As part of a broader study of ceramic nuclear waste-forms, four different lanthanide titanates were fabricated; La0.1Sm0.1Gd0.1Tb0.1Dy0.3Ho0.1Er0.2YbTiO5, Sm0.3Gd0.3Dy0.3Yb1.1TiO5, Sm0.1Gd0.4Dy0.4Yb1.1TiO5, and Sm0.2Gd0.2Dy0.2Yb1.4TiO5. The aim was to produce single-phase novel materials with cubic symmetry, capable of incorporating a wide variety of cations and with acceptable radiation tolerance. The chemistry flexibility and radiation tolerance are some of the major desirable properties for nuclear waste-form materials. By using multiple lanthanides the average lanthanide radius can be controlled and consequently the structure, along with properties such as radiation tolerance. The radiation tolerance was assessed using in situ 1 MeV krypton irradiation and transmission electron microscopy characterization. Those materials for which cubic symmetry was achieved displayed better radiation tolerance; a greater critical fluence of ions (F-c) was required for the crystalline to amorphous transition, and a lower temperature was required to maintain crystallinity (T-c) during irradiation.

Samples based on the general composition Ca1-xLa2/3xTiO3 have been irradiated in-situ, at the IVEM-TANDEM facility, through the crystalline-amorphous transition with 1 MeV Kr ions, across a range of temperatures. The results suggest that there is a correlation between the availability of vacancies to aid recovery, with enhanced recovery at low values of x, with the lowest value for the critical temperature of amorphisation (T-c) being found between x = 0.2 and x = 0.3, to be similar to 250 K. However, at higher values of x, vacancies are detrimental to rapid recovery, with an increase in T-c to over 700 K for x = 0.8. These changes are seemingly independent of the observed change in crystal symmetry, which has been previously found for these compositions. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license. (http://creativecommons.org/licenses/by/4.0/)

We refine the crystal structures of a systematic series of compounds with the general composition Ho(x)Yb(2-x)TiO5 (x = 2, 1.6, 1.2, 1, 0.8, 0.4, 0) and Er2TiO5 and find a transition from defect-pyrochlore to defect-fluorite structure with increasing ytterbium content, decreasing lanthanide radius. Short-range structure modulations consisting of pyrochlore-like nano-domains are systematically characterised using transmission electron microscopy. We test the Kr2+ 1 MeV ion-irradiation response of Ho2TiO5, HoYbTiO5, Yb2TiO5, and Er2TiO5, via the crystalline to amorphous transition observed by using the in-situ TEM approach. The critical dose of amorphisation, D-c, was measured at various temperatures and used to calculate the critical temperature for maintaining crystallinity, T-c. A trend of lower T-c values with decreasing lanthanide radius is found. We describe a new approach for determining T-c values using cross-sectional TEM analysis of ex-situ bulk irradiated, 1 MeV Se+, samples; Ho2TiO5, HoYbTiO5 and Yb2TiO5. The results of D-c and T-c values using the two approaches vary; however the trends across the sample system remain the same. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.

The Ln2TiO5 (Ln = lanthanides and yttrium) compounds find use in nuclear, electronic and battery applications but there is a lack of information correlating the synthesis, structure and properties of these materials. We give an in depth review of the current available literature on the Ln2TiO5 compounds. New structural findings of the TbxYb2−xTiO5 (x = 0, 0.4, 0.8, 1, 1.2, 1.6, 2) compounds are detailed and their response to 1 MeV krypton ion exposure is discussed. A single phase Tb2TiO5 sample with hexagonal symmetry is the first bulk, single phase, mono-lanthanide with this stoichiometry and symmetry to be reported. Thus this work brings together the current literature and illustrates the synthesis-structure-property relationships that are observed in this family.

This ion-irradiation study covers the four major crystal structure types in the Ln(2)TiO(5) series (Ln = lanthanide), namely orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. This is the first systematic examination of the complete Ln(2)TiO(5) crystal system and the first reported examination of the hexagonal structure. A series of samples, based on the stoichiometry Sm(x)Yb(2-x)TiO5 (where x = 2, 1.4, 1, 0.6, and 0) have been irradiated using 1 MeV Kr2+ ions and characterised in-situ using a transmission electron microscope. Two quantities are used to define ion-irradiation tolerance: critical dose of amorphisation (D-c), which is the irradiating ion dose required for a crystalline to amorphous transition, and the critical temperature (T-c), above which the sample cannot be rendered amorphous by ion irradiation. The structure type plus elements of bonding are correlated to ion-irradiation tolerance. The cubic phases, Yb2TiO5 and Sm0.6Yb1.4TiO5, were found to be the most radiation tolerant, with Tc values of 479 and 697 K respectively. The improved radiation tolerance with a change in symmetry to cubic is consistent with previous studies of similar compounds. (C) 2016 Elsevier B.V. All rights reserved.

Garnets have been considered as host phases for the safe immobilisation of high-level nuclear waste, as they have been shown to accommodate a wide range of elements across three different cation sites, such as Ca, Y, Mn on the a-site, Fe, Al, U, Zr, and Ti on the b-site, and Si, Fe, Al on the c-site. Garnets, due to their ability to have variable composition, make ideal model materials for the examination of radiation damage and recovery in nuclear materials, including as potential waste forms. Kimzeyite, Ca3Zr2FeAlSiO12, has been shown naturally to contain up to 30wt% Zr, and has previously been examined to elucidate both the structure and ordering within the lattice. This study examines the effects of radiation damage and recovery using in-situ ion beam irradiation with 1MeV Kr ions at the IVEM-TANDEM facility, Argonne National Laboratory. The complementary Hf containing system Ca3Hf2FeAlSiO12 was also examined, and found to have a different response to irradiation damage. A sample of irradiated Ca3Zr2FeAlSiO12, at 1000K, was characterised using aberration corrected (S)TEM and found to contain discreet, nano-sized, crystalline Fe rich particles, indicating a competing process during recovery is occurring.

The response of Ln(2)TiO(5) (where Ln is a lanthanide) compounds exposed to high-energy ions was used to test their suitability for nuclear-based applications, under two different but complementary conditions. Eight samples with nominal stoichiometry Ln(2)TiO(5) (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy), of orthorhombic (Pnma) structure were irradiated, at various temperatures, with 1 MeV Kr2+ ions in-situ within a transmission electron microscope. In each case, the fluence was increased until a phase transition from crystalline to amorphous was observed, termed critical dose D-c.At certain elevated temperatures, the crystallinity was maintained irrespective of fluence. The critical temperature for maintaining crystallinity, T-c, varied non-uniformly across the series. The T-c was consistently high for La, Pr, Nd and Sm2TiO5 before sequential improvement from Eu to Dy2TiO5 with T-c's dropping from 974 K to 712 K.In addition, bulk Dy2TiO5 was irradiated with 12 MeV Au+ ions at 300 K, 723 K and 823 K and monitored via grazing-incidence X-ray diffraction (GIXRD). At 300 K, only amorphisation is observed, with no transition to other structures, whilst at higher temperatures, specimens retained their original structure. The improved radiation tolerance of compounds containing smaller lanthanides has previously been attributed to their ability to form radiation-induced phase transitions. No such transitions were observed here. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.

Using transmission electron microscopy we show that planar deformation lamellae occur within quartz in the substrate of a rock fulgurite, i.e., a lightning-derived glass. These lamellae exist only in a narrow zone adjacent to the quartz/fulgurite boundary and are comparable to planar deformation features ("shock lamellae") caused by hypervelocity impacts of extra-terrestrial objects. Our observations strongly suggest that the lamellae described here have been formed as a result of the fulgurite-producing lightning strike. This event must have generated a transient pressure pulse, whose magnitude, however, is uncertain at this stage.

A series of single phase compounds with nominal stoichiometry Sm(x)Yb(2−x)TiO5 (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln2TiO5 series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P63/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types.

Zirconolites based on the replacement of Ti4+ with equimolar amounts of Nb5+ and Fe3+, CaZrTi2-2x, NbxFexO7, have been jointly refined using high-resolution neutron powder diffraction and resonant X-ray powder diffraction data, with extra information provided from Fe-57 Mossbauer spectroscopy, to determine cation disorder. The results indicate that the CaZrTi2-2xNbxFeO7 series adopts the zirconolite-2M polytype across the range, with the replacement of Ti4+ by Fe3+ and Nb5+ located within the hexagonal tungsten bronze (HTB) layers. Mossbauer spectroscopy shows that Fe3+ preferentially fills the Ti split (C2) site, with secondary filling of the C I site and no observable occupancy of the C3 site. This has been confirmed by neutron and resonant X-ray diffraction. Niobium has been found to occupy both the Cl and C3 sites with no evidence for occupancy of the C2 site.

The critical dose for amorphisation, Dc, of two end-member zirconolites (CaZrTi2O7) with different stacking fault densities, was measured as a function of irradiation temperature from 20 K to 623 K using the HVEM-Tandem Facility at Argonne National Laboratory (ANL). Below 473 K, the Dc, values of both samples are identical within experimental error, showing only a small increase in Dc, from (2.5 to 4.6) × 1018 ions m−2 between 20 K and 473 K. At temperatures above 473 K, the data for the zirconolite containing many stacking faults is bracketed by two data sets from almost crystallographically perfect end-member zirconolites: one collected in this study and one collected in a previous study. The raw Dc versus temperature data from the zirconolites in this and a previous study suggest that the critical temperature above which samples cannot be amorphised and/or recrystallisation is complete, Tc, is between 600 and 1000 K. The data sets collected in this study are discussed in relation to a current model.

Polished tiles (7×7×2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of ≥ 1 × 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0–1, 1–7, 7–14 and 14–28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.

Recent climate analyses indicate that average global temperature is rising and both global drought occurrence and precipitation intensity are increasing. The nature of climate change is unique to each location, and its impact, both positive and negative, is predicted to be widespread. One area to be potentially affected includes management and use of outdoor natural resources such as the Appalachian Trail (AT), a 3500 km continuous hiking trail in the eastern United States. Observed historical (1895-2008) and projected future (to 2099) seasonal temperature and precipitation trends were examined along the AT. The AT has generally warmed since 1895, with greater warming occurring more recently. The warming has been greatest in the northern part of the AT and during winter. Precipitation trends show wide spatial variation depending upon the season, but generally precipitation has increased more in the northern than southern AT. Temperature and precipitation are projected to increase for all regions during all seasons in the future. Implications of these changes are discussed with respect to hiker experience and trail management.

Using electron energy-loss filtered transmission electron microscopy (EFTEM), we have observed the formation of silicon-rich zones on the corroded surface of a West Valley (WV6) glass. This layer is approximately 100-200 nm thick and is directly underneath a precipitated smectite clay layer. Under conventional (C)TEM illumination, this layer is invisible; indeed, more commonly used analytical techniques, such as x-ray energy dispersive spectroscopy (EDS), have failed to describe fully the localized changes in the boron and silicon contents across this region. Similar silicon-rich and boron-depleted zones were not found on corroded Savannah River Laboratory (SRL) borosilicate glasses, including SRL-EA and SRL-51, although they possessed similar-looking clay layers. This study demonstrates a new tool for examining the corroded surfaces of materials.

Rising global energy demand and the adverse environmental impact of energy use have led to renewed interest in nuclear power. Novel materials and approaches are needed to advance the utilization of nuclear energy in a manner consistent with the goals of proliferation resistance, energy security and waste reduction. This book brings together experimenters, theoreticians and modelers to discuss the innovations needed to develop the next generation of nuclear materials, and to understand the performance of existing materials under extreme operating conditions. Presentations explore the fabrication (melting, rolling sol gel, sintering, hot-pressing), characterization (microscopy, diffraction, thermal and electrical property measurements), modeling (ranging from nano- to mesoscale, and spanning timeframes ranging from fractions of femtoseconds to hundreds or millions of years), and performance predictions of various nuclear fuel cycle materials.

Thin crystals of La2O3, LaAlO3, La2/3TiO3, La2TiO5, and La2Ti2O7 have been irradiated in situ using 1 MeV Kr2+ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La2O3 remained crystalline to a fluence greater than 3.1×1016 ions cm−2 at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline–amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (Tc) of 647 K for LaAlO3, 840 K for La2Ti2O7, 865 K for La2/3TiO3, and 1027 K for La2TiO5. The Tc values observed in this study, together with previous data for Al2O3 and TiO2, are discussed with reference to the melting points for the La2O3–Al2O3 and La2O3–TiO2 systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between Tc and melting temperature (Tm) in the two systems. More complex relationships exist between Tc and crystal structure, with the stoichiometric perovskite LaAlO3 being the most resistant to amorphisation.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.