Critical Amorphization Temperature Research Articles

Ion-irradiation-induced structural transitions in orthorhombic Ln2TiO5

The response of a material to a high radiation field is important when selecting materials for nuclear applications, such as structural materials, nuclear waste forms and inert matrix fuels. In the present study, the radiation response of orthorhombic, rare-earth titanates, Ln2TiO5 (Ln=La, Nd, Sm, Gd, Dy and Y), was investigated by 1MeV Kr2+ ion bombardment at temperatures ranging from 50 to 1073K. In situ transmission electron microscopy revealed that the radiation tolerance and irradiation-induced structural transitions vary largely with composition. Y2TiO5 exhibits the lowest critical amorphization temperature (Tc=623K), above which crystals cannot be amorphized, which is consistent with its use in the form of nanoclusters in radiation-resistant oxide-dispersion-strengthened steels. The disordered fluorite structure type of Ln2TiO5, with smaller Ln cations, formed as an intermediate phase prior to becoming fully amorphous. The fluorite structure type of Ln2TiO5, containing more vacancies as compared with that of Ln2Ti2O7, may exhibit enhanced ionic conductivity, which highlights an effective way of using ion beams to modify the properties of materials.

Influence of radiation damage on diffusion of fission products in silicon carbide

AbstractThe influence of irradiation induced damage on the transport of implanted species in poly and single crystalline silicon carbide is investigated. For this purpose published diffusion results of strontium, silver, iodine and cesium are compared with the associated evolution of defect profiles determined by α‐particle channelling in a backscattering geometry. Strong diffusion takes place in the amorphized surface layer of room temperature implanted 6H‐SiC during annealing at 1100 °C, which drops below the detection limit of 10‐21 m2 s‐1 as soon as re‐crystallization is completed. Diffusion in samples implanted above the critical amorphization temperature is only observed when simultaneously a significant reduction of defect density occurs. No diffusion into the undamaged bulk is detected at temperatures up to 1500 °C. The observed diffusion behaviour is explained by a defect related trapping and release mechanism. Normal grain boundary diffusion of silver and iodine occurs in CVD‐SiC. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of recombination kinetics and grain size in radiation-induced amorphization

Using a rate theory model for a generic one-component material, we investigated interactions between grain size and recombination kinetics of radiation-induced defects. Specifically, by varying parametrically nondimensional kinetic barriers for defect diffusion and recombination, we determined the effect of these parameters on the shape of the dose to amorphization versus temperature curves. We found that whether grain refinement to the nanometer regime improves or deteriorates radiation resistance of a material depends on the barriers to defect migration and recombination, as well as on the temperature for the intended use of the material. We show that the effects of recombination barriers and of grain refinement can be coupled to each other to produce a phenomenon of interstitial starvation. In interstitial starvation, a significant number of interstitials annihilate at the grain boundary, leaving behind unrecombined vacancies, which in turn amorphize the material. The same rate theory model with material-specific parameters was used to predict the grain-size dependence of the critical amorphization temperature in SiC. Parameters for the SiC model were taken from ab initio calculations. We find that the fine-grained SiC has a lower radiation resistance when compared to the polycrystalline SiC due to the presence of high-energy barrier for recombination of carbon Frenkel pairs and due to the interstitial starvation phenomenon.

Dynamic recovery in silicate-apatite structures under irradiation and implications for long-term immobilization of actinides

The irradiation responses of Ca2La8(SiO4)6O2 and Sr2Nd8(SiO4)6O2 with the apatite structure are investigated to predict their long-term behaviour as host phases for immobilization of actinide elements from the nuclear fuel cycle. Different ions and energies are used to study the effects of dose, temperature, atomic displacement rate and ionization rate on irradiation-induced amorphization and recrystallization. The dose for amorphization increases with temperature in two stages, below and above 150 K. In the high temperature stage relevant to actinide immobilization, the increase of amorphization dose with temperature exhibits a strong dependence on the ratio of ionization rate to displacement rate for the different ions. Data analysis using a dynamic model for amorphization reveals that ionization-induced processes, with activation energy of 0.15 ± 0.02 eV, dominate dynamic recovery for ions from Ne through Xe. For heavier Au ions or for alpha-recoil nuclei emitted in alpha decay of actinides, ionization becomes less dominant and dynamic recovery is controlled primarily by thermally-driven processes. In post-irradiation annealing studies of amorphous samples, epitaxial thermal recrystallization is observed at 1123 K, and irradiation-enhanced nucleation of nanocrystallites is observed under irradiation with heavier ions. The recrystallization temperature under irradiation decreases with increasing ion mass to a value of ∼823 K, which also defines the thermally-driven critical temperature for amorphization under irradiation with heavy ions. Some partial recovery due to alpha particle irradiation at 300 K is observed that suggests a self-healing mechanism in apatite phases containing actinides. Based on the results and dynamic model, the temperature and time dependences of amorphization in silicate-apatite host phases for actinide immobilization are predicted.

Damage formation and recovery in Fe implanted 6H–SiC

Silicon carbide doped with magnetic ions such as Fe, Mn, Ni or Co could make this wide band gap semiconductor part of the diluted magnetic semiconductor family. In this study, we report the implantation of 6H–SiC single crystals with magnetic 56Fe+ ions with an energy of 150keV. The samples were implanted with 5×1014Fe+/cm2 and 1×1016Fe+/cm2 at different temperatures to study the damage formation and lattice site location. The samples were subsequently annealed up to 1500°C in vacuum in order to remove the implantation damage. The effect of the annealing was followed by Rutherford Backscattering/Channeling (RBS/C) measurements. The results show that samples implanted above the critical amorphization temperature reveal a high fraction of Fe incorporated into regular sites along the 〈0001〉 axis. After the annealing at 1000°C, a maximum fraction of 75%, corresponding to a total of 3.8×1014Fe+/cm2, was measured in regular sites along the 〈0001〉 axis.A comparison is made between the observed annealing behavior and the one measured in low fluence (2×1013cm−2) β emission channeling experiments using the radioactive isotope 59Fe (t1/2=45d), where the sample was implanted at room temperature and the β− emission channeling yield was measured by means of a position-sensitive detector.

Ion irradiation of novel yttrium/ytterbium-based pyrochlores: The effect of disorder

Pyrochlores based on the general composition Ln2TiO5 (Ln2.67Ti1.33O6.67) and Ln2Ti2O7, where Ln=Y or Yb, have been irradiated through the crystalline–amorphous transition with 1MeV Kr ions at the IVEM-TANDEM facility, Argonne National Laboratory. The results show that the Tc (critical temperature for amorphization) differs significantly between each series, e.g. for Y2TiO5 it is 589±18K and for Y2Ti2O7 665±33K. The difference suggests that recovery from damage is more rapid with increasing Ln content, i.e. a lower Tc for amorphization. These results are discussed in the context of the melting points of each phase, atomic disorder, the pyrochlore–fluorite order–disorder transition and the implications for oxide dispersion-strengthened additives.

Role of Electronic Excitation in the Amorphization of Ge-Sb-Te Alloys

First-principles molecular dynamics simulation reveals the effects of electronic excitation in the amorphization of Ge-Sb-Te. The excitation makes the phase change an element-selective process, lowers the critical amorphization temperature considerably, for example, to below 700K at a 9% excitation, and reduces the atomic diffusion coefficient with respect to that of melt by at least 1 order of magnitude. Noticeably, the resulting structure has fewer wrong bonds and significantly increased phase-change reversibility. Our results point to a new direction in manipulating ultrafast phase-change processes with improved controllability.

Response of synthetic coffinite to energetic ion beam irradiation

Coffinite, USiO 4, is one of the two most abundant and important naturally occurring U 4+ phases (the other is UO 2), and it is an alteration product of the UO 2 in spent nuclear fuel when in contact with silica-rich groundwater under reducing conditions. Despite its ubiquity, there are very limited data on the response of coffinite to radiation. Here, we present the results of the first systematic investigation of energetic ion beam irradiation (1 MeV Kr 2+) of ultra-fine, synthetic coffinite (20–50 nm). In situ transmission electron microscopy (TEM) showed that the crystalline-to-amorphous transformation occurs at a relatively low dose, ∼0.27 displacements per atom (dpa) at room temperature. The critical temperature, T c , above which coffinite cannot be amorphized, is low (∼608 K). Synthetic coffinite is more stable as compared with isostructural zircon (ZrSiO 4; T c = 1000 K) and thorite (ThSiO 4; T c above 1100 K) upon ion beam irradiation at elevated temperature, suggesting enhanced defect annealing behavior in nano-sized synthetic coffinite. Irradiation was found to decrease the temperature required to induce phase decomposition process in coffinite upon thermal annealing. A good correlation among the critical amorphization temperature, T c , phase decomposition temperature, T f , and the temperature range of the two-phase (ZrO 2 and SiO 2) co-existed region was identified.

Effects of dynamic recovery on amorphization kinetics in 6H-SiC

Irradiation-induced amorphization in 6H-SiC has been previously studied as a function of irradiation temperature for electrons and ions ranging from Ne to Au. Analysis of these data with a dynamic model for amorphization reveals that the amorphization dose increases and the critical amorphization temperature decreases as the ratio of in-cascade ionization to displacement rates increases. Model fits to the data yield the ratio of radiation-induced recovery cross section, σr, to damage cross section, σd, and an activation energy of 0.12±0.01eV for irradiation-induced recovery. The critical temperature exhibits a linear dependence on 1/ln(σr/σd), consistent with the model.

Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation–anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation–anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

Irradiation behavior of SrTiO3 at temperatures close to the critical temperature for amorphization

Damage accumulation on both the Sr and Ti sublattices in strontium titanate (SrTiO3) has been investigated under 1.0MeV Au+ irradiation at 360 and 400K, close to the critical temperature for amorphization (∼370K). Under irradiation at 360K, the relative disorder on both sublattices follows a nonlinear dependence on ion dose. Amorphization starts from the damage peak region (at a depth of 60nm) and grows toward the surface and into the bulk. At 400K, the evolution of point defects to extended defects occurs as ion fluence increases. The disorder initially peaks at a depth of 60nm, saturates at a disorder level of ∼0.75, and then decreases with further irradiation. At an ion fluence of 6.0×1015cm−2, an amorphous layer of ∼10nm thickness is formed at the sample surface. After annealing at 375K for 1h, the buried amorphous layer formed during irradiation at 360K is recrystallized with planar defects and dislocation loops. However, the surface amorphous layer formed at 400K irradiation remains amorphous, and fewer defects are observed in the irradiated region. Irradiation-enhanced recrystallization due to high flux electron energy deposition is observed.

Ion beam irradiation of U-, Th- and Ce-doped pyrochlores

U-, Th- and Ce-doped pyrochlores were synthesized by solid state reaction: (Ca 0.62Gd 0.97U 0.23)(Zr 0.84Ti 1.34)O 6.9, (Ca 0.47Gd 0.95Th 0.40)(Zr 1.29Ti 0.89)O 7.05, (Ca 0.44GdTh 0.42)Zr 2.13O 7.05, Ca 0.91Th 0.84Zr 2.25O 7.09 and Ca 1.04Ce 0.97Ti 1.99O 6.96. Their response to the radiation damage from the recoils of the alpha-decay events has been simulated by 1 MeV Kr 2+ ion irradiation and studied by in situ transmission electron microscopy (TEM). An ion beam-induced pyrochlore-to-fluorite structural transition occurred for all compositions, and independent kinetics were observed for cation and anion disordering processes. Similar to the stoichiometric titanate and zirconate pyrochlores, the Ti-rich compositions are susceptible to ion beam-induced amorphization; while, the Zr-rich compositions are more radiation “resistant”. A close relation between the critical amorphization temperature of the U-, Th- and Ce-doped pyrochlores and the average cation ionic radius ratio of the A- and B-sites was observed.

Radiation Damage in Pyrochlore and Related Compounds

ABSTRACTThe radiation damage properties of synthetic pyrochlore-defect fluorite compounds containing lanthanides on the A-site and Ti, Zr, Sn, and Hf on the B-site have been studied extensively using Kr ion irradiation. Using statistical analysis, we show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and defect energies. The best current model is able to predict the critical temperature to within about 80 degrees Kelvin. The model indicates that radiation tolerance is correlated with an increase in the X anion coordinate toward the value characteristic of the defect fluorite topology, a smaller unit cell dimension, and lower defect energies. Our analysis also demonstrates that radiation tolerance is promoted by an increase in the Pauling cation-anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. Of the two possible cation sites in ideal pyrochlore, the B-site cation appears to play the major role in bonding. This result is supported, for a subset of pyrochlore compounds, by ab initio calculations, which reveal a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

Ion-beam-induced amorphization and order-disorder transition in the murataite structure

Murataite (A3B6C2O22−x∕2,F4¯3m), a derivative of an anion-deficient fluorite structure, has been synthesized as different polytypes as a result of cation ordering. Ion-beam-induced amorphization has been investigated by 1-MeV Kr2+ ion irradiation with in situ transmission electron microscopy. The critical amorphization dose was determined as a function of temperature and the degree of structural disordering. A lower critical amorphization temperature (∼860K) was obtained for the disordered murataite as compared with that of the murataite superstructure (930to1060K). An ion-beam-induced ordered murataite to a disordered fluorite transition occurred in the murataite superstructure, similar to that observed in the closely related pyrochlore structure-type, A2B2O7. The ion-beam-induced defect fluorite structure is more energetically stable in the murataite structure with a higher degree of structural disordering, as compared with the murataite superstructure. This suggests that the degree of intrinsic structural disorder has a significant effect on the energetics of structural disordering process; this affects the tendency toward the order-disorder structural transition for fluorite-related compounds and their response to ion-beam-induced amorphization.

Temperature dependence of disorder accumulation and amorphization in Au-ion-irradiated6H−SiC

Disorder accumulation and amorphization in $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ single crystals irradiated with $2.0\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ ${\mathrm{Au}}^{2+}$ ions at temperatures ranging from $150\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been investigated systematically based on $0.94\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ ${D}^{+}$ channeling analyses along the ⟨0001⟩ axis. Physical models have been applied to fit the experimental data and to interpret the temperature dependence of the disordering processes. Results show that defect-stimulated amorphization in ${\mathrm{Au}}^{2+}$-irradiated $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ dominates the disordering processes at temperatures below $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while formation of clusters becomes predominant above $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Two distinctive dynamic recovery stages are observed over the temperature range from $150\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, resulting from the coupled processes of close-pair recombination and interstitial migration and annihilation on both sublattices. These two stages overlap very well with the previously observed thermal recovery stages. Based on the model fits, the critical temperature for amorphization in $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ under the ${\mathrm{Au}}^{2+}$ ion irradiation conditions corresponds to $501\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect

Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.

Effects of implantation temperature on damage accumulation in Al-implanted 4H–SiC

Damage accumulation in 4H–SiC under 1.1 MeV Al22+ irradiation is investigated as a function of dose at temperatures from 150 to 450 K. Based on Rutherford backscattering spectroscopy and nuclear reaction analysis channeling spectra, the damage accumulation on both the Si and C sublattices have been determined, and a disorder accumulation model has been fit to the data. The model fits indicate that defect-stimulated amorphization is the primary amorphization mechanism in SiC over the temperature range investigated. The temperature dependence of the cross section for defect-stimulated amorphization and the critical dose for amorphization indicate that two different dynamic recovery processes are present, which are attributed to short-range recombination and long-range migration of point defects below and above room temperature, respectively. As the irradiation temperature approaches the critical temperature for amorphization, cluster formation has an increasing effect on disorder accumulation, and ion flux plays an important role on the nature and evolution of disorder. Dislocation loops, which are mostly formed under high ion flux, act as sinks for point defects, thereby reducing the disorder accumulation rate.

Ion beam irradiation in La 2Zr 2O 7–Ce 2Zr 2O 7 pyrochlore

Generally, zirconate pyrochlores do not experience a radiation-induced transformation from the crystalline-to-amorphous state, but rather disorder to a defect fluorite structure-type. Thus Gd 2Zr 2O 7 has been proposed as a nuclear waste form for the immobilization of plutonium because of its radiation “stability”. In contrast, La 2Zr 2O 7 can be amorphized by a 1.5 MeV Xe + ion irradiation (∼5.5 dpa at room temperature), and the critical amorphization temperature is low (∼310 K). In this study we present data on ion beam irradiations of compositions in the solid solution: (La 1− x Ce x ) 2Zr 2O 7 ( x=0, 0.1, 0.2, 1). Ce is used as an analogue element for Pu because of similarities in charge and size. La 2Zr 2O 7 can be amorphized by a 1.0 MeV Kr + ion irradiation at 25 and 293 K at doses of ∼1.19 and ∼3.42 dpa, respectively, confirming that La 2Zr 2O 7 is susceptible to ion irradiation-induced amorphization. With the addition of 10 mol% Ce in lanthanum–zirconate pyrochlore structure, no ion irradiation-induced amorphization has been observed at room temperature. The critical amorphization dose for (La 0.9Ce 0.1) 2Zr 2O 7 at 25 K is ∼3.55 dpa, and with increasing Ce-content, a higher dose (∼5.20 dpa) is required to fully amorphize (La 0.8Ce 0.2) 2Zr 2O 7 at 25 K. No amorphization occurred for Ce 2Zr 2O 7 at 25 K at a dose of ∼7 dpa. These results suggest that the addition of Ce into the La 2Zr 2O 7 structure increases the stability of the La 2Zr 2O 7 waste form in a radiation environment, which may be attributed to the decreasing average radius of cations in the A-site, resulting from the smaller ionic radii of Ce 3+ (0.114 nm) and Ce 4+ (0.097 nm) as compared to La 3+ (0.116 nm). An ion beam-induced anion-disordered pyrochlore was observed prior to the final transformation to a disordered fluorite structure. The local structural evolution upon ion irradiation was also investigated for Ce-doped La 2Zr 2O 7 pyrochlores with different average A-site cation sizes and valence states using electron energy-loss spectrometry (EELS).

Correlation of Formation Enthalpies with Critical Amorphization Temperature for Pyrochlore and Monazite

AbstractSystematic studies of a series of RE-titanate pyrochlore single crystals determined the formation enthalpies, ΔH0f-ox, and the critical amorphization temperature, Tc, the temperature above which the crystal can no longer be amorphized. A negative linear correlation was observed between ΔH0f-ox and Tc. In general as the cation radius ratio, RA/RB, decreases the formation enthalpy becomes less exothermic, i.e. the pyrochlore structure becomes less thermodynamically stable while the susceptibilty to ion beam-induced amorphization decreases. The opposite relation, i.e. a positive linear correlation between ΔH0f-ox and Tc was observed for a series of REPO4 single crystals with both the monazite and xenotime structures. Both relations can be understood in terms of pyrochlore and orthophosphate crystal chemistry, their formation of point defects, and the role of ion-irradiation induced phase transformations.

Radiation-induced amorphization of rare-earth titanate pyrochlores

Single crystals of the entire series of ${A}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ $(A=\mathrm{Sm}$ to Lu, and Y) pyrochlore compounds were irradiated by 1-MeV ${\mathrm{Kr}}^{+}$ ions at temperatures from 293 to 1073 K, and the microstructure evolution, as a function of increasing radiation fluence, was characterized using in situ transmission electron microscopy (TEM). The critical amorphization temperature, ${T}_{c},$ generally increases from \ensuremath{\sim}480 to \ensuremath{\sim}1120 K with increasing A-site cation size (e.g., 0.977 \AA{} for ${\mathrm{Lu}}^{3+}$ to 1.079 \AA{} for ${\mathrm{Sm}}^{3+}).$ An abnormally high susceptibility to ion beam damage was found for ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ (with the highest ${T}_{c}$ of \ensuremath{\sim}1120 K). Factors influencing the response of titanate pyrochlores to ion irradiation-induced amorphization are discussed in terms of cation radius ratio, defect formation, and the tendency to undergo an order-disorder transition to the defect-fluorite structure. The resistance of the pyrochlore structure to ion beam-induced amorphization is not only affected by the relative sizes of the A- and B-site cations, but also the cation electronic configuration and the structural disorder. Pyrochlore compositions that have larger structural deviations from the ideal fluorite structure, as evidenced by the smaller $48f$ oxygen positional parameter, x, are more sensitive to ion beam-induced amorphization.