Frenkel Pair Formation Research Articles

Influence of temperature gradients on helium diffusion and clustering in tungsten: A molecular dynamics study

Tungsten and its alloys, used as plasma-facing materials in fusion reactors, endure high-flux, low-energy helium ion irradiation and temperature gradients induced by thermal load and shocks. This study utilizes molecular dynamics (MD) simulations to explore the diffusion and clustering behavior of helium (He) in tungsten (W) under temperature gradients. The migration behavior of isolated helium atoms, small helium clusters, and helium self-interstitial atom (He-SIA) clusters within tungsten is analyzed. It is revealed that both helium and He-SIA clusters tend to migrate towards higher temperature regions, exhibiting negative thermophoresis. The nucleation of helium clusters, the formation of Frenkel pairs, and the interactions between self-interstitial clusters and helium clusters are also investigated. The results indicate that helium concentration significantly impacts He nucleation behavior. Directional diffusion may lead to areas of high concentration over time, potentially promoting the formation of He clusters and bubbles. These insights enhance the understanding of tungsten's performance and durability as a plasma-facing component in fusion reactors.

Effect of grain boundaries on the helium degradation mechanisms of alloy 800H: A molecular dynamics study

Alloy 800H is currently used as structural material in light-water cooled nuclear reactors, and it is also considered as candidate materials for various advanced reactor designs. Under operation, the exposure of alloy 800H to neutron irradiation results in the formation of helium (He), mainly from Ni by the transmutation reaction (n, α). In this work, we present a molecular dynamics (MD) simulation study of the behavior and effects of He on the microstructure and mechanical properties of alloy 800H. Our results show that the population of clusters made of 5 to 9 He atoms is nearly constant throughout the simulation time (10 ns), while the population of larger clusters increases as the simulation time increases. The growth of clusters is controlled by either the dissociation and diffusion of smaller clusters towards nearby larger clusters or the merging of larger clusters initially located nearby to each other. A significant accumulation of He is observed at the grain boundaries (GB), while a depletion zone is found at the neighboring regions. As a result, the density of He cluster is significantly higher at the GBs as compared to the intra-granular regions. The nucleation and growth of He clusters also results in the formation of Frenkel pairs (FP), whose associated self-interstitial atoms (SIA) agglomerate into interstitial clusters in the alloy 800H matrix. As a consequence, dislocation segments, mostly of the Shockley type, are generated in the microstructure, and often located next to He clusters. The combination of the aforementioned defect structures and the high density of He clusters at the GBs results in a substantial degradation of the mechanical properties of alloy 800H single crystal and bicrystals.

Threshold displacement energy map of Frenkel pair generation in [formula omitted]-Ga2O3 from machine-learning-driven molecular dynamics simulations

β-gallium oxide (β-Ga2O3) shows great promise for electronics applications, particularly, in future space operating devices exposed to harsh radiation environments for extended times. This study focuses on crucial, yet not fully explored, aspects of radiation damage in this material, such as threshold displacement energies and formation of various radiation-induced Frenkel pairs. Analyzing over 5,000 molecular dynamics simulations based on our machine-learning potentials, we conclude that the threshold displacement energies for two Ga sites, the tetrahedral (22.9 eV) and octahedral (20 eV) ones, differ stronger than the same values for three different O sites, which range only between 17 eV and 17.4 eV. Mapping of threshold displacement energies unveils significant differences in displacements for all five atomic sites. Our newly developed defect identification methodology successfully classified multiple Frenkel pair types in β-Ga2O3, with over ten different Ga and two primary O ones with a predominant O split interstitial at the O1 site. Finally, the calculated recombination energy barriers suggest that O Frenkel pairs are more likely to recombine upon annealing than Ga. These insights are pivotal for understanding the radiation damage and defect formation in Ga2O3, providing the basis for design of Ga2O3-based electronics with high radiation resistance.

Flash sintering of tungsten at room temperature (without a furnace) in <1 min by injection of electrical currents at different rates

AbstractSintering of tungsten nominally requires several hours at ultrahigh temperatures. We show this refractory metal can be sintered quickly by direct injection of current into dog bone shaped specimens. The current rate was varied from 10 A s−1 (fast) to 0.1 A s−1 (slow), leading to sintering in 2–200 s, respectively. Sintering occurred at the same current density, regardless of the current rate. In all instances, the samples sintered when they reached 1000°C. The phenomenological behavior of flash sintering of metals is described by three stages: an incubation time followed by electroluminescence, and finally by abrupt sintering to full density. It is conjectured that rapid sintering is instigated by the formation of Frenkel pairs (vacancies and interstitials), as well as electrons and holes. The point defects accelerate mass transport, whereas electrons and holes recombine to form photons. Calorimetric measurements show an endothermic reaction attributed to the creation of defects. Estimates suggest an unusually large concentration of Frenkel pairs. PS: Flash sintering is different than electro‐discharge‐sintering where a capacitor is discharged in a few milliseconds to sinter a metal. Here, instead of dumping large amount of energy at once, a power supply is programed to control the rate of current injection.

Frenkel pair formation energy for cubic Fe3O4 in DFT + U calculations

The cubic phase of magnetite is stabilized above the Verwey transition temperature of about 120 K via a complex electron–phonon interaction that is still not very well understood. In this work using the DFT + U method we describe our attempt to calculate point defect formation energies for this cubic phase in the static approximation. The electronic structure calculations and atomic relaxation peculiarities are discussed in this context. Only the cubic phase model with a small band gap and charge disproportionation (Fe2+/Fe3+) gives an adequate point defect formation energies, not the semi-metallic model. The relaxation of the local defect atomic structure and the relaxation of the surrounding crystal matrix are analyzed. Point defects cause only local perturbations of atomic positions and charge-orbital order. After analysis of the supercell size effects for up to 448 atoms, we justify the use of small supercells with 56 atoms to make calculations for the cubic phase. The extensive experimental results of Dieckmann et al on defects in magnetite at high temperature are deployed for comparison of our DFT + U results on Frenkel pair formation energies.

Defect behaviour in the MoNbTaVW high entropy alloy (HEA)

The intrinsic defect formation in the MoNbTaVW HEA is investigated using atomic scale modelling, the goal being to assess its response to radiation damage in its possible use as plasma facing material for fusion reactors. When interstitial defects were modelled, a strong preference to form split interstitial defects containing vanadium was observed, even forming when other interstitial elements were initially placed into the structure. Vacancies and Frenkel pair formation are also modelled and discussed. The distinct behaviour of defects in this HEA has implications for this system as well as related HEAs and their use in nuclear components.

Ab Initio Study of HfO2/Ti Interface VO/Oi Frenkel Pair Formation Barrier and VO Interaction With Filament

Frenkel pair (FP) formation at HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti interface and oxygen vacancy interaction with the filament in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti-based resistive random access memory (ReRAM) are the important mechanisms in the ReRAM operations. In this article, the formation barriers, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{A}$ </tex-math></inline-formula> , and formation energies of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{O}/{O}_{i}$ </tex-math></inline-formula> FP across the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti interface with and without preexisting vacancy are calculated using the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ab initio</i> tool under zero external electric field. It is found that both the formation barrier and energy can be increased or decreased due to a preexisting oxygen vacancy. The spread of the effect increases inversely with the distance between the FP and the preexisting vacancy. A compact model is then proposed. The parallel migration barrier of an oxygen vacancy at the nearest and the second nearest neighbors (NNs) of a filament is also studied. It is found that the second NN has the lowest barrier due to its ability to retain a +2 charge state. The merging and dissolution of an oxygen vacancy with a filament (equivalent to perpendicular migration) is found to have a similar behavior that +2 states enhance the interaction. It is found that the fourfold coordinated oxygens (4C) and threefold coordinated oxygens (3C) filaments behave similarly during dissolution. However, 4C and 3C filaments prefer merging with 4C and 3C vacancy, respectively. The barrier of merging a +2 4C vacancy with a 4C filament is only 0.56 eV. The data and model obtained from this study may be used to implement device-level simulators, such as kinetic Monte Carlo (KMC) simulators, for ReRAM.

Influence of ion irradiation-induced defects on phase formation and thermal stability of Ti0.27Al0.21N0.52 coatings

The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.

Visible light emissions during flash sintering of 3YSZ are thermal radiation

Flash sintering enables the densification of ceramics at low furnace temperatures in a few seconds by the application of an electric field to the specimen. One of the earliest mechanisms proposed to explain the rapid densification involves the generation of Frenkel defects. Light emission during flash sintering is often interpreted as electroluminescence from electron-hole pair recombination, in support of this mechanism. In this work, experimental investigation of the emissions during flash sintering of the most widely studied ceramic, 3YSZ, shows that the visible spectrum can be explained completely in terms of black body (thermal) radiation resulting from the Joule heating of the specimen rather than electroluminescence. Apparent peaks in the spectrum are experimental artefacts associated with the equipment. There is no evidence in the visible emission spectrum during flash sintering of 3YSZ for electron-hole pair recombination associated with Frenkel pair formation or for any mechanism other than Joule heating.

Carbon driven oxygen retention in uranium oxycarbide (UCO) fluorite phase

The behavior of oxygen in uranium oxycarbide (UCO) has been investigated using density functional theory. To assess the role of carbon on the stability of oxygen point defects, we first determined the formation energies of oxygen vacancy and interstitial for different carbon configurations. Subsequently, we evaluated the barriers for migration of various oxygen defects in the UCO matrix. Our results show that carbon atoms create strong attraction centers for oxygen atoms, as a consequence the spontaneous formation of oxygen Frenkel pairs in the fluorite structure is promoted. Moreover, the strong affinity of oxygen atoms for sites in the first coordination shell of a carbon center leads to the formation of CO, and thus reducing the mobility of oxygen atoms in the UCO fuel. Upon formation, the CO remains fixed at the carbon site due to significant hybridization with uranium atoms, therefore behaving as trap sites for oxygen atoms. Our findings indicate that the formation of CO molecules increases the retention of oxygen atoms inside the UCO matrix. Consequently, carbon atoms in the UCO matrix contribute to mitigating the deleterious effects that oxygen release from the fuel kernel has on the structural integrity of the silicon carbide layer in tristructural isotropic (TRISO) particles.

Molecular dynamics simulations to assess the radiation resistance of different crystal orientations of diamond under neutron irradiation

The evolution of defects in diamond under neutron irradiation was studied via molecular dynamics simulation, with under temperatures of 300–1600 K, primary knock-on atom (PKA) energies of 1–5 keV, and incident orientations in [111], [110], and [100]. The results reveal that the formation of Frenkel pairs is insensitive to temperature but strongly dependent on PKA energy and direction. While interstitials are difficult to cluster in diamond, the size and number of vacancy clusters correlate positively with the PKA energy. Moreover, a decrease in thermal spikes is observed, which is ascribed to the fact that most interstitials can bond with surrounding carbon atoms, which prevents them from moving back to the vacancy in the [111] and [100] directions. Consequently, thermal spikes decrease or disappear as the energy increases. This trend shows directional differences. The radiation resistance of diamond with respect to the direction is [110] > [111] > [100] below 1000 K, and [110] > [111] ≈ [100] at temperatures higher (1600 K). This research can be applied in radiation damage prediction and the radiation-related defect interpretation of diamonds.

Dimensionality of mobile electrons at x-ray-irradiated LaAlO3/SrTiO3 interfaces

Electronic structure of LaAlO3/SrTiO3 (LAO/STO) samples, grown at low oxygen pressure and post-annealed ex situ, was investigated by soft-x-ray ARPES focussing on the Fermi momentum (k F) of the mobile electron system (MES). X-ray irradiation of these samples at temperatures below 100 K creates oxygen vacancies (VOs) injecting Ti t 2g-electrons into the MES. At this temperature the oxygen out-diffusion is suppressed, and the VOs should appear mostly in the top STO layer. The x-ray generated MES demonstrates, however, a pronounced three-dimensional (3D) behavior as evidenced by variations of its experimental k F over different Brillouin zones. Identical to bare STO, this behavior indicates an unexpectedly large extension of the x-ray generated MES into the STO depth. The intrinsic MES in the standard LAO/STO samples annealed in situ, in contrast, demonstrates purely two-dimensional (2D) behaviour. The relevance of our ARPES data analysis is supported by model calculations to compare the intensity vs gradient methods of the k F determination as a function of the energy resolution ratio to the bandwidth. Based on self-interaction-corrected DFT calculations of the MES induced by VOs at the interface and in STO bulk, we discuss possible scenarios of the puzzling 3D-ity. It may involve either a dense ladder of quantum-well states formed in a long-range interfacial potential or, more likely, x-ray-induced bulk metallicity in STO accessed in the ARPES experiment through a short-range interfacial barrier. The mechanism of this metallicity may involve remnant VOs and photoconductivity-induced metallic states in the STO bulk, and even more exotic mechanisms such as x-ray induced formation of Frenkel pairs.

Unravelling the ion-energy-dependent structure evolution and its implications for the elastic properties of (V,Al)N thin films

Ion irradiation-induced changes in the structure and mechanical properties of metastable cubic (V,Al)N deposited by reactive high power pulsed magnetron sputtering are systematically investigated by correlating experiments and theory in the ion kinetic energy (Ek) range from 4 to 154 eV. Increasing Ek results in film densification and the evolution from a columnar (111) oriented structure at Ek ≤ 24 eV to a fine-grained structure with (100) preferred orientation for Ek ≥ 104 eV. Furthermore, the compressive intrinsic stress increases by 336 % to -4.8 GPa as Ek is increased from 4 to 104 eV. Higher ion kinetic energy causes stress relaxation to -2.7 GPa at 154 eV. These ion irradiation-induced changes in the thin film stress state are in good agreement with density functional theory simulations. Furthermore, the measured elastic moduli of (V,Al)N thin films exhibit no significant dependence on Ek. The apparent independence of the elastic modulus on Ek can be rationalized by considering the concurrent and balancing effects of bombardment-induced formation of Frenkel pairs (causing a decrease in elastic modulus) and evolution of compressive intrinsic stress (causing an increase in elastic modulus). Hence, the evolution of the film stresses and mechanical properties can be understood based on the complex interplay of ion irradiation-induced defect generation and annihilation.

Computational study of crystal defect formation in Mo by a machine learning molecular dynamics potential

In this work, we study the damage in crystalline molybdenum material samples due to neutron bombardment in a primary knock-on atom (PKA) range of 0.5–10 keV at room temperature. We perform classical molecular dynamics (MD) simulations using a previously derived machine learning (ML) interatomic potential based on the Gaussian approximation potential (GAP) framework. We utilize a recently developed software workflow for fingerprinting and visualizing defects in damaged crystal structures to analyze the Mo samples with respect to the formation of point defects during and after a collision cascade. As a benchmark, we report results for the total number of Frenkel pairs (a self-interstitial atom and a single vacancy) formed and atom displacements as a function of the PKA energy. A comparison to results obtained using an embedded atom method (EAM) potential is presented to discuss the advantages and limits of the MD simulations utilizing ML-based potentials. The formation of Frenkel pairs follows a sublinear scaling law as ξ b where b is a fitting parameter and ξ = E PKA/E 0 with E 0 as a scaling factor. We found that the b = 0.54 for the GAP MD results and b = 0.667 for the EAM simulations. Although the average number of total defects is similar for both methods, the MD results show different atomic geometries for complex point defects, where the formation of crowdions by the GAP potential is closer to the DFT-based expectation. Finally, ion beam mixing results for GAP MD simulations are in a good agreement with experimental mixing efficiency data. This indicates that the modeling of atom relocation in cascades by machine learned potentials is suited to interpret the corresponding experimental findings.

Electrochemically controlled defect chemistry: From oxygen excess to deficiency

Fundamental understanding of materials properties requires the study and development of defect chemical reactions and models. In-depth analysis of the defect chemistry of metal oxides, however, has generally been limited to the oxygen deficient regime. Here, we present a defect chemical study covering both oxygen deficient and excess regimes using donor-doped layered cuprate (La1.85Ce0.15CuO4+δ) thin films over a wide range of oxygen activity levels. Unlike perovskite- or fluorite-structured oxides, layered cuprates can readily accommodate oxygen interstitials as well as vacancies. The scope of the defect chemical study was extended to include highly oxidizing conditions equivalent to many orders of magnitude higher oxygen activities than ambient pressure. This was achieved by electrochemical pumping of oxygen through an ionically conducting yttria-stabilized zirconia (YSZ) substrate with oxygen nonstoichiometry values derived from in situ chemical capacitance measurements between 500°C and 650°C. The defect chemical model was correlated with in-plane conductivity at different oxygen pumping levels in the temperature regime of 275~350°C. Thermodynamic parameters - thermal band gap, enthalpy of reduction and anion Frenkel-pair formation - were derived from a defect chemical analysis. The approach taken here can be extended to other layered oxides and opens up the possibility of accessing much broader oxygen activity limits and thereby an extended range of defect regimes and neighboring phases.

Monte Carlo simulation of the passage of γ-rays and α-particles in CsI

Theoretical and computational methods for simulating the creation of ionization tracks by fast ions in solids were applied to the passage of α-particles in CsI, an inorganic scintillator commonly used for radiation detection. The methods were implemented in a Monte Carlo program to simulate the interaction of α-particles, with incident energies of up to 1 MeV, with CsI. The simulations followed the fate of individual electron-hole pairs and included the formation of Frenkel pairs and thus allowed for a detailed description of the microscopic structure of ionization and defect tracks created by incident radiation. Simulations were also performed with γ-rays of the same energy to compare and contrast the ionization tracks obtained with both types of particles. Intrinsic properties such as the mean energy per electron-hole pair, Fano factor, maximum theoretical light yield, and spatial distributions of electron-hole pairs were computed for both α-particles and γ-rays. α-Particles created cylindrical tracks that were initially aligned with the incident direction and with initial radii of a few tens of nanometers, whereas γ-rays showed significant scattering, resulting in probability distributions with lower intensities and much greater radial extents.

Comprehensive Study on Microstructure and Mechanical Properties of Si Ion Irradiated Al 6063

Aluminum alloys are widely used as fuel cladding materials for research reactors. Neutron irradiation damage is a major concern in the nuclear industry, and it is of great interest to study irradiation damage by ions, which can cause similar damage to materials. In this study, Al 6063 alloy was irradiated with Si ions at 333 K up to 90 dpa to mimic the effects of both elemental Si, a transmutation product of thermal neutrons, and Frenkel pair formation, an effect of fast neutrons. The irradiation damage was investigated by transmission electron microscopy and the nanoindentation technique. The irradiated sample's net dislocation density was about 1.2·1014 m−2 in the vicinity of the voids and 2·1014 m−2 in the vicinity of the Si peak. The implanted Si peak concentration was ~ 6 at% at a depth of ~ 1100 nm. The formation of voids with an average size of 8 nm, and a peak number density of ~1022 m−3 was identified, and the void annihilation time dependence was found to be proportional to (1t)0.25.The amorphous to crystalline structure transformation temperature of the intermetallic Al8Fe2Si occurs at 723 K and follows the Johnson-Mell-Avrami diffusion control model. The hardness and estimated ultimate tensile strength of the irradiated surface layer, obtained by the nanoindentation measurements, increased by ~65% in comparison to the unirradiated sample. A similar increase in yield stress and ultimate tensile strength published values of neutron irradiation damage, occurs at much higher dpa (~260 dpa). This difference is suggested to be related to the high dpa rate and higher at%Si/dpa ratio in the present ion irradiation experiments.

Elastic Softening in Zirconia During Stage III of Flash Experiments

We report in-situ measurements of the elastic modulus of dense polycrystalline zirconia held in a constant state of flash, i.e. during Stage III of flash experiments. The modulus, measured with and without flash, is lowered by 10 - 35 %, in the temperature range of 1100-1400 oC. The specimen temperature, measured with a pyrometer, was consistent with the estimate from a black body radiation model. These measurements echo recent publications on the role of phonons in the onset of flash, namely the Debye temperature as a lower bound for flash, and the formation of Frenkel pairs in molecular dynamics calculations by the proliferation of phonons above the Debye temperature. However, several questions remain, for example, the rise in electrical conductivity, and electroluminescence that are nearly universal signatures of the flash phenomenon.

Effects of applied strain on defect production and clustering in FCC Ni

The structural materials of nuclear reactors are subjected to an environment of complex mechanical stresses, which has been reported to significantly affect the primary radiation damage suffered by the structure. In this study, we perform classical molecular dynamics simulations to study the effects of mechanical strains on the defect morphology due to primary radiation damage for a collision energy of 5 keV in face-centered cubic Ni. We consider the effects of four representative types of mechanical strains: hydrostatic, tetragonal shear, monoclinic shear, and uniaxial strains, and we discuss three aspects of the defect morphology: 1) time evolution of generated Frenkel pairs, 2) size distribution of defect clusters, and 3) fraction of clustered defects. We find that volumetric and anisotropic strains differently and significantly influence the primary radiation damage to the structure via affecting the intrinsic physical properties associated with the generation, formation, migration, or binding of Frenkel pairs; this result is generally consistent with those of many simulations. We believe our current study can aid in providing a fundamental mechanistic understanding of primary radiation damage in Ni-based alloys (for e.g., high-entropy alloys) as well as the subsequent long-term microstructural evolution.

Defect Chemistry and Oxygen Ionic Conducting Properties of Gd2O3 at Elevated Temperatures

Gadolinium sesquioxide (Gd2O3), an oxide exhibiting outstanding thermal stability, wide bandgap (>5eV) and relatively high dielectric constant (>14), is attractive as a high-k dielectric for CMOS, particularly when high temperature processing is required. However, in contrast to its conventional application as an electrical insulator, recent research on magneto-ionic devices suggests that Gd2O3 can serve as a solid electrolyte for field driven devices operating near room temperature. [1][2] According to previous research, proton transport in Gd2O3 is relatively swift among the binary oxides. [3] However, the defect chemistry and the oxygen ion conductivity of Gd2O3 have only been researched to a limited extent. In this work, we investigate the defect chemistry and electrical properties of doped and phase controlled Gd2O3 by impedance spectroscopy, as a function of oxygen partial pressure and temperature.Nominally undoped, as well as intentionally donor and acceptor doped cubic and monoclinic phase polycrystalline specimens of Gd2O3 were prepared. Studies on the two different phases were possible since the monoclinic phase can stabilize below ~1420 K as a metastable phase. Ca2+& Sr2+and Ce4+& Zr4+ were used as acceptor and donor dopants respectively. 2-point impedance measurements were conducted on samples over a wide range of pO2 (10-12 atm – 1 atm) and for 5 isotherms ranging from 695 – 895 ˚C.By analyzing the measured conductivity in terms of the developed defect model, relevant thermodynamic and kinetic parameters were extracted. In bixbyite (defective fluorite) phase Gd2O3, the formation enthalpy of anion Frenkel pairs was found to be 1.8 eV, lower than those of related fluorite oxides. The monoclinic phase Gd2O3 showed a somewhat higher, but still a rather low anion Frenkel pair formation enthalpy (2.1eV). Surprisingly, the more close-packed monoclinic structured Gd2O3 exhibited higher oxygen ion conductivity than more open cubic structured Gd2O3. The most highly ionic conducting samples of each phase were Ce doped cubic and Sr doped monoclinic Gd2O3, although the donor doped cubic had a significantly lower ionic conductivity and larger migration energy (1.7 eV and 0.8 eV respectively). All the Gd2O3 specimens showed mixed ionic and p-type conduction in synthetic air within the measured temperature range, implying excess oxygen nonstoichiometry.