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Lithium doping has been widely employed to modulate the photoluminescence of lanthanide-doped crystals. Despite its effectiveness, the underlying mechanism remains debatable. Using cubic sodium yttrium fluoride as a model crystal, first-principles calculations indicate that neutral substitution of host sodium with lithium ions can occur, irrespective of synthesis conditions. Unlike lithium substitution, the formation of lithium interstitials is likely accompanied by the substitution of F- with O2- ions. The lithium substitution shows negligible influence on the electronic structures of the host crystal and the 4f orbital energies of lanthanide ions. By comparison, the lithium-oxygen defect pairs could induce self-absorption losses in lanthanide luminescence or produce ligand-to-metal charge transfer states that could populate lanthanide 4f states.

AlxGa1−xN nanowires are the key materials for next-generation ultraviolet (UV) detectors. However, such devices have a low quantum efficiency caused by the introduction of defects and impurities throughout the preparation process of nanowires. Herein, the effects of different interstitial defects and vacancy defects on the electronic structure of Al0.5Ga0.5N nanowires are investigated using density functional theory calculations. Our results successfully discovered that only the formation of an N interstitial defect is thermally stable. In addition, the introduction of different defects makes the different nanowires exhibit n-type or p-type characteristics. Additionally, different defects lead to a decrease in the conduction band minimum in band structures, which is the major cause for the decrease in work function and increase in electron affinity of Al0.5Ga0.5N nanowires. What is more, the calculation of the partial density of states also proved that the interstitial defects contribute to a re-hybridization of local electron orbitals and then cause more significant movement of the electron density. Our investigations provide theoretical guidance for the pursuit of higher-quantum-efficiency ultraviolet (UV) detectors. [ABSTRACT FROM AUTHOR]

The intrinsic point defect influence data for β –Si3N4 by far are incomplete and experimental clarification is not easy. In this contribution, the effects of vacancy ( V 2c , V 6h and V Si ) and interstitial ( I N and I Si ) defects on the electronic properties of H-passivated β –Si3N4 (0001) surface are explored based on density functional theory (DFT) calculation. The results show that it is easier to form N 6h vacancy defects in the surface layer under Si-rich conditions. The existence of N vacancies makes the bottom of conduction bands shift downwards, and the top of valance band is away from Fermi level. The presence of V Si makes the system have the characteristics of p-type semiconductor, and the closer to the inner layer, the narrower the range of additional energy bands and the greater the degree of localization of electrons. The closer the Si atom vacancy is to the surface, the smaller the photon energy corresponding to the maximum absorption coefficient is. Compared with the N vacancy system, the Si vacancy system has higher reflection ability in the low energy region. For the interstitial defect systems, I N is easy to form on the surface layer, and I Si is easy to produce in the inner layer. The I N system has a new additional energy level at the Fermi level, and as the I N is closer to the inner layer, the energy range of the additional energy level is also narrower. In the I Si system, the new additional energy levels appear at the Fermi level and the intermediate band. The results have positive significance for the design of this advanced structural and functional integrated ceramics. The absorption coefficient and reflection coefficient of I Si - 3 system are much higher than those of other systems when the energy is greater than 2.5 eV. [ABSTRACT FROM AUTHOR]

Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti 1–x Hf x NiSn 1–y Sb y half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti 0.5 Hf 0.5 NiSn 0.98 Sb 0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects. Image 1 [ABSTRACT FROM AUTHOR]

The beltline region of the reactor pressure vessel (RPV) is subject to an extreme radiation, temperature, and pressure environment over several decades of operation; therefore it is necessary to understand the mechanisms through which radiation damage occurs and how it affects the mechanical and chemical properties of the RPV steel. Chemical rate theory is a mean field rate theory simulation model which applies chemistry to the evaluation of irradiation-induced embrittlement. It presents one method of analysis that may be coupled to other distinct methods, in order to analyze defect formation, ultimately providing useful information on strength, ductility, toughness and dimensional stability changes for effects such as embrittlement, reduction in ductility and toughness, void swelling, hardening, irradiation creep, stress corrosion cracking, etc. over time as materials are subjected to reactor operational irradiation. This paper serves as a brief review of rate theory fundamentals and presents several examples of research that exemplify the application and importance of rate theory in examining the effects of radiation damage on RPV steel.

Previous investigations on the thermal recovery of irradiated uranium dioxide crystals defects related to each recovery stage have been estimated based on indirect observations such as activation energy calculations of ion migration during crystal lattice recovery. In this study, thermal recovery mechanisms for fission- and α-particle-irradiated uranium dioxide lattices were investigated by atomistic simulation methods. Isochronal and isothermal annealing methods were applied to the irradiated samples. Following reported procedures, the activation energies were calculated by molecular dynamics simulations and compared with empirical data, which showed good agreement with the experimental results. The migration energy barrier and the recovery energies of the obstruction type defects were calculated using molecular dynamics simulations and density functional theory. The irradiated crystal recovery stages are also discussed. Contrary to the reported predictions, direct, visual observation of the simulated data show that fission- and α-particle-irradiated crystals are not thermally recovered via the same mechanism. All the three recovery stages of the fission-irradiated uranium dioxide crystal are attributed to oxygen interstitials, while the first, second, and third recovery stages of the α-particle-irradiated crystals are attributed to oxygen–uranium, uranium, and uranium interstitial defects respectively. Finally, in a previous study of the present author it was shown that fission- and α-particle induced lattice swelling of uranium dioxide occurs due to obstruction-type defects. In this study, the main driving force of thermal recovery of irradiated crystals was once more observed as obstruction-type defects, which initiated all other types of defect recovery at each lattice recovery stage. [ABSTRACT FROM AUTHOR]

The geometric structure, electronic structure and formation energy of CaCu 3 Ti 4 O 12 (CCTO) with interstitial copper atom have been studied using the density-functional method within the GGA approximation. Result of structural optimization shows that the interstitial Cu-atom (Cu 7 ) prefers to occupy a special location which is symmetrical with an intrinsic copper atom (Cu 13 ) deviated from the normal site. The mulliken analysis indicates the loss of electrons from interstitial atom (Cu 7 ) and Cu 13 are only half more of the losing in other copper atom, which reveals a characteristics of covalent bonding between Cu 7 /Cu 13 and surrounding oxygen atoms respectively. Meanwhile, it is found from electron density difference (EDD) and orbital analysis that the introduction of interstitial Cu atom causes prominent structural reconstruction of a new “CuO 4 ” quadrilateral. Moreover, the new “CuO 4 ” planar leads to a corresponding electronic reconstruction in the hybridization between Cu 7 /Cu 13 3d and O 2p at the vicinity of fermi surface, for which a new conductive filament channel comes into being. Besides, the formation energies of the interstitial defects in various charge states are corrected with the value of 2.18, −4.17 and −9.46 eV for charge of 0, 1+ and 2+, respectively. [ABSTRACT FROM AUTHOR]

The beltline region of the reactor pressure vessel (RPV) is subject to an extreme radiation, temperature, and pressure environment over several decades of operation; therefore it is necessary to understand the mechanisms through which radiation damage occurs and how it affects the mechanical and chemical properties of the RPV steel. Chemical rate theory is a mean field rate theory simulation model which applies chemistry to the evaluation of irradiation-induced embrittlement. It presents one method of analysis that may be coupled to other distinct methods, in order to analyze defect formation, ultimately providing useful information on strength, ductility, toughness and dimensional stability changes for effects such as embrittlement, reduction in ductility and toughness, void swelling, hardening, irradiation creep, stress corrosion cracking, etc. over time as materials are subjected to reactor operational irradiation. This paper serves as a brief review of rate theory fundamentals and presents several examples of research that exemplify the application and importance of rate theory in examining the effects of radiation damage on RPV steel. [ABSTRACT FROM AUTHOR]

In current study molecular dynamics (MD) simulations are performed to reveal effects of pre-existing defects on nanoimprint of single crystal aluminum () thin films. Simulation results suggest that critical force required for plastic deformation initiation decreases with the increase of vacancy volume fraction, but increases with the interstitial volume fraction. It is found that adsorption phenomenon is affected by pre-existing defects, as adsorption phenomenon weakens with the increasing interstitial volume fraction. Pre-existing defects have significant influence on the deformation mechanisms and imprint forces during the nanoimprint processes. Simulation results also show that surface deformation is strongly affected by pre-existing defects. [ABSTRACT FROM AUTHOR]

The Ti-Ni-Sn half-Heusler compound exhibits a high power factor but relatively low figure of merit due to its high thermal conductivity. In this paper, we propose an effective and inexpensive way to reduce the thermal conductivity, and independently increase the power factor. To this end, we have systematically synthesized off-stoichiometric Ti-Ni-Sn half-Heusler compounds and introduced Y-Sb dilute co-doping at Ti-Sn sites. Excess Ni introduces interstitial defects in the half-Heusler crystal and results in significant reduction of the thermal conductivity, which drops below 3 W/mK at room temperature. In addition, the Y-Sb dilute co-doping at Ti-Sn sites improves the power factor while the thermal conductivity remains reasonably small. As a result, the figure of merit at room temperature is eight times larger than that of nondoped Ti-Ni-Sn. [ABSTRACT FROM AUTHOR]

Abstract: We report the annealing temperature dependence of global and local structures of hydrothermally grown KNbO3 nanorods. With increasing annealing temperature, the amount of hydroxyl groups in the KNbO3 nanorods decreased and finally disappeared at about 800°C. Morphology of the nanorods seemed not to change significantly, however, X-ray intensity ratio between (022) and (200) planes, i.e., I 022/I 200, increased with decreasing lattice hydroxyl group contents. This result could be attributed to the fact that the hydroxyl groups were desorbed more effectively along the [011] direction than in other directions due to the elongated nanorod along the [011] direction. The frequencies of external and bending modes, besides stretching mode, showed red-shift with decreasing lattice hydroxyl group contents. This result implied that the lattice hydroxyl groups existed as interstitial defects near K ions. [Copyright &y& Elsevier]

Abstract: Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008). [Copyright &y& Elsevier]

The complex oxide system of Ba-Ta-O presents interesting cases of tetragonal tungsten bronze (TTB) phases being formed over a wide compositional range of Ba 1− x Ta 2 O 6− x , 0 ≤ x ≤ 0.65 [X. Kuang et al., Inorg. Chem. , 2013, 52 , 13244−13252]. Compared to the common formalism of the TTB phase AB 2 O 6 , Ba 1− x Ta 2 O 6− x is highly off stoichiometric and is presumed to involve substantial structural modifications. In this study, Ba 1− x Ta 2 O 6− x samples were prepared with the chemical compositions of BaTa 2 O 6 (x = 0), Ba 0.8 Ta 2 O 5.8 (x = 0.2), and Ba 0.5 Ta 2 O 5.5 (x = 0.5). Rietveld refinement and Fourier electron density calculation, using high-resolution synchrotron X-ray diffraction data, revealed that the above three compositions have distinct local structural features, and the irregularity escalates with the increase of x. In all the three compositions, the Ba position inside the pentagonal tunnel (A2 site) was lightly split, reflecting the oversize polyhedral cavity. Additionally, Ba 0.8 Ta 2 O 5.8 and Ba 0.5 Ta 2 O 5.5 contained segments of (TaO)3+ chains in the pentagonal tunnel and Ta5+ in the triangular tunnel (A3 site). Hence, the unit cell structure of BaTa 2 O 6 , Ba 0.8 Ta 2 O 5.8 , and Ba 0.5 Ta 2 O 5.5 can be represented as Ba 5 [Ta 10 O 30 ], Ba 4.26 (TaO) 0.04 Ta 0.22 [Ta 10 O 30 ], and Ba 2.84 (TaO) 1.19 Ta 0.18 [Ta 10 O 30 ], respectively. Dielectric constants (κ) of Ba 1− x Ta 2 O 6− x were measured in the temperature range of 30–450 K and at frequencies of 100 kHz−1 MHz. Compared to BaTa 2 O 6 , which displayed high κ (≈100 at 300 K) and broad relaxation peaks below 50 K, Ba 0.8 Ta 2 O 5.8 and Ba 0.5 Ta 2 O 5.5 exhibited markedly reduced lattice polarizations. • TTB phases from extraordinarily wide compositional range of Ba 1− x Ta 2 O 6− x. • Unusual interstitial defects are clearly identified by Fourier density mapping. • Ba 1− x Ta 2 O 6− x maintains the TTB lattice by creating interstitial Ta5+ and (TaO)3+. • Interstitial species impact immediately the polarization of Ba 1− x Ta 2 O 6− x. [ABSTRACT FROM AUTHOR]

The ability to incorporate high content of interstitial oxygen ions (Oi) in La1+xSr1‐xGa3O7+0.5x melilite owing to the good size match between La3+ and Sr2+ ions is well documented. Here, the complete substitution of Sr2+ by Pb2+ lone‐pair cations results in a significant loss of this ability, even though Sr2+ and Pb2+ have almost the same effective ionic radius. To explore the fundamental mechanism underlying this result, density functional theory (DFT) calculations are performed on both the LaSrGa3O7‐based and LaPbGa3O7‐based materials, revealing a new chemical bonding effect on the incorporation of mobile oxygen interstitial defects in melilites. For LaSrGa3O7‐based melilites, the interstitial oxygens have cooperatively weak antibonding interactions with the framework oxygen atoms (Of). This antibonding Oi‐Of interaction pushes Oi toward a 3‐linked Ga ion, enhancing the covalent bonding between this Ga ion and Oi. In addition, the antibonding Oi–Of interaction makes the oxygen interstitial defects and framework atoms highly active, benefiting the migration of defects. In contrast, for LaPbGa3O7‐based materials, the 6s2 electrons of Pb2+ point toward the c‐axis and form antibonding with framework O2−. This antibonding projects into the tunnel void, thereby directly hindering the entrance of interstitial oxygen atoms into the pentagonal rings. [ABSTRACT FROM AUTHOR]