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1.
Point defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE2B2O7 (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first-principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO2 excess, the formation of the B-RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE-B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE2O3 excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B-site and/or RE-site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect-related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.  相似文献   

2.
《Ceramics International》2017,43(13):10347-10352
With a motivation to investigate the simultaneous effect of sintering temperature and sintering time in tandem on defect distributions in polycrystalline undoped ZnO a new technique called Multi-step sintering (MSS) has been deliberately designed. Systematic investigations on structural parameters suggest the rigorous defect propagation along a-axis of the unit cell. We report the unwavering nature of Zinc interstitials (Zni) and the complexes of Zni with oxygen vacancies (Vo) with sintering temperature. The enhanced and broad green emission in Photoluminescence spectroscopy reveals the existence of surplus Vo in the sample. The band gap narrowing at 800 °C and enhanced Raman modes in Raman scattering experiment confirms the existence of excess oxygen vacancies and make MSS successful technique to tailor the intrinsic defects without any extrinsic doping.  相似文献   

3.
《Ceramics International》2020,46(7):8958-8970
A series of charge compensated Ga–V co-doped TiO2 samples (Ti(1-x)(Ga0.5V0.5)xO2) have been synthesized by a modified sol-gel process. X-ray diffraction pattern shows that the anatase to rutile (A→R) onset temperature (TO) shifts to a higher temperature, whereas the complete phase transformation temperature (TC) shifts to a low-temperature region as compared to pure TiO2, due to Ga–V incorporation. Ga–V co-doping helps in the transformation of some smaller sized Ti4+ to a relatively larger Ti3+. In the anatase phase, oxygen content also increases with increasing doping concentration, which along with the larger size of Ti3+ results in lattice expansion and thereby delays the TO. In the rutile phase, oxygen vacancy increases with increasing doping concentration, which results in lattice contraction and accelerates phase transition. Grain growth process is hindered in the anatase phase (crystallites size reduces from ~15 nm (x = 0.00) to 8 nm (0.10)), whereas it is accelerated in the rutile phase as compared to pure TiO2. In both phases bandgap (Eg) reduces to the visible light region (anatase: Eg = 3.16 eV (x = 0.00) to 2.19 eV (x = 0.10) and rutile: 3.08 eV (x = 0.00) to 2.18 eV (x = 0.10)) in all co-doped samples. The tail of the absorption edge reveals lattice distortion and increase of Urbach energy proofs the same due to co-doping. All these changes (grain growth, phase transition, and optical properties) are due to lattice distortion created by the combined effect of substitution, interstitials, and oxygen vacancies due to Ga–V incorporation in TiO2.  相似文献   

4.
The technological usefulness of a semiconductor often depends on the types, concentrations, charges, spatial distributions, and mobilities of the atomic‐scale defects it contains. For semiconducting metal oxides, defect engineering is relatively new and involves complex transport and reaction networks. Surface‐based methods hold special promise in nanostructures where surface‐to‐volume ratios are high. This work uses photoreflectance augmented by X‐ray photoelectron spectroscopy to show that the surface potential VS for Zn‐terminated ZnO(0001) can be manipulated over a significant range 54.97–79.08 kJ/mol (0.57–0.82 eV) via temperature and the partial pressure of O2. A defect transport model implies this variation in VS should affect the injection rate of oxygen interstitials by a factor of three. Such injection plays an important role in controlling the concentrations of oxygen vacancies deep in the bulk, which often prove troublesome as trapping centers in photocatalysis and photovoltaics and as parasitic emitters in light‐emitting devices. © 2015 American Institute of Chemical Engineers AIChE J, 62: 500–507, 2016  相似文献   

5.
RE2SiO5 (RE = Yb and Lu) are significant environmental barrier coating (EBC) materials, in which surface and oxygen vacancy play crucial roles in their structural stability and functionality. In this work, the structural configuration and thermodynamics of (1 0 0), (0 1 0), and (0 0 1) surfaces of RE2SiO5 are investigated by first-principles calculations. The (0 0 1) surface is preferred energetically, which is attributed to the weak bond broken environment and large rare-earth polyhedron distortion on this surface. Moreover, the formation energies of various oxygen vacancies on the stable (0 0 1) surface are estimated and the optimal location for oxygen vacancies is held by the [SiO4] tetrahedron. The oxygen vacancies are more likely to segregate on the surface because of the lower formation energies on the surfaces compared with those in the bulk. These findings are expected to enable the development of RE2SiO5-based EBCs by tuning grain size and/or thin film growth orientation.  相似文献   

6.
The surface structures, bond variations, and segregation of oxygen vacancies play crucial roles in the structural stability and functionality of nanocrystalline rare-earth zirconate pyrochlores. In this work, the stabilities of (1 0 0), (1 1 0), and (1 1 1) surfaces of pyrochlore A2Zr2O7 (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd) are investigated by first-principles calculations. Surface reconstruction occurs on (1 1 0) surface with a transition of ZrO6 octahedron to ZrO4 tetrahedron, leading to their large relaxation energies. In combination with the small amount of broken bonds during the surface formation process, the (1 1 0) surfaces are identified having the lowest surface formation energies than the (1 0 0) and (1 1 1) surfaces. Moreover, the reconstructed (1 1 0) surface has characteristics of the segregation of oxygen vacancies. The surface oxygen vacancies have the low migration barriers (<1.2 eV), which are comparable with those in bulk and ensure the long-distance diffusion of oxygen vacancies in A2Zr2O7. These discoveries provide fundamental insight to the surface structure and related oxygen vacancy behavior, which are expected to guide the optimization of the surface related properties for nanocrystalline rare-earth zirconates.  相似文献   

7.
《Ceramics International》2020,46(11):18690-18697
Bi0.9Er0.1Fe1−xMnxO3 (BEFMxO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe2+ in BEFMxO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFMxO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm2, and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFMxO films obtain good stability and reliability in the application of ferroelectric memory devices.  相似文献   

8.
《Ceramics International》2021,47(18):25567-25573
The Li4TiO4 ceramic is a promising solid-state tritium breeder material for the production of tritium fuel in the designs of fusion reactors. Tritium extraction efficiency is one of the key factors that determines the performance of the breeder material. Vacancies are known to trap tritium and adversely affect tritium extraction. Understandings of tritium-trapped defect configurations in Li4TiO4 are limited so far. In this work, the oxygen/lithium vacancy (VO/Li) and the vacancy-tritium defect complex (VO/Li + T) in Li4TiO4 are studied by first-principles density functional theory. The atomic configurations, formation energies and electronic structures of various charged defect species are obtained. We find that 2+ and 0 are the dominate charge states of VO, 1- is the dominate charge state of VLi, 1+ is the dominate charge state of the defect complex (VO + T), and 0 is the dominate charge state of (VLi + T). Tritium atoms trapped in VO are bonded to Ti atoms, and those trapped in VLi are bonded to O atoms.  相似文献   

9.
In this article, the electroluminescence (EL) spectra of zinc oxide (ZnO) nanotubes/p-GaN light emitting diodes (LEDs) annealed in different ambients (argon, air, oxygen, and nitrogen) have been investigated. The ZnO nanotubes by aqueous chemical growth (ACG) technique on p-GaN substrates were obtained. The as-grown ZnO nanotubes were annealed in different ambients at 600°C for 30 min. The EL investigations showed that air, oxygen, and nitrogen annealing ambients have strongly affected the deep level emission bands in ZnO. It was concluded from the EL investigation that more than one deep level defect is involved in the red emission appearing between 620 and 750 nm and that the red emission in ZnO can be attributed to oxygen interstitials (Oi) appearing in the range from 620 nm (1.99 eV) to 690 nm (1.79 eV), and to oxygen vacancies (Vo) appearing in the range from 690 nm (1.79 eV) to 750 nm (1.65 eV). The annealing ambients, especially the nitrogen ambient, were also found to greatly influence the color-rendering properties and increase the CRI of the as - grown LEDs from 87 to 96.  相似文献   

10.
《Ceramics International》2022,48(16):22789-22798
(1-x)Li2Zn3Ti4O12-xSr3(VO4)2 (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO3 and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τf value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti3+, and V4+, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li2Zn3Ti4O12-0.3Sr3(VO4)2 ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of εr = 17.8, Q × f = 41,891 GHz, and τf = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.  相似文献   

11.
Lead scandium tantalate (PbSc0.5Ta0.5O3, PST), an order/disorder ferroelectric, is a potential candidate for electrocaloric cooling and pyroelectric infrared (IR) detector. In this work, we report the phase transformation kinetics from two series of samples containing pure amorphous and mixture of amorphous and pyrochlore to desired perovskite phase using postdeposition rapid thermal processing (RTP) as well as growth mechanism of RF sputtered PST thin films using excess lead target on platinized silicon (Pt/Ti/SiO2/Si) substrates. We find that small changes in the temperature ramp have a large effect on the degree of perovskite conversion (ferroelectric phase), orientation (crystallographic texture), and long-range order parameter (< S111 >). Through isothermal annealing, we obtained optimal perovskite phase at ≥700°C temperature. The phase transformation is characterized by spontaneous formation of center-type in-plane radial rosette-like structures revealed by scanning electron microscopy. The PST perovskite crystallites were found to coexist with pyrochlore in RTP annealed films. The volume fractions for perovskite and pyrochlore phase were obtained from the analysis of “rosettes” and respective X-ray diffraction intensities which helped to determine various parameters associated with phase kinetics (n, k, and activation energy, Ea) and accompanying growth. The effective activation energies of perovskite transition and growth were found to be 332 ± 11 kJ/mol (345 ± 11 kJ/mol) and 114 ± 10 kJ/mol (122 ± 10 kJ/mol), respectively, for pure amorphous only (and mixed amorphous and pyrochlore) phase following nucleation-growth controlled Avrami's equation. A linear growth rate (n∼1) for the perovskite phase indicates predominant interface-controlled process and diffusion-limited phenomena thus inhibiting rosette size owing to reactant depletion and soft impingement at the grain boundary. However, the growth behavior is isotropic in two-dimension parallel to the plane of the substrates for both sample series. Lead loss was severe for in-situ growth and RTP combined with conventional furnace annealing than those of RTP only films, which were closer to stoichiometric albeit with excess lead and marginal oxygen vacancies (Vo).  相似文献   

12.
New heteronuclear (NH4)REIII[FeII(CN)6nH2O complexes (RE = La, Ce, Pr, Nd, Sm, Gd, Dy, Y, Er, Lu) were synthesized and their thermal decomposition products were investigated. The crystal structure of (NH4)RE[FeII(CN)6nH2O would be a hexagonal unit cell (space group: P63/m), which was the same as that of La[FeIII(CN)6]·5H2O. The hydration number n = 4 was estimated by TG results for all the RE complexes. The lattice constants depended on the ionic radius of the RE3+ ion for the heteronuclear complexes. The single phase of the perovskite type materials was directly obtained by decomposition of the heteronuclear complexes for RE = La, Pr, Nd, Sm, and Gd. A mixture of CeO2 and Fe2O3 was formed for RE = Ce because of its oxidation to Ce4+. In the case of RE = Dy, Y, Er, and Lu complexes, the perovskite type materials formed at higher temperature via. mixed oxides such as RE2O3 and RE4Fe5O13 due to the small RE3+ ionic radius.  相似文献   

13.
High-entropy pyrochlore has become a promising immobilization matrix due to its ability to immobilize multiple nuclides especially for high-level radioactive waste. This work adopts first principle method to analyze the formation possibility of (Lu0.25Y0.25Eu0.25Gd0.25)2Ti2O7 (HTP-1). By calculating the Gibbs free energy of possible chemical reactions, we predict that the driving force to synthesize HTP-1 is greater than the driving force to form single-component pyrochlore at temperatures above 1225 K. The characterization results show that the HTP-1 sample is successfully prepared by the solid-state reaction at the predicted temperature. To investigate the radiation resistance of HTP-1, the sample is irradiated by 800 keV Kr2+ ions, and the microstructure evolution is characterized by in-situ TEM. HTP-1 sample achieves complete amorphous at 0.26 dpa, indicating that its radiation resistance is between Eu2Ti2O7 and Y2Ti2O7, which is proved by the calculation results of xO48f and antisite defect formation energy.  相似文献   

14.
《Ceramics International》2022,48(5):6218-6224
Gallium-based SrREGa3O7 (RE = La, Pr) melilite ceramics were prepared and selected to modify their microwave dielectric properties. Sintered at 1425 °C for 6 h, SrLaGa3O7 (SLGO) and SrPrGa3O7 (SPGO) ceramics exhibited high relative densities of 98.33 and 95.23%, low εr values of 11.8 and 10.9, Q×f values of 32,500 GHz (at 12.1 GHz) and 26,400 GHz (at 12.7 GHz), negative τf values of ?32 and ?54 ppm/°C. As a compensator, CaTiO3 can tune the τf values of SLGO and SPGO to near zero (+2 and ?4 ppm/°C). In SrREGa3O7 melilite ceramics, the εr and τf values are mainly dependent on ionic polarizability, crystal structure and the “rattling” effect. The micromorphology, XPS, Raman spectrum and A-site bond valence (VRE) of SrREGa3O7 (RE = La, Pr) microwave dielectric ceramics have also been comprehensively reported.  相似文献   

15.
Pyrochlore structure material (A2B2O7) has gained interest in diverse applications like catalysis, nuclear waste encapsulation, sensors, and various electronic devices due to the unique crystal structure, electrical property, and thermal stability. This review deals with the ionic/electronic conductivity of numerous pyrochlore structure materials (titanates, zirconates, hafnates, stannates, niobates, ruthenates, and tantalite based pyrochlore) as electrolyte and electrode materials for solid oxide fuel cells (SOFCs). The impact of cation radius ratio (rA/rB) on the lattice constant and oxygen ‘x’ parameter of different pyrochlore structure materials obtained by various synthesis methods are reported. Higher ionic conductivity is essential for better ion transport in an electrolyte, and mixed ionic and electronic conductivity in electrode is essential for attaining higher efficiency in a typical SOFC. GdxTi2O7-δ, Gd2-xCaxTi2O7-δ, Nd2-yGdyZr2O7, Y2Zr2O7, Y2Zr2-xMnxO7-δ, SmDy1-xMgxZr2O7-x/2, Gd2-xCaxTi2O7-δ pyrochlore are reported as electrolytes for fuel cell applications. Some pyrochlore material (La2-xCaxZr2O7, Sm2-xMxTi2O7 (M = Mg, Co, and Ni) pyrochlore) shows protonic conductivity at lower temperatures and ionic conductivity at higher temperature condition. Also, the mixed ionic-electronic conductivity behavior is reported in electrode materials for SOFC such as R2MnTiO7 (R = Er and Y), R2MnRuO7 (R = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), R2Ru2O7 (R = Bi, Pb and Y), Y2-xPrxRu2O7, Ni-(Gd0.9Ca0.1)2Ti2O7-δ, (Gd0.9Ca0.1)2Ti2O7-δ, Gd2(Ti0.8Ru0.2)2O7-δ, (Sm0.9Ca0.1)2Ti2O7-δ and (Y0.9Ca0.1)2Ti2O7-δ pyrochlore. The detailed study of the electronic behavior of these pyrochlore system confirms the necessity of defect structure with high oxygen mobility, lower activation energy, ionic radii ratio criterion should satisfy, and possess notable ion-ion interaction. Ionic conductivity in pyrochlore is increased by enhancing the oxygen migration through 48f-48f site with the formation of oxygen vacancy. Vacancy formation can be achieved by adding a suitable dopant that creates oxygen vacancy by charge compensation mechanism or as anion Frenkel defects. Similarly, the electrical conductivity is improved while adding suitable dopant (Ce, Pr, Ru, etc.) due to disordered structure and anti-Frenkel defect formation which leads to oxygen vacancy formation and thus improves conductivity.  相似文献   

16.
《Ceramics International》2017,43(7):5668-5673
Si addition in ZnO lattice significantly improves electrical conductivity. The extra charge of Si4+ ion (in comparison to Zn2+) attracts more oxygen in the lattice and reduces oxygen vacancies. Reduction of oxygen vacancies (defects) reduces strain in the lattice. Transparency of visible light (<3.0 eV) improves due to reduction of these defects in the wide bandgap (~3.3 eV: UV) of ZnO. Extra charge of Si4+ enhances carrier density in the ZnO lattice. Improved carrier density, reduced strain facilitate transport of carriers and therefore conductivity increases. Si incorporation also makes the samples moisture resistant. The material becomes more robust to operate in adverse humid conditions. An ideal transparent conductive oxide (TCO) should be conductive, transmit visible light and able to sustain humid conditions. All these properties are observed in Zn(1−x)SixO material.  相似文献   

17.
Phase stability and macroscopic performances of ZrC are closely related to the behavior of native point defects. In this study, structures and stabilities of native point defects in ZrC as well as diffusion of C‐related defects are investigated by first‐principles calculations. It is shown that the carbon vacancy (VC) and interstitials (Cis) are the dominant native point defects in ZrC. Six types of Ci configurations: two C‐trimers, one C‐tetrahedron, and three C‐dimers are identified with low defect formation energies. The VC has a high migration energy (4.39 eV) which suggests its low mobility in ZrC. The Cis have low diffusion energy barriers (from 0.26 to 1.29 eV) which lead to their high mobility. In addition, the impact of defects (VC, VZr, ZrC, and CZr) on bonding strengths of neighboring Zr–C bonds is discussed. Especially near VC, the 1NN (nearest neighboring) and 2NN Zr–C bonds are strengthened but 4NN Zr–C bonds are weakened. Interestingly, the 3NN Zr–C bonds are almost not affected by the presence of VC. These results may be closely related to the short‐range interactions and ordering of VC in nonstoichiometric ZrC.  相似文献   

18.
We have determined the energetics of defect formation and migration in Mn+1AXn phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti3SiC2 structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti3AlC2 structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti3AlC2 is different and the energy is lower compared with the Ti3SiC2 structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti3SiC2, will have important implications for radiation damage response in these materials.  相似文献   

19.
In this work, the orthorhombic structured SrIn2O4 ceramics with a space group Pnam were synthesized by a solid-state reaction method. A high relative density (96.5%) coupled with excellent microwave dielectric properties (εr ∼ 12.3, Q × f = 96,900 GHz, τf ∼ −61.6 ppm/°C) were obtained as sintered 1300 °C for 4 h. The bond valence analysis demonstrates that the large sized cation Sr2+ exhibits a compressed state, while the In3+ exhibits a weaken rattling effect. The P-V-L chemical bond theory analysis indicates that the In-O bonds play a key role in affecting the dielectric loss. The thermally conductivity activation energy Edc (0.94 eV) of SrIn2O4 was obtained by the dielectric spectroscopy, indicating that the Edc was contributed to the double ionized oxygen vacancies. Furthermore, the intrinsic dielectric properties (εr ∼ 11.2, Q × f = 148,900 GHz) of SrIn2O4 were obtained by infrared reflectivity spectroscopy.  相似文献   

20.
Computer simulation techniques are used to develop defect models for the non-stoichiometric spinel with composition Zn1+xCr2–xO4 system. The results are used to provide an interpretation of the variation with composition of the catalytic activity of the monophasic material. We suggest that the increase in the catalytic activity with the Zn/Cr ratio is associated with an increased concentration of oxygen vacancies.  相似文献   

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