Nonstoichiometric Defects Research Articles

Assessment of uranium nitride interatomic potentials

Uranium mononitride (UN) is a promising nuclear fuel due to its high fissile density, high thermal conductivity, and suitability for reprocessing. In this study, two uranium nitride interatomic potentials are assessed: Tseplyaev and Starikov's angular-dependent potential and Kocevski et al.'s embedded atom model potential. Predictions of the thermophysical and elastic properties of UN, ▪, and α- and β-▪ computed using both potentials are assessed and compared to available experimental data. The Tseplyaev potential performs better with the energetic aspects of UN, e.g., specific heat capacity and point defect formation energies, whereas the Kocevski potential performs better with the structural aspects of UN, e.g., thermal expansion as well as with the elastic properties. The reasons why the Kocevski potential underestimates the UN specific heat are explained by examining the UN phonon properties modeled using both potentials. The Kocevski potential shows better identification of the mechanical stability ranges of UN, ▪, and α- and β-▪, reasonably predicting the melting point of UN and predicting stable structures for ▪ and α- and β-▪. On the other hand, the Tseplyaev potential predicts a premature phase change of both UN and ▪ and cannot stabilize α- nor β-▪. However, the Kocevski potential cannot predict a stable α-U phase and is thus not suitable for the calculation of formation energies for non-stoichiometric point defects.

A first principles investigation of defect energetics and diffusion in actinide dioxides

In this work, we have studied the formation and migration of various neutral intrinsic defects in AnO2. The non-stoichiometric defects such as vacancy defect, interstitial defect, and antisite defect as well as stoichiometric defects such as Schottky defect and Frenkel pairs has been considered for investigation. The formation energies of point defects are much higher in ThO2 as compared to UO2 and PuO2. The binding energy of defect clusters indicate that formation of Frenkel pair and Schottky defect is favorable over individual defects in ThO2. In case of UO2 and PuO2, hyper-stoichiometric defects are more stable than hypo-stoichiometric defects. The results obtained in this work are in good agreement with experimental results.

Structure and magnetic properties of epitaxial Sr-LaMnO3 thin films obtained by polymer assisted deposition

Epitaxial La1-xSrxMnO3 (LSMO, where x = 0, 0.1, 0.3 and 0.5) films on single crystal SrTiO3 (001) substrate were obtained by water-based chemical solution deposition method, so-called polymer assisted deposition (PAD). The as-prepared films (heated at 750?C for 1 h) have thicknesses of ?30 nm, high uniformity, clear and well-defined interface and crack-free surfaces. In addition, they are characterized by the formation of an imperfect crystal structure with some disoriented areas, small amount of non-stoichiometric phase and defects created just to support the epitaxial film growth. During multiple annealing at different temperatures up to 900?C epitaxial nature was preserved in all LSMO films. The structure rearrangement through elimination of defects and formation of unit cell closer to the corresponding bulk stoichiometric phase were observed. The magnetic properties of the LSMO thin films were measured using SQUID magnetometer in the temperature range of 5-400K with the field applied parallel (in-plane) and perpendicular (out-of-plane) to the film surface. The angle dependence of the magnetic moment in the LSMO thin films at the room temperature was also measured by a vibrating sample magnetometer.

Ionic conductivity of Li3TaO4 depending on polymorphism and non-stoichiometric defects

Ionic conductivity of complex rock-salt Li3TaO4 was studied using low-temperature (LT; disordered) and high-temperature (HT; ordered) polymorphs modified with Li/Ta non-stoichiometry. For each of LT and HT derivatives, samples were prepared with four different nominal compositions of Li3.05Ta0.99O4, Li3TaO4, Li2.99Ta1.002O4, and Li2.98Ta1.004O4. The synchrotron X-ray diffraction analysis showed that the ordered HT phases have larger lattice volume than the disordered LT phases by ≈0.5%. Among both LT and HT sample groups, the lattice volume was smallest in the stoichiometric phases Li3TaO4. The X-ray diffraction line broadening analysis implied that the disordered LT phases contain local distortions that resemble the ordered HT phases. The solid-state magic-angle-spinning 7Li nuclear magnetic resonance spectroscopy revealed that LT phases have the larger LiO6 octahedra and the shorter Li–Li distances than HT phases. According to the ac impedance measurement and equivalent circuit analysis, the disordered LT phases had higher ionic conductivities (σi) than the ordered HT phases, by more than two orders of magnitudes. Particularly, the σi of Li2.98Ta1.004O4 (at ≈390 °C) was evaluated to be 10−2.4 S/cm and 10−4.8 S/cm for LT and HT phases, respectively. Meanwhile, the Li/Ta non-stoichiometry contributed to increase the σi of Li3TaO4, by more than an order.

Defect characteristics of cadmium oxide nanocrystallites synthesized via a chemical precipitation method

In this study, we synthesized cadmium oxide nanocrystallites measuring 16 nm–30 nm via a chemical precipitation method at synthesis temperatures ranging from 400 °C to 800 °C. X-ray diffraction analysis identified well-defined peaks with no indication of mixed or impurity phases. The samples were also characterized by transmission electron microscopy. The band gap energies measured by optical absorption varied between 2.32 eV and 2.38 eV, thereby indicating quantum confinement effects over the entire range of nanocrystallite sizes. Fourier transform-infrared spectroscopic measurements demonstrated the localized surface plasmonic resonance behavior of the samples. Positron annihilation studies were conducted to identify and quantify the vacancy type defects, sizes, and concentrations. Increased annihilation of positrons occurred on the surfaces of nanocrystallites with dimensions less than the thermal diffusion length of positrons (∼25.4 nm). In crystallites larger than this limit, positrons were trapped in the non-stoichiometric defects within the crystallites, as demonstrated by both the positron lifetime and coincidence Doppler broadening measurements. Defects played a major role in determining the favorable properties of these cadmium oxide nanocrystallites because defects and parameters such as the synthesis temperature were strongly correlated.

Enhanced zT due to non-stoichiometric induced defects for bismuth telluride thermoelectric materials

Thermoelectric materials have potential applications in energy conversion and waste heat recovery. Thermoelectric materials with non-stoichiometric composition may exhibit improved thermoelectric properties compared to the stoichiometric composition material. Bi2Te3 is one of the best-known thermoelectric materials having optimum performance near room temperature. In the present study, non-stoichiometric bismuth telluride (Bi2±x Te3), x = (0, 0.01, 0.02, 0.03), was synthesized using the co-precipitate method to create imperfections and strains in the crystal structure of the material. The crystal structure of the synthesized bismuth telluride revealed significant changes in the c direction when examined using X-ray diffraction (XRD). Scanning electron microscopy (SEM) revealed the formation of micro-particles. Energy dispersive spectroscopy (EDX) also confirmed the non-stoichiometric elemental composition of Bi and Te rich bismuth telluride. Thermogravimetric analysis (TGA) was performed in a non-inert environment, and an oxidation temperature of 685 ​K was recorded in non-stoichiometric samples. The post-oxidation of stoichiometric and non-stoichiometric Bi Rich and Te Rich bismuth tellurides was also confirmed by XRD and TGA comparison. The thermal conductivity was reduced by the imperfections and defects for all non-stoichiometric compositions, and a zT value of 0.54 at 420 ​K was obtained in the Tellurium Rich bismuth telluride, which is about a 12% improvement compared to the pure sample.

Controlling the carrier and phonon transport behavior of SnSe via stoichiometric adjustment

In this paper, we designed and fabricated SnSe1+x by vacuum melting with spark plasma sintering, and explored effect of stoichiometric change on energy band structure, bond anharmonicity and thermoelectric transport behavior under same process condition. Se overdose results in increase of electronical conductivity due to narrowing bandgap. Sn overdose causes obviously attenuated thermoelectric properties since the Fermi level moves up into the conduction band. The maximum powder factor value of 528 μWm−1K−2 for the SnSe1.01 sample at 773K results from energy gap reduction and steep density of states on valence band top. Non-stoichiometric defect causes enhancement of thermal conductivity due to evolution of bond characteristics between interlayers. The maximum ZT of the SnSe1.01 sample achieves 0.87 at 773K along “⊥” direction via stoichiometric adjustment.

Modification of the Ni-NiO-oxidant system by thermo-chemical oxidation with accelerated growth of the p-type dielectric oxide layer, by the action of substituted V2O5 on the system

Nickel is covered with very thin oxide film and is stable against corrosion under normal conditions. Nickel, as Ni-NiO-oxidant system, can make much thicker (even a thousand times) and more corrosion unstable oxides. With certain new oxidation process parameters, nickel can undergo so-called accelerated oxidation. The subject and goal of this work is to find the most favorable conditions for performing of accelerated - catastrophic Ni oxidation process, when substitution of V2O5 is carried out into NiO (2V (Ni)), with significant electron concentration increase and oxygen surplus. Unlimited main and secondary charge carriers transfer is significantly facilitated if oxide film with non-stoichiometric composition and defects is formed. The aim is to avoid compact oxide film formation by chemical-thermal treatments and to obtain porous and defective NiO. By using oxidized system Ni - NiO - oxidant in glass, ceramics and evaporated metal technology, extraordinary usable value of this work is ensured.

Enhancing the thermoelectric performance of Ag2Se by non-stoichiometric defects

Ag2Se is an attractive candidate for room temperature thermoelectric applications because of its extremely low thermal conductivity, large power factor, and excellent mechanical properties. However, its performance is difficult to be improved effectively by doping other atoms. The traditional synthesis processes have the disadvantages of low yield, complex steps, and long period. In this study, the Ag2Se1+x samples were rapidly synthesized by the melting method via changing the ratio of Ag and Se. The carrier concentration and mobility of Ag2Se are optimized by adding excess Se, which could regulate non-stoichiometric defects in the process of synthesis. A high Seebeck coefficient of ∼136.0 μV K−1, a large power factor of ∼3000.0 μW m−1 K−2, and the maximum figure of merit (ZT) of ∼1.02 of the Ag2Se1.015 sample have been achieved at 375 K. The ZT value is 2.3 times than that of the pristine sample (Ag2Se). The mechanical properties of the molting sample are comparable to state-of-the-art thermoelectric materials. Finally, the highest ZT value of the material is predicted by factor B. This work provides an idea for further optimizing Ag2Se-based and other non-stoichiometric thermoelectric semiconductors by a slight excess of Se. Moreover, the fast synthesis technology can significantly save time and energy, which manifests great advantages to the wholesale manufacturing industry.

Effects of temperature and oxygen partial pressure on electrical conductivity of Fe-doped β-Ga2O3 single crystals

In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2¯01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance.

Simultaneously achieving high performance of energy storage and transparency via A-site non-stoichiometric defect engineering in KNN-based ceramics

Ceramic-based transparent dielectric materials are regarded as the best candidates for advanced energy storage and conversion materials because of their outstanding optical and electric properties. Nevertheless, due to the presence of low density, low band gap energy and large grain size, it is difficult to simultaneously obtain high energy storage density and high optical transmittance in lead-free based ceramics, which limiting their further development of practical applications. In this work, the relaxor ferroelectric ceramics of x mol% Er3+-doped 0.91 K0.5Na0.5NbO3-0.09BamSrnTiO3 (xEr-SrmBan) were constructed via A-site non-stoichiometric defect engineering. It is worth noting that excessive addition of Sr and Ba, especially Sr, can significantly refine the grain size and domain size on account of vacancy-related defect pinning. Finally, high energy storage density (W = 6.39 J/cm3, Wrec = 3.42 J/cm3) together with high optical transmittance (∼72% at 900 nm) can be achieved simultaneously in 0.25Er-Sr1Ba0.5 due to the exist of dense structure, ultrafine grain size (<100 nm), small-sized nano-microdomains (i.e., PNRs, < 50 nm) and large bandgap energy (∼3.10 eV). Furthermore, a fast discharge time of 42 ns can also be realized. The above results indicate that the present study helps to promote the development of advanced transparent energy storage and conversion materials.

Reactive-Vacancy Mechanism of Pore Growth in Oxide Films

It is known that deviation in the stoichiometry of the composition of chemical compounds is associated with nonstoichiometric point defects, namely vacancies and interstitial ions. Until recently, only the behavior of point defects in the bulk of a solid was considered, while the question of the appearance of point defects in chemical compounds, and where and how they appear, remained uninvestigated. Previously, we showed that nonstoichiometric point defects arise exclusively on the surface of a solid, and the mechanisms of their occurrence were given. However, phase boundaries can also appear inside a solid, and this is due to the presence of pores inside them. Here we propose that a reactive-vacancy mechanism for pore formation and growth in chemical compounds is based on a quasi-chemical reaction that occurs on the pore surface between nonstoichiometric point defects; vacancies in one sublattice and interstitial ions in the other. This reaction leads to the formation of vacancies in the second sublattice and, together with vacancies from the first sublattice, to the growth of pores.

Time-Domain Observation of Spectral Diffusion in Defective ZnO.

Defects can affect all aspects of materials by altering their electronic structures and mediating the carrier dynamics. However, in the past decades, most research efforts were restricted to nonstoichiometric defects, while the effects of high-density defects on the carrier dynamics of semiconductors remained elusive. In this work, using transient absorption spectroscopy, we have observed for the first time a hybrid carrier relaxation dynamics with the feature of a Poisson-like retard shoulder in a time-domain profile in highly defective ZnO crystals. This novel behavior has been attributed to the spectral diffusion within continuum defect states, which is further confirmed by a proposed diffusion (in energy space) controlled carrier dynamic model. Our results thus reveal an alternative energy decay channel in highly defective crystals and may provide a new route for defect engineering.

On the pinning force in high density MgB2 samples

An analysis of the field dependence of the pinning force in different, high density sintered samples of MgB2 is presented. The samples were chosen to be representative for pure MgB2, MgB2 with additives, and partially oriented massive samples. In some cases, the curves of pinning force versus magnetic field of the selected samples present peculiar profiles and application of the typical scaling procedures fails. Based on the percolation model, we show that most features of the field dependence of the critical force that generate dissipation comply with the Dew-Hughes scaling law predictions within the grain boundary pinning mechanism if a connecting factor related to the superconducting connection of the grains is used. The field dependence of the connecting function, which is dependent on the superconducting anisotropy, is the main factor that controls the boundary between dissipative and non-dissipative current transport in high magnetic field. Experimental data indicate that the connecting function is also dependent on the particular properties (e.g., the presence of slightly non-stoichiometric phases, defects, homogeneity, and others) of each sample and it has the form of a single or double peaked function in all investigated samples.

Study on Preparation and Ns-Laser Damage of HfO&amp;lt;sub&amp;gt;2 &amp;lt;/sub&amp;gt;Single Layers

The effects of the main parameters of argon flux, oxygen flux and beam voltage on the surface morphology, transmittance spectrum and laser damage of the HfO2 single layers prepared by ion beam sputtering are studied. The HfO2 amorphous single layers have porous surface morphologies. Different processes will cause differences in coatings absorption and surface morphology, which in turn will cause changes in the spectral transmittance curve. The ion beam sputtering HfO2 single layers have high content of argon (4.5% - 8%). The laser damage of HfO2 single layers is related to argon inclusions and non-stoichiometric defects. The changes of argon flux and beam voltage have a greater impact on argon content and O/Hf ratio. When the argon content in the coatings is lower and the O/Hf ratio is higher, the laser damage thresholds of the HfO2 single layers are higher.

Stoichiometry-modulated dual epsilon-near-zero characteristics of niobium nitride films

Metal nitride is an important new type of Epsilon-near-zero (ENZ) materials applicable in visible and near infrared. In this study, NbNx films with different nitrogen content Rn were prepared by magnetron sputtering. The results show that the films are stoichiometry-tunable on the lattice constant, chemical state, and plasmonic properties. Importantly, NbNx films exhibit dual ENZ behavior tunable in the range of 400 to 1000 nm, and the absolute value of the negative real part of epsilon is below 1.0. Increased Rn reduces the screened plasma frequency and narrows the ENZ range by introducing different non-stoichiometric defects. The result of first-principle calculation show that non-stoichiometric defects affect the band structure and the density of states, and the cation vacancies are the main defects tailoring the ENZ behavior of the NbNx films. As examples of the application, we provide conceptual designs of angular filter and perfect absorber, the performances of which can be enhanced by NbNx films. NbNx is proved to be an ENZ material with simple composition, easy preparation and high tunability in a wide range. The tunable dual ENZ characteristics of NbNx films highlight their significant future in visible and near infrared nanophotonic applications.

Effect of nonstoichiometric defects and Cr ions on the photoconductivity of Bi12SiO20 crystals

The results of an experimental study of the effect of nonstoichiometric defects in oxygen and the Bi:Si ratio, as well as Cr ions, on the photoconductivity of Bi12SiO20 crystals are presented. It is shown that varying the annealing conditions allows one to modify the spectral distribution and quantitative characteristics of photoconductivity substantially.

Defect accommodation in off-stoichiometric (SrTiO3)nSrO Ruddlesden–Popper superlattices studied with positron annihilation spectroscopy

The low dielectric loss underlying the record performance of strained (SrTiO3)nSrO Ruddlesden–Popper films as tunable microwave dielectrics was postulated to arise from (SrO)2 faults accommodating local non-stoichiometric defects. Here, we explore the effect of non-stoichiometry on (SrTiO3)nSrO using positron annihilation lifetime spectroscopy on a composition series of 300 nm thick n = 6 (Sr1+δTiO3)nSrO thin films. These films show titanium-site vacancies across the stoichiometry series, with evidence that TiOx vacancy complexes dominate. Little change in defect populations is observed across the series, indicating the ability of Ruddlesden–Popper phases to accommodate ± 5% off-stoichiometry. This ability for defect accommodation is corroborated by scanning transmission electron microscopy with electron energy loss spectroscopy.

Effects of non-stoichiometric defects on the dielectric properties of (Ba0.5Sr0.5)xTiO3-ZnGa2O4 composite ceramics

In order to explore the effects of non-stoichiometric defects on the dielectric properties of composite ceramics, 70 wt% (Ba0.5Sr0.5)xTiO3-30 wt%ZnGa2O4 ((BS)xT50-ZG, x = (Ba + Sr)/Ti = 0.99, 1.00, 1.01 and 1.05) composite ceramics were fabricated by the traditional sintering technique. The association between structure and dielectric properties has been studied. The results show that the distortion of the crystal lattice brought by the partial Schottky defects, namely [V″Ba,Sr–V˙˙O]× and [V′′′′Ti–2V˙˙O]×, induces a decrease in the Curie temperature of (BS)xT50-ZG composite ceramics. The orientation of the elastic dipoles brought by oxygen vacancies causes the pinning of the domain walls, which reduces the dielectric loss at low frequencies. Tunability is related to the dipole polarization caused by [V″Ba,Sr–V˙˙O]× and [V′′′′Ti–2V˙˙O]× defect complexes. In addition, compared with the composite ceramic with x = 1, the Q values of the composite ceramics with x < 1 and x > 1 decreases due to the deterioration of the microstructure homogeneity and the enhancement of the disorder of the B-site cations in (BS)xT50.

Evidences of inner Se ordering in topological insulator PbBi2Te4-PbBi2Se4-PbSb2Se4 solid solutions

In topological insulators (TIs), carriers originating from non-stoichiometric defects hamper bulk insulation. In (Bi,Sb)2(Te,Se)3 TIs (BSTS TIs), however, Se atoms strongly prefer specific atomic sites in the crystal structure (Se ordering), and this ordering structure suppresses the formation of point defects and contributes to bulk insulation. It has accelerated the understanding of TIs’ surface electron properties and device application. In this study, we select Pb(Bi,Sb)2(Te,Se)4 (Pb-BSTS) TIs, which are reported to have larger bandgap compared to counterpart compound BSTS TIs. The Se ordering geometry was investigated by combining state-of-the-art scanning transmission electron microscopy and powder X-ray diffractometry. We demonstrated the existence of inner Se ordering in PbBi2(Te,Se)4 and also in Pb-BSTS TIs. Quantitative analysis of Se ordering and a qualitative view of atomic non-stoichiometry such as point defects are also presented. Pb-BSTS TIs’ Se ordering structure and their large gap nature has the great potential to achieve more bulk insulation than conventional BSTS TIs.