Simulating Radiation-Induced Defect Formation in Pyrochlores

2013 ◽  
Vol 1514 ◽  
pp. 15-20
Author(s):  
David S.D. Gunn ◽  
John A. Purton ◽  
Ilian T. Todorov
Keyword(s):  
Point Defects ◽  
Oxygen Vacancies ◽  
Defect Formation ◽  
Migration Energy ◽  
Cation Disorder ◽  
Pair Defect ◽  
Radiation Induced ◽  
Ab Inito ◽  
Formation Energies ◽  
Selection Of

ABSTRACTThe accuracy and robustness of new Buckingham potentials for the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 is demonstrated by calculating and comparing values for a selection of point defects with those calculated using a selection of other published potentials and our own ab inito values. Frenkel pair defect formation energies are substantially lowered in the presence of a small amount of local cation disorder. The activation energy for oxygen vacancy migration between adjacent O48f sites is calculated for Ti and Zr pyrochlores with the energy found to be lower for the non-defective Ti than for the Zr pyrochlore by ∼0.1 eV. The effect of local cation disorder on the VO48f → VO48f migration energy is minimal for Gd2Ti2O7, while the migration energy is lowered typically by ∼43 % for Gd2Zr2O7. As the healing mechanisms of these pyrochlores are likely to rely upon the availability of oxygen vacancies, the healing of a defective Zr pyrochlore is predicted to be faster than for the equivalent Ti pyrochlore.

Charged Defect Formation Energies in TiO2 Using the Supercell Approximation

2006 ◽  
Vol 45 ◽  
pp. 1-8 ◽  
Author(s):  
Jun He ◽  
Mike W. Finnis ◽  
Elizabeth C. Dickey ◽  
Susan B. Sinnott
Keyword(s):  
Point Defects ◽  
Density Functional ◽  
Oxygen Vacancies ◽  
Defect Formation ◽  
Density Functional Theory Calculations ◽  
Wide Band ◽  
Wide Band Gap ◽  
Influence Of Temperature On ◽  
Formation Energies ◽  
Positively Charged

TiO2 has been intensively studied as a wide band-gap transition metal oxide partially due to the multi-valence nature of its cation. Here, density-functional theory calculations within the supercell approximation are carried out to determine the preferred charge state of point defects in rutile TiO2. The first component of this work is to investigate the dependence of the defect formation energies on supercell size and the electrostatic Makov-Payne correction. The results show that the Makov-Payne correction improves the convergence of defect formation energies as a function of supercell size for positively charged titanium interstitials and negatively charged titanium vacancies. However, in the case of positively charged oxygen vacancies, applying the Makov-Payne correction gives the wrong sign for the defect formation energy correction. This is attributed to the shallow nature of the transition levels for this defect in TiO2. Finally, we combine the calculated defect formation energies with thermodynamic data to evaluate the influence of temperature on the relative stabilities of point defects. The results indicate that when the Makov- Payne correction is applied, a stable charge transition occurs for titanium interstitials. In addition, as the temperature increases, the dominant point defect in TiO2 changes from oxygen vacancies to titanium interstitials.

Hg1-xCdxTe: Defect Structure Overview

1990 ◽  
Vol 216 ◽  
Author(s):  
M.A. Berding ◽  
A. Sher ◽  
A.-B. Chen
Keyword(s):  
Point Defects ◽  
Defect Structure ◽  
Defect Formation ◽  
Activation Energies ◽  
Mass Action ◽  
Native Defect ◽  
Native Point Defects ◽  
Vacancy Migration ◽  
Formation Energies ◽  
Diffusion Activation

ABSTRACTNative point defects play an important role in HgCdTe. Here we discuss some of the relevant mass action equations, and use recently calculated defect formation energies to discuss relative defect concentrations. In agreement with experiment, the Hg vacancy is found to be the dominant native defect to accommodate excess tellurium. Preliminary estimates find the Hg antisite and the Hg interstitial to be of comparable densities. Our calculated defect formation energies are also consistent with measured diffusion activation energies, assuming the interstitial and vacancy migration energies are small.

Point-defects in magnesium sulfide

1994 ◽  
Vol 9 (1) ◽  
pp. 132-134
Author(s):  
Upendra Puntambekar ◽  
Sunder Veliah ◽  
Ravindra Pandey
Keyword(s):  
Optical Absorption ◽  
Shell Model ◽  
Point Defects ◽  
Formation Energy ◽  
Vacancy Mechanism ◽  
Migration Energy ◽  
Absorption Energy ◽  
Defect Energies ◽  
Formation Energies ◽  
Magnesium Sulfide

The results of a study of point defects in MgS are presented. First we obtain empirical interionic potentials in the framework of a shell model and then calculate defect energies using the HADES and ICECAP simulation procedures. The calculated Schottky formation energy is 10.9 eV in comparison to the cation and anion Frenkel formation energies of 11.9 and 25.1 eV, respectively. The migration energy by the vacancy mechanism of the Mg2+ and S2− ions is predicted to be 2.5 and 3.4 eV, respectively. One-electron ICECAP calculations yield the optical absorption energy of 3.1 eV for the F+ center in MgS.

scholarly journals The Effect of Defect Disorder on the Electronic Structure of Rutile TiO2-x

2006 ◽  
Vol 251-252 ◽  
pp. 1-12 ◽  
Author(s):  
Faruque M. Hossain ◽  
Graeme E. Murch ◽  
L. Sheppard ◽  
Janusz Nowotny
Keyword(s):  
Electronic Structure ◽  
Band Gap ◽  
Point Defects ◽  
Density Functional ◽  
Defect Formation ◽  
Energy Levels ◽  
Defect Energy ◽  
Defect Disorder ◽  
Effect Of Oxygen ◽  
Formation Energies

The purpose of this work is to study the effect of bulk point defects on the electronic structure of rutile TiO2. The paper is focused on the effect of oxygen nonstoichiometry in the form of oxygen vacancies, Ti interstitials and Ti vacancies and related defect disorder on the band gap width and on the local energy levels inside the band gap. Ab initio density functional theory is used to calculate the formation energies of such intrinsic defects and to detect the positions of these defect induced energy levels in order to visualize the tendency of forming local mid-gap bands. Apart from the formation energy of the Ti vacancies (where experimental data do not exist) our calculated results of the defect formation energies are in fair agreement with the experimental results and the defect energy levels consistently support the experimental observations. The calculated results indicate that the exact position of defect energy levels depends on the estimated band gap and also the charge state of the point defects of TiO2.

Ab initio Simulation of Vacancy Processes in Ni3Al

1998 ◽  
Vol 552 ◽  
Author(s):  
H. Schweiger ◽  
E. Moroni ◽  
W. Wolf ◽  
W. Püischl ◽  
W. Pfeiler ◽  
...  
Keyword(s):  
Activation Energy ◽  
Ab Initio ◽  
Point Defects ◽  
Nearest Neighbor ◽  
Defect Formation ◽  
Temperature Dependent ◽  
Ab Initio Simulation ◽  
Jump Model ◽  
Migration Barriers ◽  
Formation Energies

ABSTRACTProperties of point defects such as antisites and vacancies in Ni3A1 are studied by means of ab initio calculations for supercells. Temperature dependent quantitities such as defect formation energies are derived by means of a grandcanonical ensemble. Stimulated by experiments of residual resistivities suggesting an outstandingly large activation energy of 4.6 eV due to Al vacancies, several models for point like defects are treated in combination with calculated migration barriers for nearest neighbor jumps and also the six-jump model.

First Principles Study of Point Defects in Uranium Dioxide

2007 ◽  
Vol 561-565 ◽  
pp. 1971-1974 ◽  
Author(s):  
Ying Chen ◽  
Misako Iwasawa ◽  
Yasunori Kaneta ◽  
Toshiharu Ohnuma ◽  
Hua Yun Geng ◽  
...  
Keyword(s):  
Point Defects ◽  
First Principles ◽  
Defect Formation ◽  
Lattice Relaxation ◽  
Atomic Displacement ◽  
Grain Structure ◽  
First Principles Calculations ◽  
Fine Grain ◽  
Unit Cells ◽  
Formation Energies

To clarify the origin of a characteristic fine grain structure formed under the high burn-up of the nuclear fuel, the comprehensive first-principles calculations for UO2 containing various types of point defect have been performed by the PAW-GGA+U with lattice relaxation for supercells containing 1, 2 and 8 unit cells of UO2. The electronic structure, the atomic displacement and the defect formation energies of defective systems are obtained, and the effects of supercell size on these properties are discussed. Based on this relatively high precise self-consistent formation energies dataset, thermodynamic properties of various types of point defects in UO2 are further investigated in the framework of the point defects model.

scholarly journals Моделирование радиационно-индуцированной сегрегации в сплавах Fe-Cr-Ni

2019 ◽  
Vol 61 (12) ◽  
pp. 2281
Author(s):  
Р.В. Скороход ◽  
А.В. Коропов
Keyword(s):  
Steady State ◽  
Point Defects ◽  
Kirkendall Effect ◽  
Concentration Profiles ◽  
Migration Energy ◽  
Dose Rates ◽  
Dislocation Densities ◽  
Ni Alloy ◽  
Different Temperatures ◽  
Radiation Induced

In the model of radiation-induced segregation, based on the first and second Fick’s laws and with inverse Kirkendall effect considered, the concentration profiles of the components of the concentrated Fe-Cr-Ni alloy and radiation point defects are obtained at different temperatures, dislocation densities, dose rates. The concentration profiles were calculated from the start of irradiation to the steady state. The sensitivity of concentrations of alloy components near the surface is analyzed as function of input parameters (vacancies migration energy for Cr, Ni, Fe, interstitial migration energy, etc.).

Nonstoichiometry and Defect Mechanism in Intermetallics with L12-Structure

2000 ◽  
Vol 646 ◽  
Author(s):  
Herbert Ipser ◽  
Olga P. Semenova ◽  
Regina Krachler ◽  
Agnes Schweitzer ◽  
Wenxia Yuan ◽  
...  
Keyword(s):  
Intermetallic Compounds ◽  
Point Defects ◽  
Thermodynamic Model ◽  
Defect Formation ◽  
Statistical Thermodynamic ◽  
Thermodynamic Activities ◽  
Defect Mechanism ◽  
First Time ◽  
Formation Energies ◽  
L12 Structure

ABSTRACTA statistical-thermodynamic model was derived which allows to describe thermodynamic activities in intermetallic compounds with L12-structure as a function of composition and temperature. The energies of formation of the four types of point defects (anti-structure atoms and vacancies on both sublattices) were used as adjustable parameters. The model was applied to the three compounds Ni3Al, Ni3Ga, and Pt3Ga, and it permitted to estimate for the first time the defect formation energies for Ni3Ga and to provide initial estimates for Pt3Ga.

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