Oxygen permeability, stability and electrochemical behavior of \Pr _{2} {\text{NiO}}_{{4 + \delta }} -based materials

The high-temperature electronic and ionic transport properties, thermal expansion and stability of dense $ \Pr _{2} {\text{NiO}}_{{4 + \delta }} ,\Pr _{2} {\text{Ni}}_{{0.9}} {\text{Fe}}_{{0.1}} {\text{O}}_{{4 + \delta }} $ and $ \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $ ceramics have been appraised in comparison with K₂NiF₄-type lanthanum nickelate. Under oxidizing conditions, the extensive oxygen uptake at temperatures below 1073–1223 K leads to reversible decomposition of Pr₂NiO₄-based solid solutions into Ruddlesden–Popper type Pr₄Ni₃O₁₀ and praseodymium oxide phases. The substitution of nickel with copper decreases the oxygen content and phase transition temperature, whilst the incorporation of iron cations has opposite effects. Both types of doping tend to decrease stability in reducing atmospheres as estimated from the oxygen partial pressure dependencies of total conductivity and Seebeck coefficient. The steady-state oxygen permeability of $ \Pr _{2} {\text{NiO}}_{{4 + \delta }} $ ceramics at 1173–1223 K, limited by both surface-exchange kinetics and bulk ionic conduction, is similar to that of $ {\text{La}}_{2} {\text{NiO}}_{{4 + \delta }} $ . The phase transformation on cooling results in considerably higher electronic conductivity and oxygen permeation, but is associated also with significant volume changes revealed by dilatometry. At 973–1073 K, porous $ \Pr _{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $ electrodes deposited onto lanthanum gallate-based solid electrolyte exhibit lower anodic overpotentials compared to $ {\text{La}}_{2} {\text{Ni}}_{{0.8}} {\text{Cu}}_{{0.2}} {\text{O}}_{{4 + \delta }} $ , whilst cathodic reduction decreases their performance.     

Effects of the gas flow rate on the ionic conductivity of {\text{CaZr}}_{0.95} {\text{In}}_{0.05} {\text{O}}_{3 - \delta } ceramics

With coarse CaCO₃, nano ZrO₂ and In₂O₃ as raw materials, fine ${\text{CaZr}}_{0.95} {\text{In}}_{0.05} {\text{O}}_{3 - \delta } $ powders were synthesized at 1000°C by an optimized solid-state method. With the powders, ceramics with relative density as high as 98% were successfully fabricated at the temperature as low as 1400°C. The effects of gas flow rate on the conductivity of the ${\text{CaZr}}_{0.95} {\text{In}}_{0.05} {\text{O}}_{3 - \delta } $ ceramics under wet air conditions were first studied. The results showed that with the increase of temperature, the effects became more and more significant. In order to gain insight into the ion transfer mechanism of the electrolyte, the absorption and diffusion processes were analyzed. It was suggested that at lower temperature, the diffusion step was the rate-determining step. However, with the increase of the temperature, the adsorption process became the rate-determining step at lower flow rates.     

Filament-to-dielectric band alignments in $$\hbox {TiO}_{2}$$ and $$\hbox {HfO}_{2}$$HfO2 resistive RAMs

The next-generation nonvolatile memory storage may well be based on resistive random access memories (RRAMs). \(\hbox {TiO}_{2}\) and \(\hbox {HfO}_{2}\) have been widely used as the resistive switching layer for RRAM devices. However, the electronic properties of the filament-to-dielectric interfaces are still not well understood yet, compared to those of the electrodes and the dielectric. In this work, we study the electronic structures of three typical filament and dielectric structures, \(\hbox {Ti}_{4}\hbox {O}_{7}/\hbox {TiO}_{2}\), \(\hbox {Hf}_{2}\hbox {O}_{3}/\hbox {HfO}_{2}\) and \(\hbox {Hf}/\hbox {HfO}_{2}\), using ab initio calculations. We implement the GGA-1/2 method, which rectifies the band gaps of GGA through self-energy correction. Our calculation predicts an ohmic contact for the \(\hbox {Ti}_{4}\hbox {O}_{7}/\hbox {TiO}_{2}\) interface, where the defective \(\hbox {Ti}_{4}\hbox {O}_{7}\) phase was experimentally identified as the filament composition in \(\hbox {TiO}_{2}\). However, there is a finite Schottky barrier existing in either \(\hbox {Hf}_{2}\hbox {O}_{3}/\hbox {HfO}_{2}\) interface (1.96 eV) or \(\hbox {Hf}/\hbox {HfO}_{2}\) interface (0.61 eV), the two probable filament–dielectric configurations in hafnia-based RRAM. Our results suggest that the distinct filament-to-dielectric band alignments in \(\hbox {TiO}_{x}\) and \(\hbox {HfO}_{x}\) systems account for the much larger resistance window for the latter.     

Superhigh frequency magnetic properties and electromagnetic attenuation applications for \mathrm { BaMn_{x}Ti_{x}Fe_{12-2x}O_{19}} composites

Superhigh frequency microwave magnetic properties and attenuation characteristics at K and Ka bands have been studied for M-type barium hexaferrite $\mathrm {BaMn_{x}Ti_{x}Fe_{12-2x}O_{19}}$ ( $x=0.4-1.0$ ) composites with c-axis anisotropy. The complex permeability of the composites has resonancelike dispersion with relatively large imaginary permeability $\mu ''_{\max }$ of 1.1 and high resonance frequency $f_{R}$ of 20–28 GHz. $f_{R}$ is determined by the anisotropy fields $H_{a}$ . The composites show good attenuation properties with percentage bandwidth of 30–40 % and small thickness of 0.08–0.1 cm at K and Ka bands, thus being potential candidates for electromagnetic attenuation applications.     

First-principle calculations of electronic and ferromagnetic properties of $$\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}$$

We have used the first-principle calculations of density functional theory within full-potential linearized augmented plane-wave method to investigate the electronic and ferromagnetic properties of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\) alloys. The electronic structures of \(\hbox {Al}_{0.25}\hbox {V}_{0.75}\hbox {Sb}, \hbox {Al}_{0.5}\hbox {V}_{0.5}\hbox {Sb}\) and \(\hbox {Al}_{0.75}\hbox {V}_{0.25}\hbox {Sb}\) exhibit a half-metallic ferromagnetic character with spin polarization of 100 %. The total magnetic moment per V atom for each compound is integral Bohr magneton of 2 \(\mu _{\mathrm{B}}\), confirming the half-metallic feature of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\). Therefore, these materials are half-metallic ferromagnets useful for possible spintronics applications.     

Effect of annealing temperature on the electrostrictive properties of 0.94(Na0.5Bi0.5)TiO3-0.06BaTiO3 thin films

0.94(Na_0.5Bi_0.5)TiO₃-0.06BaTiO₃ (NBT-BT6) thin films were fabricated by metal-organic decomposition (MOD) at the different annealing temperatures. Based on the electrostrictive effect and converse piezoelectric effect, the phenomenological approach is provided to characterize the electrostrictive properties of the perovskite relaxor, and it is used to determine the effective electrostriction coefficients $ Q_{33}^{\mathrm{eff}} $ and electrostrictive strains $ {S_3} $ of NBT-BT6 thin films annealed at the range of 650?C800?°C. After the microstructure, ferroelectric, dielectric and piezoelectric properties of the thin films were determined, the maximum values of $ Q_{33}^{\mathrm{eff}} $ and $ {S_3} $ of NBT-BT6 thin film annealed at 750?°C are respectively determined as 0.0289?m⁴/C² and 0.26?% under the bipolar driving field of 391?kV/cm. They are strongly influenced by annealing temperature due to the bismuth evaporation and crystallization of perovskite phase, and the enhanced electrostrictive properties could make NBT-based thin film a promising candidate to the design and application of stacked actuators, microangle-adjusting devices, and oil pressure servo valves.     

Analog and RF performance of doping-less tunnel FETs with $$\hbox {Si}_{0.55} \hbox {Ge}_{0.45}$$ source

This paper reports studies of a doping-less tunnel field-effect transistor (TFET) with a \(\hbox {Si}_{0.55} \hbox {Ge}_{0.45}\) source structure aimed at improving the performance of charge-plasma-based doping-less TFETs. The proposed device achieves an improved ON-state current (\(I_{{\mathrm{ON}}} \sim {4.88} \times {10}^{-5}\,{\mathrm{A}}/\upmu {\mathrm{m}}\)), an \(I_\mathrm{ON}/I_\mathrm{OFF}\) ratio of \({6.91} \times {10}^{12}\), an average subthreshold slope (\(\hbox {AV-SS}\)) of \(\sim \) \({64.79}\,{\mathrm{mV/dec}}\), and a point subthreshold slope (SS) of 14.95 mV/dec. This paper compares the analog and radio of frequency (RF) parameters of this device with those of a conventional doping-less TFET (DLTFET), including the transconductance (\(g_{{\mathrm{m}}}\)), transconductance-to-drain-current ratio \((g_\mathrm{m}/I_\mathrm{D})\), output conductance \((g_\mathrm{d})\), intrinsic gain (\(A_{{\mathrm{V}}}\)), early voltage (\(V_{{\mathrm{EA}}}\)), total gate capacitance (\( C_{{\mathrm{gg}}}\)), and unity-gain frequency (\(f_{{\mathrm{T}}}\)). Based on the simulated results, the \(\hbox {Si}_{0.55}\hbox {Ge}_{0.45}\)-source DLTFET is found to offer superior analog as well as RF performance.     

Numerical analysis of transmission coefficient,LDOS, and DOS in superlattice nanostructures of cubic $$\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}$$ resonant tunneling MODFETs

Numerical analysis of the transmission coefficient, local density of states, and density of states in superlattice nanostructures of cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) resonant tunneling modulation-doped field-effect transistors (MODFETs) using \(\hbox {next}{} \mathbf{nano}^{3}\) software and the contact block reduction method is presented. This method is a variant of non-equilibrium Green’s function formalism, which has been integrated into the \(\hbox {next}\mathbf{nano}^{3}\) software package. Using this formalism in order to model any quantum devices and estimate their charge profiles by computing transmission coefficient, local density of states (LDOS) and density of states (DOS). This formalism can also be used to describe the quantum transport limit in ballistic devices very efficiently. In particular, we investigated the influences of the aluminum mole fraction and the thickness and width of the cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N}\) on the transmission coefficient. The results of this work show that, for narrow width of 5 nm and low Al mole fraction of \(x = 20\,\%\) of barrier layers, cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) superlattice nanostructures with very high density of states of 407 \(\hbox {eV}^{-1}\) at the resonance energy are preferred to achieve the maximum transmission coefficient. We also calculated the local density of states of superlattice nanostructures of cubic \(\hbox {Al}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) to resolve the apparent contradiction between the structure and manufacturability of new-generation resonant tunneling MODFET devices for terahertz and high-power applications.     

Structural,electrical and optical properties of $$\hbox {InGaZnO}_{4}$$ and $$\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}$$In29Sn3O48: a first-principles study

First-principles calculations were performed to investigate the electrical and optical properties of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) with Sn-doped \(\hbox {In}_{2}\hbox {O}_{3}\) and \(\hbox {InGaZnO}_{4}\) (IGZO). The band structure, density of states, optical properties including dielectric function, loss function, reflectivity and absorption coefficient are calculated. The calculated total energy shows that the most stable crystal structures are type III for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and type II for \(\hbox {InGaZnO}_{4}\). The band structure indicates the both \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) are direct gap semiconductors. The intrinsic band gap of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) is much narrower than that of \(\hbox {InGaZnO}_{4}\), and results in a better electrical conductivity for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\). The density of states shows the main hybridization occurring between In-4d and O-2p states for \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) while between In-4d In-5p, Zn-4s and O-2p states for \(\hbox {InGaZnO}_{4}\) near the valence band maximum. The reflectivity index \(R({\omega })\) shows that the peak value of \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) appears only in the ultraviolet range, indicating that these two materials have all excellent transparency. In addition, the absorption coefficient \({\alpha }({\omega })\) of both \(\hbox {In}_{29}\hbox {Sn}_{3}\hbox {O}_{48}\) and \(\hbox {InGaZnO}_{4}\) is high in the ultraviolet frequency range, and therefore they show, a high UV absorption rate.     

Frequency-temperature compensation mechanism for bismuth based dielectric/PTFE microwave composites

The dielectric property and thermal expansion property of Bi₂O₃-ZnO-Nb₂O₃-based (BZN) ceramics filler reinforced composites have been investigated as a function of temperature range from ?50 to 175 °C. The composites with adjustable temperature coefficient of frequency (τ _f ) and dielectric temperature coefficient ( $ \alpha _{\varepsilon } $ ) are achieved by filling the ceramic filler with different $ \alpha _{\varepsilon } $ into polymer matrix. A series of polytetrafluoroethylene (PTFE) based composites blended with different amount of ceramic filler with different $ \alpha _{\varepsilon } $ have been studied in this paper. The results indicated that with the amount of ceramic filler increasing, both of the relative permittivity and dissipation factor of composites increased. Composite filled with positive $ \alpha _{\varepsilon } $ (245 ppm/°C) BZN ceramic filler (40 vol.%) has low $ \alpha _{\varepsilon } $ (22 ppm/°C), while filled with near-zero $ \alpha _{\varepsilon } $ (10 ppm/°C) BZN ceramic filler (40 vol.%) have low τ _f (?5 ppm/°C).     

Atomistic tight-binding simulations of quaternary-alloyed $$\hbox {Zn}_{x}\hbox {Cd}_{1-x}\hbox {S}_{y}\hbox {Se}_{1-y}$$ nanocrystals

Advancement of alloyed nanocrystals with attractive structural and optical properties for use in a wide range of physical, chemical, and biological applications represents a growing research field. Employing atomistic tight-binding theory combined with the virtual crystal approximation, the electronic structure and optical properties of quaternary-alloyed \(\hbox {Zn}_{{x}}\hbox {Cd}_{1-{x}} \hbox {S}_{{y}}\hbox {Se}_{1-{y}}\) nanocrystals with experimentally synthesized compositions (x and y) and sizes were investigated. Analysis of the results shows that the physical properties are mainly sensitive to the concentrations (x and y) and the diameter. With decreasing x and y contents, the optical bandgap is reduced because the contributions of the materials with narrower bulk bandgap (ZnSe and CdSe) is mostly promoted. The optical bandgap is reduced with increasing diameter due to the quantum confinement effect. The optical bandgap calculated based on tight-binding calculations shows discrepancy of less than 0.4 eV from experiment. Most importantly, the optical emission is continuously tunable across the entire visible spectrum. The conduction and valence bands are predominantly contributed by cation and anion atoms, respectively. The optical properties are obviously improved in Cd- and Se-rich quaternary \(\hbox {Zn}_{{x}}\hbox {Cd}_{1-{x}} \hbox {S}_{{y}}\hbox {Se}_{1-{y}}\) nanocrystals with large diameter. The atomistic electron–hole interactions can be hybrid-engineered by tuning either the contents (x and y) or diameter. The Stokes shift becomes more pronounced with decreasing alloy concentrations (x and y) and diameter, as described by the trend of the atomistic electron–hole exchange interaction. The present systematic study provides a new avenue to understand the unique size- and composition-dependent structural and optical properties of quaternary-alloyed \(\hbox {Zn}_{{x}}\hbox {Cd}_{1-{x}} \hbox {S}_{{y}}\hbox {Se}_{1-{y}}\) nanocrystals for broad use in multicolor bioimaging, biosensing, light-emitting diodes, solar cells, and other nanodevice applications.     

Theoretical insights and experimental characterization of $$\hbox {HfO}_2$$-based OxRRAMs operation

The intensive research in resistive random access memories (RRAM) field has brought in significant improvements in the performance, optimization and reliability of the devices as well as more understanding on their operation. This was made possible through the combination of different tools starting from material engineering to device characterization, modeling and simulations. In this review, we bring an overview of our recent work on RRAM through experimental characterization and first-principles calculations. We explore the effects of metal electrodes on the switching performance and conductive filament (CF) stability of \(\hbox {HfO}_2\) oxide-based RRAM (OxRRAM). With the insight gained from the experimental data, we employ first-principles calculations to have a better microscopic understanding on OxRRAM operation. We show that CF stability and device operating voltages strongly depend on the electrode material. Ti being an electrode material of high interest, we investigate the type of \(\hbox {Ti/HfO}_2\) interface that may be formed and propose a probable composition. We also study the formation and migration of extended Frenkel-pair (EFP) defect in \(\hbox {HfO}_2\) which we consider to be the prototype defect responsible for OxRRAM degradation leading to CF formation. This EFP emission occurs through a cascading migration of O atoms inside \(\hbox {HfO}_2\) lattice. Based on EFP formation and diffusion, we present a simplified CF formation model. Finally, we study low resistance data retention failure in OxRRAM through \(\hbox {HfO}_2\), \(\hbox {Hf}_{1x}\hbox {Al}_{2x}\hbox {O}_{2+x}\) (HfAlO) and \(\hbox {Hf}_{1-x}\hbox {Ti}_{x}\hbox {O}_{2}\) (HfTiO) type of cells. We link its origin to the lateral diffusion of oxygen vacancies at the constriction/tip of the conductive filament in \(\hbox {HfO}_2\)-based RRAM.     

First-principles investigation on transport properties of $$\hbox {Zn}_{2}\hbox {SnO}_{4}$$ molecular device and response toward $$\hbox {NO}_{2}$$NO2 gas molecules

The transport properties of a \(\hbox {Zn}_{2}\hbox {SnO}_{4}\) device along with adsorption properties of \(\hbox {NO}_{2}\) gas molecules on \(\hbox {Zn}_{2}\hbox {SnO}_{4}\) (ZTO) molecular devices are investigated with density functional theory using the non-equilibrium Green’s function technique. The transmission spectrum and device density of states spectrum confirm the changes in HOMO–LUMO energy level due to transfer of electrons between the ZTO-based material and the \(\hbox {NO}_{2}\) molecules. I–V characteristics demonstrate the variation in the current upon adsorption of \(\hbox {NO}_{2}\) gas molecules on the ZTO device. The findings of the present study clearly suggest that ZTO molecular devices can be used to detect \(\hbox {NO}_{2}\) gas molecules in the trace level.     

Comparative study on interfacial properties of Mo,Nb, and W contacts with monolayer $$\hbox {MoS}_{2}$$

In this work, we make a comparative study on the interfacial properties of top contact for Mo, Nb, and W metals with monolayer \(\hbox {MoS}_{2 }\,(\hbox {mMoS}_{2})\) by employing first-principles based on density functional theory (DFT) calculations. We evaluate the heights of Schottky barrier (SBH) and orbital overlap of the three models by carefully observing band structure and the density of the states relative to the Fermi level. Also, the tunnel barriers and electron densities of the three systems are analyzed. In accordance with the DFT simulations, \(\hbox {mMoS}_{2}\) forms an n-type Schottky contact with Mo, Nb, and W electrodes with electron SBH of 0.28, 0, and 0.6 eV, respectively. Besides, \(\hbox {Nb-mMoS}_{2}\) contact exhibits higher average electron density and lower tunneling barriers, demonstrating that Nb can form a better top contact with \(\hbox {mMoS}_{2}\) and should have prior electron injection efficiency and backgated regulation of current compared to the \(\hbox {mMoS}_{2}\) contacts with Mo and W.     

DFT studies of electronic structure and dielectric properties in layered perovskite $$\hbox {LaSrAlO}_{4}$$

The structural, electronic, dielectric and optical properties of tetragonal \(\hbox {LaSrAlO}_{4}\) are studied in detail using density functional theory calculations. The energy band structures and density of states are predicted by generalized gradient approximation (GGA) and local density approximation (LDA) respectively. The fundamental band gaps of \(\hbox {LaSrAlO}_{4}\) are all indirect by GGA (2.860 eV) and LDA (2.863 eV) calculations. The complex dielectric function was calculated. There are two peaks in the real part \(\varepsilon _{1}(\omega )\) and three peaks in the imaginary part \(\varepsilon _{2}(\omega )\). The optical spectra are assigned to the interband transition from O valence to La and Sr conduction bands in the low energy region. In addition, the electron energy-loss spectrum, optical conductivity, reflectivity spectrum, and refractive index, are given to support the potential applications for microwave dielectric ceramics.     

Investigation on the structural,elastic, electronic,and magnetic properties of half-metallic $$\hbox {Co}_{2}\hbox {MnSi}$$ and CoMnIrSi via first-principles calculations

We investigated the structural, elastic, electronic, and magnetic properties of \(\hbox {Co}_{2}\hbox {MnSi}\) and CoMnIrSi full-Heusler compounds by means of density functional theory based on the full-potential linearized augmented plane wave (FP-LAPW) approach. The generalized gradient approximation as proposed by Wu and Cohen (GGA-WC) was employed to treat the exchange-correlation effect. The results show that both alloys are structurally and mechanically stable. \(\hbox {Co}_{2}\hbox {MnSi}\) is almost elastically isotropic, while CoMnIrSi is anisotropic, and both alloys are ductile. The studied compounds have perfect spin polarization of 100 %, with down-spin bandgap of 0.796 eV and 0.728 eV, respectively. The calculated magnetic properties indicate that the Slater–Pauling rule is satisfied in both cases. Finally, the effect of strain on the half-metallic properties of \(\hbox {Co}_{2}\hbox {MnSi}\) and CoMnIrSi was also investigated by varying the lattice constant over a wide range.     

Transport studies of $$\hbox {CeO}_{2}$$ molecular device and adsorption behavior of CO on $$\hbox {CeO}_{2}$$ device: a first-principles investigation

Using density functional theory and the non-equilibrium Green’s function formalism, the transport and CO adsorption properties of \(\hbox {CeO}_{2}\) molecular device are studied. The band structure shows that \(\hbox {CeO}_{2}\) nanostructure exhibits semiconducting nature. The electron density is found to be more in oxygen sites rather than in cerium sites along \(\hbox {CeO}_{2}\) nanostructure. The density of states spectrum shows the variation in density of charge upon adsorption of CO on CeO\(_2\) device. The transmission spectrum provides the insights on the transition of charge in \(\hbox {CeO}_{2}\) molecular device upon adsorption of CO along the scattering region. I–V characteristics confirm the adsorption of CO with the variation of current along \(\hbox {CeO}_{2}\) molecular device. The findings show that \(\hbox {CeO}_{2}\) two probe molecular device can be efficiently used for CO detection in the atmosphere.     

Structural,electronic, and optical properties of C-type $$\hbox {Gd}_{2}\hbox {O}_{3}$$: a density functional theory investigation

Recently, \(\hbox {Gd}_{2}\hbox {O}_{3}\) has gained considerable interest in industry, and its optical applications have been of interest in optoelectronic. The band structure and optical properties of cubic \(\hbox {Gd}_{2}\hbox {O}_{3}\) are investigated using the density functional theory framework. Calculations are performed within the local density approximation and generalized gradient approximation, adding the empirical Hubbard potential U. Calculation of the electronic band structure indicates a direct \({\Gamma }\) band gap. Further, the total and partial densities of states were presented, and the contribution of different orbitals is analyzed. Moreover, the behavior of optical spectra such as real and imaginary part of dielectric function, refractive index, extinction coefficient, optical conductivity, and electron energy-loss function is analyzed. There is a good agreement between the computed results and reported experimental data.     

Electrical properties of rutile-type relaxor ferroelectric-like Fe0.9W0.05TiMO6 (M = Ta,Nb) ceramics

Electrical properties of rutile-type $\mathrm {Fe}_{0.9}{\kern 1pt} \mathrm {W}_{0.05}\\\mathrm {TiMO}_{6} (\mathrm {M} {} = {} \mathrm {Ta,Nb})$ ceramics were measured at and above room temperature and the results are compared with those gained previously on rutile-type relaxor ferroelectrics $\mathrm {FeTiMO}_{6} (\mathrm {M} {} = {} \mathrm {Ta,Nb})$ . The aliovalent ${\rm W}^{6+}$ cationsin the current compounds might change the suggested polar nanodomains, giving rise to very high dielectric constant $\epsilon (\omega )$ , and further electrical quantities can possibly shed additional light on the nature of the mechanism leading to extraordinary values in $\epsilon (\omega )$ . In part similar electrical data were established such as very high $\epsilon (\omega )$ but also different results were noted. Apart from $\epsilon (\omega )$ , the electrical response was analysed by measuring losses, dissipation factor $\tan \delta $ , DC conductivity $\sigma _{DC}$ and AC conductivity $\sigma _{AC}(\omega )$ using impedance spectroscopy, and thermopower; the results are discussed in conjunction with literature data. The role of grain boundaries and sample-electrode processes was investigated in particular with respect to the sample capacitance. Eventual microstructural local inhomogeneities were checked by means of ⁵⁷Fe Mössbauer spectroscopy. For both compounds, the temperature dependence of bulk $\sigma _{DC}$ showed Arrhenius behaviour with activation energy ${E_{A}}\sim $ 0.35 eV and $\sigma _{DC}$ (295 K) $\sim 5\times 10^{-5} \Omega ^{-1}\text{cm}^{-1}$ ; grain boundaries exhibited slightly higher ${E_{A}}$ but the value of $\sigma _{DC}$ was a factor of up to $\sim 10$ lower at all temperatures. From $\sigma _{AC}(\omega )$ data, a power law frequency dependence of grain boundary conductivity was derived. Relaxation processes were established from loss and $\tan \delta $ data. The thermopower is negative and varies weakly with temperature, pointing to long-range charge transfer by a hopping-type mechanism of electron polarons.     

Improvement in microwave dielectric properties of Sr<Subscript>2</Subscript>TiO<Subscript>4</Subscript> ceramics through post-annealing treatment

Sr₂TiO₄ ceramics were synthesized via the conventional solid-state reaction process, and the effects of post-annealing treatment in air on the microwave dielectric properties and defect behavior of title compound were investigated systematically. The Q?×?f values could be effectively improved from 107,000 GHz to 120,300 GHz for the specimens treated at 1450 °C for 16 h. The thermally stimulated depolarization currents (TSDC) revealed two kinds of defect dipoles [\( \left({\mathrm{Ti}}_{\mathrm{Ti}}^{\hbox{'}}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \) and \( \left({V}_{\mathrm{Sr}}^{"}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \)] and oxygen vacancies \( \left({V}_{\mathrm{O}}^{\bullet \bullet}\right) \) were considered the main defects in Sr₂TiO₄. Under a post-annealing treatment in air, the concentrations of such defects in the ceramics decreased. Meanwhile, the impedance spectrum revealed the activation energy of the grain boundaries increased. These evidences could account for the improvement of Q?×?f values. Accompanied with a high ε_r of 40.4 and a large τ_f of 126 ppm/°C, the enhanced high-Q Sr₂TiO₄ ceramics can be good candidates for applications in wireless passive temperature sensing.