ni Composites with Ultrafine Microstructure Obtained by Spark Plasma Sintering

Spark plasma sintering (SPS) was used to densify BaTiO3–Ni composite powders to relative densities above 92.8%. With the increasing Ni content, a decrease in relative density is observed, suggesting that Ni hampers the consolidation process. The microstructures of the BaTiO3–Ni composites were of duplex character. The crystallite size of the BaTiO3-grains was around 100 nm in average. The ceramic matrix phase of BaTiO3 surrounded Ni inclusions of approximately 1 μm in diameter that were completely incorporated without the formation of any elongated metallic filaments. The ac conductivity of these BaTiO3–Ni composites increased with increasing Ni content and with temperature. The dominant conduction mechanisms in SPSed BaTiO3–Ni composites showed quite a complicated behavior. A gradual change from band conduction of trapped electrons in oxygen vacancies to a hopping-type motion of small polarons between Ti4+ and Ti3+ is suggested to occur, when the Ni content increases. The influence of oxygen vacancies and other lattice defects on the electrical properties of BaTiO3–Ni composites is discussed.

Low Temperature Dielectric Relaxations in ZnO Varistor

The main purpose of this work is to study the behaviour of ZnO varistor by means of dielectric relaxation spectroscopy at the low temperature range. The complex frequency spectrum of dielectric losses has been studied in detail and analysed carefully by fitting of a sum of Havriliak–Negami expression. Three relaxation processes are studied here, exhibiting a very strong Cole–Davidson behaviour. The faster relaxation process shows an unusual thermal behaviour. Initially, its relaxation time increases as the temperature rises but later it becomes thermally activated. The transition temperature of thermal behaviour of this process was found very close to the characteristic value of 0.4θD=160 K, where θD is the Debye temperature of ZnO. The de-activation energy of this process has been estimated by using the Arrhenius type relation to be 78 meV, a value very close to the higher longitudinal optical (LO) phonon energy in ZnO, 72 meV. This is a strong indication that the predicted Holstein transition between large to small-polaron motion should be observed in ZnO. The two remaining relaxations are electronic processes, the slower one is associated with the doubly ionised zinc interstitial Zni••, while the other should be related to defects induced by the dopants, or of complexes of intrinsic native defect with dopant impurities.

Small-polaron formation and motion

Small-polaron formation and motion

Electron spin resonance study of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0,0.45, 1)

Electron spin resonance (ESR) spectra of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0, 0.45, 1) were studied within the temperature range110 K≤T≤470 K. The temperature dependence of the ESR intensity for the samples is described by athermally activated model in the paramagnetic regime. It is found that the activationenergy in the orthorhombic phase is higher than that in the rhombohedral phase forLa0.75(Ca0.45Sr0.55)0.25MnO3. It is suggested that a higher energy is required to destroy the correlated polarons due tothe fact that correlated polarons only exist in the orthorhombic phase. This proposition isconfirmed by the analysis of the ESR linewidth data, which can be well fitted by the modelof adiabatic hopping motion of small polarons. In addition, it is found that, ata fixed temperature, the linewidth decreases with increasing Sr doping, whichreveals that the structural tolerance factor has a significant effect on the linewidth.

Boundary between coherent and noncoherent small polaron motion: Influence of the phonon hardening

The transition from band to hopping conductivity of small polaron is examined within Holstein's molecular crystal model. The conditions under which each of these mechanisms prevail are formulated in terms of the values of the coupling constant S and adiabatic parameter B. Particular attention was paid to the possible influence of the polaron induced modification of phonon spectrum.

Small polaron transport in Li 7MnN 4 containing isolated MnN 4 tetrahedra

The electrical transport properties related to the Mn 3d-band in Li 7MnN 4, which has been considered as an electrode material of Li ion secondary battery, are studied by combining with the optical absorption and electron paramagnetic resonance (EPR). The optical absorption studies show that the empty Mn 3d-band attributed to isolated MnN 4 tetrahedra in Li 7MnN 4 lies at about 1.15 eV above the top of the valence band. An EPR signal with g -value = 1.9848 is observed at 77 K, suggesting the presence of Mn 5+ in the isolated MnN 4 tetrahedra like that in Li 3MnO 4. The temperature dependence of electrical resistivity shows the hopping conduction with the activation energy of 112 meV at higher temperature above 180 K and the tunneling conduction at lower temperature below 60 K, suggesting the small polaron motion in the Mn-3d band as predicted by Holstein.

Evolution of conductivity, structure and thermochemical stability of lanthanum manganese iron nickelate perovskites

A series of solid solutions in the La0.8Sr0.2MnO3±δ (LSM)–xLa0.95Ni0.6Fe0.4O3−δ (LNF) system were synthesised by solid-state reaction to investigate the effect of Ni/Fe substitution for Mn on their DC electrical conductivity, structure and thermochemical stability. A non-linear change in structure, conductivity, magnetic properties and thermochemical stability of solid solutions was observed with the increase in the Ni/Fe content. A new phase (100 − x)LSM·xLNF with orthorhombic symmetry was found in the concentration range 35 ≤ x < 70 mol%, and exhibits the lowest value of the conductivity and the highest activation energy in the single-phase concentration range 43 ≤ x ≤ 53. This is, most probably, caused by a low concentration of Mn3+–O–Mn4+ units available for small polaron motion through the B sublattice. The compositions containing more than 50 mol% Ni in the B sublattice shows metallic-like conductivity behaviour. The temperature of the semiconducting metallic transition decreases with increase in the Ni concentration. All compositions had good thermochemical stability in air, in contrast to that under reducing conditions. The higher the Ni/Fe content in the sample, the faster the reduction of the solid solutions occurred in non-humidified Ar–H2 (95 : 5) atmosphere. The overall oxygen stoichiometry of the solid solutions is discussed.

Valence changes of manganese and praseodymium in Pr 1−xSr xMn 1−yIn yO 3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr 1− x Sr x Mn 1− y In y O 3− δ upon cation substitution with Sr 2+ and In 3+ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L II,III and Pr M IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr 0.5Sr 0.5InO 3− δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.

Holstein light quantum polarons on the one-dimensional lattice

The polaron formation is investigated in the intermediate regime of the Holstein model by using an exact diagonalization technique for the one-dimensional infinite lattice. The numerical results for the electron and phonon propagators are compared to the nonadiabatic weak- and strong-coupling perturbation theories, as well as with the harmonic adiabatic approximation. A qualitative explanation of the crossover regime between the self-trapped and free-particle-like behaviors, not well understood previously, is proposed. It is shown that a fine balance of nonadiabatic and adiabatic contributions determines the motion of small polarons, making them light. A comprehensive analysis of spatially and temporally resolved low-frequency lattice correlations that characterize the translationally invariant polaron states is derived. Various behaviors of the polaronic deformation field, ranging from classical adiabatic for strong couplings to quantum nonadiabatic for weak couplings, are discussed.

Influence of A-site substitution on ESR spectra of lanthanum manganite perovskites

We studied electron spin resonance (ESR) spectra of La 0.7A′ 0.3MnO 3 (A′=Sr, Ba, Pb, and Cd) perovskites in a large range of temperature, from their Curie temperature to ∼500 K. Asymmetrical EPR signals at temperatures T C < T < T min , due to FM correlations, became Lorentzian at T ⩾ T min , where T min is the temperature corresponding to the narrowest ESR linewidth. The ESR linewidth vs. temperature, Δ H( T), in the region T > T min well fits to the model of adiabatic hopping motion of small polarons Δ H ( T ) = Δ H 0 + ( a / T ) exp ( - E a / k B T ) , where E a and k B are the activation energy and the Boltzmann factor, respectively; Δ H 0 and a are constants. It is worth noticing that, in this temperature range, the Lande factor g for the samples is about 2.00, indicating spin–spin interaction among electrons of Mn 3+ and Mn 4+ ions playing an important role.

Electrical and magnetic behaviors in Nd 0.7Sr 0.3MnO 3 with different annealing periods

We studied electrical and magnetic behaviors of polycrystalline samples of Nd 0.7Sr 0.3MnO 3 prepared by the solid-state reaction method at 1300 °C for 10, 20, 30, 40, and 80 h. Magnetization M( T), resistivity ρ( T) and X-band electron paramagnetic resonance (EPR) measurements were carried out. Experimental results showed that the Curie temperature T C decreased considerably with increasing the annealing time t a from 10 to 30 h, but became unchanged with t a>30 h. The insulating-metallic transition was observed at T MI. Spin-dynamic processes in the ferromagnetic and paramagnetic regimes were recorded by EPR. Electrical conductivity and the EPR linewidth (Δ H) versus temperature were analyzed by means of the adiabatic hoping motion of small polarons.

Influence of a-site substitution on the EPR parameters of La/sub 0.7/A'/sub 0.3/MnO/sub 3/ (A'= Sr, Ba) compounds

In this paper, we investigate in detail influences of A-site substitution on the EPR parameters for La/sub 0.7/Sr/sub 0.3/MnO/sub 3/ (LSM) and La/sub 0.7/Ba/sub 0.3/MnO/sub 3/ (LBM) compounds by means of electron paramagnetic resonance (EPR) study. Magnetization and resistivity measurements were also performed. The Curie temperature T/sub C/ is 365 and 322 K for LSM and LBM, respectively. In the whole temperature range studied, metallic behavior was observed. The minimum linewidth /spl Delta/H/sub min/ is 402 and 378 Oe for LSM and LBM, respectively. Temperature dependences of the EPR linewidth /spl Delta/H/sub pp/(T) for the samples were analyzed by using the model of adiabatic hopping motion of small polarons, /spl Delta/H/sub pp/(T)=/spl Delta/H/sub 0/+(A/T)exp(-E/sub a//k/sub B/T). The activation energies E/sub a//sup (1)/ and E/sub a//sup (2)/, obtained from /spl Delta/H/sub pp/(T) and EPR intensity I(T) data, are different. At temperatures T>T/sub min/, the Lande factor of both samples is the same (g/spl ap/2), indicating that spin-spin interactions among electrons of Mn/sup 3+/ and Mn/sup 4+/ ions play a main role.

ESR study of Sr‐doped LaCoO3 cobaltites

AbstractThe electron spin resonance (ESR) method was used to investigate the internal spin dynamics of La1–xSrxCoO3 (x = 0.1, 0.2, 0.3) compounds in the temperature range 170 to ∼500 K. ESR spectra of the samples over the whole temperature range studied could be divided into three regions corresponding to different linewidths. This is due to the presence of an anisotropic field arising from ferromagnetic clusters. This is similar to manganite perovskites. The peak‐to‐peak ESR linewidth vs. temperature, ΔHpp(T), for the samples has a minimum value at a temperature Tmin (in this case Tmin ≈ 1.7TC, whereas Tmin ≈ 1.1TC for manganites). Asymmetric Lorentzian ESR spectra at temperatures T &lt; Tmin become Gaussian at T &gt; Tmin. ESR signals of La1–xSrxCoO3 cobaltites are thermally damped out at ∼460 K, this value being much lower than that for manganite compounds. The explanation of the experimental ΔHpp(T) data in the T &gt; Tmin regime is based on the one‐phonon process and adiabatic hopping motion of small polarons. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hopping polaronic motion in high- Tc superconductors

The motion of small polarons is studied as a function of temperature in the context of a molecular crystal model in which the discreteness of the lattice is accounted for. The polaron bandwidth and the site jump hopping probability have been calculated vs. temperature and dimensionality. The crossover temperature T d* between band-like and hopping motion is reduced in low-dimensional systems due to the enhanced importance of the off-diagonal scattering processes. We discuss the relevance of the model to high- T c superconductors in which polaronic features in the transport properties have been pointed out [M. Zoli, Phys. Rev. B 53 (1996) 9074; M. Zoli, Phys. Rev. B 56 (1997) 111].