?? ????? ??????? ??? ????????????? ?????? ??????? ???? ??????? ????????????? ??? ???????? ?? 1978 ??? ?? ????? ?????? ???? ??? ?? 1980. ??????, ??????????? ??? 2020, ??????? 17 ???? ??? ??? ??????? 25 ?????????. ?? 1995 ?? ????? ??????????? ??? ????????? ?????? ???? ??????? ??? ??????, ???? ????????????????? ??? ???????? ??? ?????? ??? ????????? ?????? 8 ??? ??????????? ??? ??? ???? ??? ?????????. ?? ???????? ??? ????? ?????? ??? ????? ????????? ?? ????? ??????? ???? ?? ?? ????? ????????? ??????????? [1] ??? ?? ?????? ??????????? ???? ???????? [2], ???????? ??? Univ. of Southern California (???).

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Dielectric relaxation, modulus behavior and conduction mechanism in Sb2S3 thin films

Photoelectrochemical (PEC) water reduction is a long-term strategical technology for hydrogen production. In this work, we synthesize a series of compact and nano/mesoporous Nb-substituted BaBiO3 [i.e., Ba2Bi(Bi1–xNbx)O6, 0 ≤ x ≤ 0.93, BBNO] thin films using cost-effective chemical solution methods. The synthesized BBNO alloy based thin films demonstrate tunable bandgaps from 1.41 eV (x = 0) to 1.89 eV (x = 0.93) to efficiently absorb the solar spectrum and p-type conductivities suitable for hydrogen production. The photoelectrodes with a configuration fluorine-doped SnO2/BBNO (0 ≤ x ≤ 0.93)/Pt produce cathodic photocurrents of 0.05–1 mA·cm–2 at 0 VRHE (volt versus reversible hydrogen electrode) measured in a neutral (pH = 7.2) phosphate buffer and under simulated AM 1.5G illumination (100 mW·cm–2). The BaBiO3 without Nb alloying based electrode delivers the best photocurrent of 1 mA·cm–2 at 0 VRHE but is subjected to severe corrosions during the PEC related tests. Alloying Nb has an obvious influence on ...

Solution-processed Nb-substituted BaBiO3 Double Perovskite Thin Films for Photoelectrochemical Water Reduction

Tuning the composition of perovskite oxides results in double perovskites with cation ordering. One of the most important families of double perovskites is A2B′B′′O6. With great structural variability, A2B′B′′O6 are promising candidates for a broad range of applications such as magnetic, optical, electrical, and catalytic applications. Rare earth elements, as the ensemble of fifteen lanthanide elements as well as yttrium and scandium, represent an important part of the periodic table due to their unique 4f electrons. The incorporation of rare earth elements into double perovskites may open great opportunities to novel properties and applications. A significant number of rare earth A2B′B′′O6 double perovskites have been reported. However, there is no systematic overview of these materials to date. In this review, we summarize the compositions, syntheses, and applications of rare earth A2B′B′′O6 double perovskites. We hope that it will encourage more researchers to enter this emerging area.

Rare earth double perovskites: a fertile soil in the field of perovskite oxides

Here we have reported the electric and dielectric properties of a Bornite (Cu5FeS4) film using non-destructive complex impedance spectroscopy (CIS) in the frequency range 40 Hz–20 MHz and bias voltage range 0–0.8 V. A typical hydrothermal reaction was carried out to obtain the Cu5FeS4 material. Thereafter, we fabricated the film of the synthesized material and analysed its various properties as a function of frequency and dc bias voltages. The CIS analysis at room temperature (303 K) for different forward dc bias voltages shows that the bulk resistance plays a predominant role in conduction mechanism. The complex impedance (Nyquist) plots are well modelled with grain and grain boundary resistance by introducing proper ac equivalent circuit. The impedance loss spectra showed that the relaxation peak shifts towards the higher frequency with increasing bias voltages, which implies the possibility of a charge hopping between the localized charge states. Furthermore, we find that dielectric constant (
 $${\varepsilon ^\prime }$$
 ), dielectric loss (
 $${\varepsilon ^{\prime \prime }}$$
 ), electrical modulus (
 $${{\text{M}}^*}$$
 ) and ac/dc conductivity of the Cu5FeS4 film are strongly frequency dependent. The values of $${\varepsilon ^\prime }$$
 and $${\varepsilon ^{\prime \prime }}$$
 decreases whereas ac conductivity increases with increasing frequency. High frequency and low frequency dielectric constant of the material are found to be 3.48 and in the order of 104 respectively. Electrical modulus study is introduced for better explanation of conductivity relaxation phenomenon. Finally, we investigated the variation of ac/dc conductivity with forward dc bias voltages. This confirmed the presence of a hopping mechanism for electrical transport in our system, which can be best explained on the basis of jump relaxation model. Overall, this extensive study based on complex impedance spectroscopy demonstrates the dielectric relaxation behaviour of Cu5FeS4 film and also shed light on hopping induced conduction mechanism.

Bias dependent conduction and relaxation mechanism study of Cu 5 FeS 4 film and its significance in signal transport network

Dielectric and Raman spectroscopic studies of A2ErSbO6 (A = Ba, Sr and Ca)

The electrical properties of rare-earth based ordered double perovskite oxide barium erbium tantalate, Ba2ErTaO6 synthesized by solid-state reaction method are investigated. The x-ray diffraction pattern of the sample shows cubic Fm3m phase at room temperature with ordering of the B cations. Fourier transform infrared spectrum shows two primary phonon modes of the sample at around 350 cm−1 and 600 cm−1. The dielectric relaxation of the sample is investigated in the frequency range from 50 Hz to 1.1 MHz and in the temperature range from 303 K to 673 K. Electric modulus and electrical impedance data are fitted to the Cole–Cole equation. The frequency dependent conductivity spectra follow the power law. Summerfield scaling is used to explain the conduction mechanism. The scaling behavior of the imaginary part of the impedance spectra suggests that the relaxation shows the same mechanism at various temperatures. The complex impedance plane plots show that the relaxation (conduction) mechanism in this material is mainly due to grain boundary effect for all temperatures and grain effect for low temperature. The relaxation frequency corresponding to dielectric loss is found to obey Arrhenius law with activation energy of 0.50 eV. The values of activation energy indicate that the dielectric relaxation and the conduction mechanism are due to adiabatic small polaronic hole hopping mechanism.

Dielectric Relaxation of Rare Earth Ordered Double Perovskite Oxide Ba 2 ErTaO 6

Structural and electrical transport properties of a rare earth double perovskite oxide: Ba2ErNbO6

A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr2ErNbO6 (SEN) is synthesized by the solid state reaction technique. Rietveld analysis of X-ray diffraction pattern reveals that SEN has monoclinic phase with space group P21/n. Fourier transform infrared spectrum shows two phonon modes of the sample due to the antisymmetric NbO6 stretching vibration. The compound shows significant frequency dispersion in its dielectric properties. The frequency dependent ac conductivity spectra follow the Jonscher's universal power law. The conductivity at 100 Hz varies from 1.38×10−6 Sm−1 to 2.34×10−4 Sm−1 with the increase of temperature from 303 K to 653 K respectively. The increase of conductivity can be explained from the Correlated barrier hopping (CBH) model. The relaxation mechanism of the sample in the framework of electric modulus formalism is modelled by Davidson–Cole equation. The activation energy of the sample, calculated from both conductivity and modulus spectra is found to be~0.8 eV which indicates that the conduction mechanism for SEN is due to ion hopping. The scaling behavior of imaginary electric modulus suggests that the relaxation describes the same mechanism at various temperatures.

Micro-structure, optical properties and ac conductivity of rare earth double perovskite oxides: Sr2ErNbO6

Zero dimensional graphene quantum dots (GQDs) exhibit interesting physical and chemical properties due to the edge effect and quantum confinement. As the number of carbon atoms in edge is more than on basal plane, GQDs are more reactive. Room temperature XRD pattern confirms the formation of the GQDs. UV-Visible spectra confirm that GQDs show optical absorption in the visible region. The emission peaks in the photoluminescence spectra are red shifted with the increase of excitation wavelength. Dynamic light scattering (DLS) analysis shows that the average size of the particles is found to be ∼65 nm. The frequency dependent electrical transport properties of the GQDs are investigated in a temperature range from 300 to 500 K. Most interestingly, for the first time, the insulator to semiconductor transition of GQD is observed near 400K. The transition mechanism of GQD is discussed with detailed dielectric analysis. The effects of intercalated water on temperature dependent conductivity are clearly discussed. The dielectric relaxation mechanism is explained in the framework of permittivity, conductivity and impedance formalisms. The frequency dependent ac conductivity spectra follows the Jonscher’s universal power law. Cole-Cole model is used to investigate the dielectric relaxation mechanism in the sample.

Rajesh Mukherjee

Papers

Dielectric and Raman spectroscopic studies of A2ErSbO6 (A = Ba, Sr and Ca)

Dielectric Relaxation of Rare Earth Ordered Double Perovskite Oxide Ba 2 ErTaO 6

Structural and electrical transport properties of a rare earth double perovskite oxide: Ba2ErNbO6

Micro-structure, optical properties and ac conductivity of rare earth double perovskite oxides: Sr2ErNbO6

Insulator to semiconductor transition in graphene quantum dots