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The atomic structures of the lanthanide tantalates, Ln3TaO7, series (Ln = Pr, Tb, Dy, Ho, Tm, Yb) were systematically investigated using total scattering techniques.

Critical metals (CMs), including rare-earth elements (REEs), lithium (Li), and beryllium (Be), provide the foundation for the development of advanced technologies and are essential to national security and economic development in many countries. While China has most of the world’s REE processing capacity, the Democratic People’s Republic of Korea (DPRK) reportedly possesses significant untapped CM resources. With the size and total value estimated to be worth trillions of dollars, its large resources are reputedly the DPRK’s most significant internal asset. Despite the oft-cited abundance of CMs, evidence-based analyses of the mineral resources are limited. We provide a comprehensive analysis of CM resources in the DPRK based on geological analysis of potential deposits. Three main REE deposits at Ryongpo, Cheolsan, Abdong, and various Li and Be deposits are investigated. Extensive analyses indicate that previous estimates of a large CM resource base for the DPRK are exaggerated. There is no clear evidence for significant resources that are economical. While there remains a possibility of identifying new ore deposits, for the DPRK to access its CM resources, it will need to develop a nationwide exploration and exploitation strategy to attract foreign investments. A strategy of investment and investigation might provide a springboard for cooperation between the DPRK and other nations as these resources are discovered and developed.

Minerals are solid state nuclear track detectors - nuclear recoils in a mineral leave latent damage to the crystal structure. Depending on the mineral and its temperature, the damage features are retained in the material from minutes (in low-melting point materials such as salts at a few hundred degrees C) to timescales much larger than the 4.5 Gyr-age of the Solar System (in refractory materials at room temperature). The damage features from the $O(50)$ MeV fission fragments left by spontaneous fission of $^{238}$U and other heavy unstable isotopes have long been used for fission track dating of geological samples. Laboratory studies have demonstrated the readout of defects caused by nuclear recoils with energies as small as $O(1)$ keV. This whitepaper discusses a wide range of possible applications of minerals as detectors for $E_R \gtrsim O(1)$ keV nuclear recoils: Using natural minerals, one could use the damage features accumulated over $O(10)$ Myr$-O(1)$ Gyr to measure astrophysical neutrino fluxes (from the Sun, supernovae, or cosmic rays interacting with the atmosphere) as well as search for Dark Matter. Using signals accumulated over months to few-years timescales in laboratory-manufactured minerals, one could measure reactor neutrinos or use them as Dark Matter detectors, potentially with directional sensitivity. Research groups in Europe, Asia, and America have started developing microscopy techniques to read out the $O(1) - O(100)$ nm damage features in crystals left by $O(0.1) - O(100)$ keV nuclear recoils. We report on the status and plans of these programs. The research program towards the realization of such detectors is highly interdisciplinary, combining geoscience, material science, applied and fundamental physics with techniques from quantum information and Artificial Intelligence., Comment: 115 pages, many pictures of tracks. Please see the source file for higher resolution versions of some plots. v2: matches the published version

The Democratic People’s Republic of Korea (DPRK)’s only confirmed uranium mill is within the Pyongsan uranium mining complex. The ore processing pathway and the production capacity for uranium conc...

Small modular reactors (SMRs; i.e., nuclear reactors that produce elec each) have garnered attention because of claims of inherent safety features and reduced cost. However, remarkably few studies have analyzed the management and disposal of their nuclear waste streams. Here, we compare three distinct SMR designs to an 1,100-MW elec pressurized water reactor in terms of the energy-equivalent volume, (radio-)chemistry, decay heat, and fissile isotope composition of (notional) high-, intermediate-, and low-level waste streams. Results reveal that water-, molten salt–, and sodium-cooled SMR designs will increase the volume of nuclear waste in need of management and disposal by factors of 2 to 30. The excess waste volume is attributed to the use of neutron reflectors and/or of chemically reactive fuels and coolants in SMR designs. That said, volume is not the most important evaluation metric; rather, geologic repository performance is driven by the decay heat power and the (radio-)chemistry of spent nuclear fuel, for which SMRs provide no benefit. SMRs will not reduce the generation of geochemically mobile 129 I, 99 Tc, and 79 Se fission products, which are important dose contributors for most repository designs. In addition, SMR spent fuel will contain relatively high concentrations of fissile nuclides, which will demand novel approaches to evaluating criticality during storage and disposal. Since waste stream properties are influenced by neutron leakage, a basic physical process that is enhanced in small reactor cores, SMRs will exacerbate the challenges of nuclear waste management and disposal.

The diffusion of defects (e.g., vacancies and interstitials) and elements used for dating (e.g., He and Pb) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of 238U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils from the alpha-decay of 238U and 232Th. We simulate alpha-decay induced track-shortening by combining ion accelerator irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of self-irradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology.

International audience; Weberite-type oxides are a family of oxides with A3BO7 stoichiometry that can adopt different space groups depending on chemical composition. There is a discrepancy in previous studies as to whether Y3TaO7 is the orthorhombic C2221 or the Ccmm space group. Here, we describe the short- and long-range structural properties of weberite-type Y3TaO7 using neutron total scattering data, which has a high sensitivity to the oxygen sublattice. Simultaneous analysis of both short- and long-range structural data via conventional Rietveld and small box modelling demonstrates that Y3TaO7 is best modeled as C2221. While the Ccmm describes equally well the long-range structure, pair distribution function analysis revealed that the local atomic configuration can be best modelled as C2221. This is corroborated by first-principles calculations that confirm the energetic preference of the C2221 over Ccmm. Neutron total scattering data are reported for the first time for the Y3TaO7 weberite-type ceramics.

A key property for high-frequency inductive applications is the magnetic softness, ideally characterized by complete reversibility of the hysteresis curve. Magnetic softness of alloys depends on the dimensional ratio of grain size to the correlation length. The random anisotropy model predicts the optimum magnetic softness if the crystallite size is well below the correlation length. Based on this theory, a common strategy in processing is to obtain extremely fine nanoscale grains via rapidly solidified alloys, followed by a controlled vacuum anneal. As is well known, this conventional approach requires two demanding experimental conditions: 1) an extremely high quench rate, >105 K/s, and 2) a high vacuum anneal near 600 °C to avoid oxidation. Furthermore, the completely amorphous ribbon is extremely elastic and mechanically strong and is not easily made into the fine powder form for net-shaping and 3-D printing of the soft magnets. Therefore, annealing to induce crystallization is a necessary step before using any conventional powdering techniques, such as ball milling. In this article, we report a new processing strategy of making Fe77Ni5.5Co5.5Zr7B4Cu powders directly from the as-spun ribbons without any crystallization annealing step. This is achieved by melt spinning of the alloy at a relatively low quenching rate, resulting in partially crystallized ribbons that deform plastically, a behavior that is in sharp contrast to its amorphous counterpart. The ribbon samples are ball milled into fine powders by tungsten carbide (WC) at various time intervals. Magnetization measurements show extremely low coercive field (Hc) of 0.51 Oe for the powder that is ball milled for 60 min. However, Hc consistently increases with ball milling time, as the samples are strain-hardened, introducing more magneto anisotropy. The coercive fields of these samples are easily recovered during the high-temperature magnetization measurement at 900 K, and an Hc of 3.78 Oe is obtained upon recovery. This strategy paves a new way for straightforwardly making soft magnetic powders while avoiding high quenching rate requirement and the possibility of oxidation during annealing.

The mining and milling of uranium ore is the first step in the production of fissile material and is a rate-limiting step for the indigenous production of nuclear weapons in the Democratic People's Republic of Korea (DPRK). This study reports a geologic analysis of uranium mines in the DPRK in order to bound the state's potential uranium production. The analysis suggests that the uranium deposits of the possible mines are of four types: (1) black shale (metamorphosed organic shale); (2) limestone; (3) granite/metasomatic; and (4) metamorphic deposits. Comparison with geologically-related, uranium-bearing host rocks in the Republic of Korea (ROK) indicate that DPRK uranium mines are associated with medium-to-high quantities of average low-grade ore (0.001-0.04 wt.% uranium). Using this low-grade ore, expansion of the state's nuclear arsenal would require the extraction of larger quantities of uranium ore than has been previously assumed. The DPRK's geology could, therefore, limit the future development of its nuclear weapons program.

Recent efforts have been made toward the integration of the back-end of the nuclear fuel cycle in the United States. The back-end integration seeks to address several management challenges: 1) current storage practices are not optimized for transport and disposal; 2) the impact of interim storage on the disposal strategy needs to be evaluated; and 3) the back-end is affected by—and affects—nuclear fuel cycle and energy policy choices. The back-end integration accounts for the various processes of nuclear waste management—onsite storage, consolidated storage, transport and geological disposal. Ideally, these processes should be fully coupled so that benefits and impacts can be assessed at the level of the full fuel cycle. The paper summarizes the causes and consequences of the absence of integration at the backend of the nuclear fuel cycle in the U.S., critically reviews ongoing integration efforts, and suggests a framework that would support the back-end integration.

Microcrystalline CeO2, ThO2, and UO2 were irradiated with 198 MeV 132Xe ions to the same fluence at temperatures ranging from 25 °C to 700 °C then characterized by synchrotron X-ray diffraction and X-ray absorption spectroscopy. All samples retain crystallinity and their nominal fluorite-type phase at a fluence of 1.5 × 1013 ions/cm2. Both CeO2 and ThO2 display defect-induced unit cell expansion after irradiation at room temperature (∼0.15% and ∼0.10%, respectively), yet as irradiation temperature increases, the maximum swelling produced decreases to ∼0.02%. Alternatively, UO2 shows an initial contraction in unit cell parameter (approximately −0.05%), for room temperature irradiation most likely related to irradiation-enhanced annealing or irradiation-induced oxidation. At higher temperatures (above 200 °C), UO2 begins to swell, surpassing its unit cell parameter prior to irradiation (∼0.05%), an effect which could be attributed to minor reduction in uranium oxidation state in vacuum. However, while CeO2 irradiated at room temperature undergoes partial reduction, both UO2 and ThO2 exhibit no measurable change in cation oxidation state as evidenced by X-ray absorption spectroscopy. All samples display a decrease in irradiation-induced heterogeneous microstrain as a function of increasing irradiation temperature.

Non-ideal thermodynamics of solid solutions can greatly impact materials degradation behavior. We have investigated an actinide silicate solid solution system (USiO4–ThSiO4), demonstrating that thermodynamic non-ideality follows a distinctive, atomic-scale disordering process, which is usually considered as a random distribution. Neutron total scattering implemented by pair distribution function analysis confirmed a random distribution model for U and Th in first three coordination shells; however, a machine-learning algorithm suggested heterogeneous U and Th clusters at nanoscale (~2 nm). The local disorder and nanosized heterogeneous is an example of the non-ideality of mixing that has an electronic origin. Partial covalency from the U/Th 5f–O 2p hybridization promotes electron transfer during mixing and leads to local polyhedral distortions. The electronic origin accounts for the strong non-ideality in thermodynamic parameters that extends the stability field of the actinide silicates in nature and under typical nuclear waste repository conditions.

Boron carbide control rods remain in the fuel debris of the damaged reactors in the Fukushima Daiichi Nuclear Power Plant, potentially preventing re-criticality; however, the state and stability of the control rods remain unknown. Sensitive high-resolution ion microprobe analyses have revealed B-Li isotopic signatures in radioactive Cs-rich microparticles (CsMPs) that formed by volatilization and condensation of Si-oxides during the meltdowns. The CsMPs contain 1518-6733 mg kg

International audience; Orthosilicates adopt the zircon structure-types (I41/amd), consisting of isolated SiO4 tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development and material science. Stetindite (CeSiO4) and coffinite (USiO4) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 °C to ~850 °C. Stetindite maintains its I41/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high temperature Raman and FTIR spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C, and then thermally decompose into a mixture of UO2 and SiO2. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high temperature structures of two coffinite-thorite solid solutions, uranothorite (UxTh1-xSiO4), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO4, USiO4, U0.46Th0.54SiO4, and U0.9Th0.1SiO4 were determined to be αV = 4.21 × 10-6 °C-1 , 14.29 × 10-6 °C-1 , 17.21 × 10-6 °C-1 , and 17.23 × 10-6 °C-1 , respectively.

Orthosilicates adopt the zircon structure types (

Zircon (ZrSiO4, I41/amd) can accommodate actinides, such as thorium, uranium, and plutonium. The zircon structure has been determined for several of the end-member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO4), a surrogate of PuSiO4. For the first time, the standard enthalpy of formation of CeSiO4 was obtained by high temperature oxide melt solution calorimetry to be -1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO2 and SiO2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO4 from this work, along with those previously reported for USiO4 and ThSiO4, we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO4 are then determined with a discussion of future strategies for efficiently immobilizing Pu or minor actinides in the zircon structure.

International audience; Pyrochlore (A 2 B 2 O 7) is an important, isometric structure-type because of its large variety of compositions and structural derivatives that are generally related to different disordering mechanisms at various spatial scales. The disordering is key to understanding variations in properties, such as magnetic behavior or ionic conduction. Neutron and X-ray total scattering methods were used to investigate the degree of structural disorder in the Ho 2 Ti 2-x Zr x O 7 (x = 0.0-2.0, dx =0.25) solid solution series as a function of the Zr-content, x. Ordered pyrochlores (Fd 3m) disorder to defect fluorite (Fm 3m) via cation and anion disordering. Total scattering experiments with sensitivity to the cation and anion sublattices provide unique insight into the underlying atomic processes. Using simultaneous Rietveld refinement (long-range structure) and small-box refinement PDF analysis (short-range structure), we show that the series undergoes a rapid transformation from pyrochlore to defect fluorite at x ~ 1.2, while the short-range structure exhibits a linear increase in a local weberite-type phase, C222_1 , over the entire composition range. Enthalpies of formation from the oxides DH f;ox determined using high temperature oxide melt solution calorimetry support the structural data and provide insight into the effect of local ordering on the energetics of disorder. The measured enthalpies of mixing are negative and are fit by a regular solution parameter of W =-31.8 +/- 3.7 kJ mol-1. However, the extensive short-range ordering determined from the structural analysis strongly suggests that the entropies of mixing must be far less positive than implied by the random mixing of a regular solution. We propose a local disordering scheme involving the pyrochlore 48f to 8a site oxygen Frenkel defect that creates 7-coordinated Zr sites contained within local weberite-type coherent nanodomains. Thus, the solid solution is best described as a mixture of two phases, with the weberite-type nanodomains triggering the long-range structural transformation to defect fluorite after accumulation above a critical concentration (50% Ti replaced by Zr).

Zircon (ZrSiO

Short-range structure in disordered materials can be predicted by application of Pauling’s rules., Disordered crystalline materials are used in a wide variety of energy-related technologies. Recent results from neutron total scattering experiments have shown that the atomic arrangements of many disordered crystalline materials are not random nor are they represented by the long-range structure observed from diffraction experiments. Despite the importance of disordered materials and the impact of disorder on the expression of physical properties, the underlying fundamental atomic-scale rules of disordering are not currently well understood. Here, we report that heterogeneous disordering (and associated structural distortions) can be understood by the straightforward application of Pauling’s rules (1929). This insight, corroborated by first principles calculations, can be used to predict the short-range, atomic-scale changes that result from structural disordering induced by extreme conditions associated with energy-related applications, such as high temperature, high pressure, and intense radiation fields.

Pyrochlore (A

Most of the highly radioactive spent nuclear fuel (SNF) around the world is destined for final disposal in deep-mined geological repositories. At the end of the fuel’s useful life in a reactor, about 96% of the SNF is still UO2. Thus, the behaviour of UO2 in SNF must be understood and evaluated under the weathering conditions of geologic disposal, which extend to periods of hundreds of thousands of years. There is ample evidence from nature that many uranium deposits have experienced conditions for which the formation of coffinite, USiO4, has been favoured over uraninite, UO2+x, during subsequent alteration events. Thus, coffinite is an important alteration product of the UO2 in SNF. Here, we present the first evidence of the formation of coffinite on the surface of UO2 at the time scale of laboratory experiments in a solution saturated with respect to amorphous silica at pH = 9, room temperature and under anoxic conditions.

Diamondoids are promising precursors for facile diamond synthesis using laser-induced high-pressure, high-temperature conditions., Carbon-based nanomaterials have exceptional properties that make them attractive for a variety of technological applications. Here, we report on the use of diamondoids (diamond-like, saturated hydrocarbons) as promising precursors for laser-induced high-pressure, high-temperature diamond synthesis. The lowest pressure and temperature (P-T) conditions that yielded diamond were 12 GPa (at ~2000 K) and 900 K (at ~20 GPa), respectively. This represents a substantially reduced transformation barrier compared with diamond synthesis from conventional (hydro)carbon allotropes, owing to the similarities in the structure and full sp3 hybridization of diamondoids and bulk diamond. At 20 GPa, diamondoid-to-diamond conversion occurs rapidly within

Plutonium association and transport by ferrihydrite has been confirmed in the groundwater system of Lake Karachai at the Mayak Production Association, Russia (Novikov et al. 2006). Since this initial description, there has been substantial progress in understanding the mechanisms that control the concentrations and distribution of Pu at the Mayak site. This chapter is a concise review of work during the past 10 years that utilizes to advantage recent advances in nanoscale, analytical techniques. The structure of ferrihydrite (P63mc) was determined using a pair-distribution function analysis. Scanning transmission electron microscopy of Mayak colloids resolved the occurrence of mixed silica nano-colloids and ferrihydrite. Thermodynamic properties for aqueous Pu-species have been updated. Inner-sphere surface complexation was determined for adsorbed Pu using synchrotron-based X-ray absorption spectroscopy. The adsorbed Pu can transform into Pu-dioxide nanoparticles as revealed by high-resolution transmission electron microscopy. Plutonium polymerization can also proceed by aggregation of cubic Pu-dihydroxy building blocks of Pu(OH)22+•6H2O. Plutonium desorption occurs associated with re-adsorption at neutral pH. To establish comprehensive model of Pu transport at the Mayak site, the detailed mechanisms described above must be considered. Rigorous characterization of colloids is necessary and these results must be compared with results obtained from experimental studies. The state-of-the-art knowledge on the Pu association with Fe hydroxides is now well enough understood that safety assessments no longer need to rely on the classical Kd approach.

Oxide materials find use throughout the nuclear fuel cycle, from actinide-bearing ores and commercial reactor fuels to wasteforms for radionuclide disposal. In geological, reactor, and waste repository conditions, these materials are often exposed to ionizing radiation, high temperatures, and mechanical stresses. Recently, a large body of work has investigated the response of actinide oxides and analogue compounds to extreme environments, including the individual and combined effects of radiation, temperature, and pressure. Study of the phase behavior of these materials under such conditions can lead to improved understanding of their stability throughout the nuclear fuel cycle, as well as strategies for the mitigation of associated performance degradation. This article reviews some recent experimental work on this topic, highlighting advanced techniques developed for the exposure of materials to extreme environments, and for the in situ characterization of their structural and chemical responses. The study of two classes of nuclear materials is reviewed: binary oxides typical of nuclear fuels, and complex oxides typical of geological materials and wasteforms. Particular emphasis is placed on the individual and combined effects of modifications to the atomic and electronic structures of materials by exposure to extreme environments.

Nuclear forensics involves determination of the origin and history of interdicted nuclear materials based on the detection of signatures associated with their production and trafficking. The surface oxidation undergone by UO2 when exposed to air is a potential signature of its atmospheric exposure during handling and transport. To assess the sensitivity of this oxidation to atmospheric parameters, surface sensitive grazing-incidence x-ray diffraction (GIXRD) measurements were performed on UO2 samples exposed to air of varying relative humidity (34%, 56%, and 95% RH) and temperature (room temperature, 50 °C, and 100 °C). Near-surface unit cell contraction was observed following exposure, indicating oxidation of the surface and accompanying reduction of the uranium cation ionic radii. The extent of unit cell contraction provides a measure of the extent of oxidation, allowing for comparison of the effects of various exposure conditions. No clear influence of relative humidity on the extent of oxidation was observed, with samples exhibiting similar degrees of unit cell contraction at all relative humidities investigated. In contrast, the thickness of the oxidized layers increased substantially with increasing temperature, such that differences on the order of 10 °C yielded readily observable crystallographic signatures of the exposure conditions.

The structural evolution of lanthanide A2TiO5 (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy2TiO5 and Gd2TiO5 form P21am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy2TiO5 and Er2TiO5 as well as defect-fluorite-type Yb2TiO5 form Pnma at ∼21 GPa, followed by Im3m. Hexagonal Dy2TiO5 forms Pnma directly, although a small amount of remnants of hexagonal Dy2TiO5 is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy2TiO5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er2TiO5. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A2BO5 composition at high pressure.

Phase transitions induced in hcp Mn+1AXn phases (Ti2AlN, Ti2AlC, and Ti4AlN3) by 1 MeV Au+ ion irradiation were investigated, over a series of ion fluences ranging from 1 × 1014 to 2 × 1016 ions cm−2, by transmission electron microscopy (TEM) and synchrotron grazing incidence X-ray diffraction (GIXRD). Irradiation-induced structural evolutions were observed using high-resolution TEM (HRTEM) imaging and selected area electron diffraction (SAED). Based on phase contrast imaging and electron diffraction pattern (EDP) simulations, the atomic-scale mechanisms for the phase transitions were determined. Transformations of the initial hcp phases to the intermediate γ-phases and fcc phases were driven by the formation of Ti/Al antisite defects and extended stacking faults induced by ion irradiation. By comparing the transformation behavior of Ti2AlN with that of Ti2AlC and Ti4AlN3 under the same irradiation conditions, using both the experimental data and first-principles calculations, the role of the X and n parameters in the radiation responses of Mn+1AXn phases were elucidated. The susceptibilities of materials in this Ti-Al-X (X = C, N) system to irradiation-induced phase transitions were determined with respect to the bonding characteristics and compositions of these MAX phases. Ti2AlC is slightly less susceptible to the radiation-induced phase transformation than Ti2AlN, which is attributed to the stronger Ti-Al bond covalency in Ti2AlN. Ti4AlN3 is more resistant to radiation-induced phase transformations than is Ti2AlN, due to the lower Al content and lower anion vacancy ratio in the irradiation-induced solid solution phases.

A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, or become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity.

The structural responses of A 2 BO 5 (A = Nd, Gd, and Yb; B = Ti) compositions irradiated by high-energy Au ions (2.2 GeV) were investigated using transmission electron microscopy, synchrotron X-ray diffraction and Raman spectroscopy. The extent of irradiation-induced amorphization depends on the size of the A-site cation, with smaller lanthanides having less susceptibility to the accumulation of radiation damage. In the track-overlapping regime, complete amorphization is observed in all three compounds, despite the ability of Yb 2 TiO 5 to incorporate a great deal of structural disorder into its initial defect-fluorite structure ( Fm-3m ). This is attributed to the high cation radius ratio (A:B = 2:1), which reduces the stability of the structure upon ion irradiation. The fully-amorphized samples were subsequently isochronally heated at temperature intervals from 100 °C to 850 °C. X-ray diffraction analysis indicated a similar damage recovery process in Nd 2 TiO 5 and Gd 2 TiO 5 , where both compositions recover their original structures ( Pnma ) at 850 °C. In contrast, Yb 2 TiO 5 exhibited recrystallization of a metastable, non-equilibrium orthorhombic phase at ~ 550 °C, prior to a transformation to the stable defect-fluorite phase ( Fm-3m ) at 625 °C. These compositional variations in radiation tolerance and thermal recovery processes are described in terms of the energetics of disordering during the damage and recrystallization processes.