Karl R. Whittle

Temperature dependence of ion irradiation damage in the pyrochlores La2Zr2O7 and La2Hf2O7

Synthetic samples of the pyrochlores La2Zr2O7 and La2Hf2O7 were irradiated in situ in the intermediate voltage electron microscope (IVEM-Tandem Facility) at Argonne National Laboratory using 1.0 MeV Kr2+. Results of this study demonstrate that both pyrochlores pass through the crystalline–amorphous transformation albeit with significantly different critical amorphization dose curves. The critical dose values extrapolated to 0 K (Dc0) are 11 ± 3 × 1014 ions cm−2 ( dpa) for La2Zr2O7 and 5.5 ± 0.7 × 1014 ions cm−2 ( dpa) for La2Hf2O7. Non-linear least squares analysis of the dose–temperature curves gave values of the critical temperature (Tc) of 339 ± 49 K for La2Zr2O7 and 563 ± 10 K for La2Hf2O7. This analysis also gave values of the activation energy (Ea) for thermal recovery of damage of 0.02 ± 0.01 and 0.05 ± 0.01 eV for the zirconium pyrochlore and the hafnium pyrochlore, respectively. These results demonstrate that there is a major difference in the dose–temperature response dependent upon the nature of the B-site cation of these two pyrochlores. Results are discussed in terms of the pyrochlore structural parameters rA/rB and x(48f) as well as the stopping powers, displacement energies, and defect energies of the materials.

In situ Raman spectroscopy of A-site doped barium titanate

Raman spectroscopy has been used to study the ferroelectric tetragonal to paraelectric cubic phase transition Tc in undoped BaTiO3 (BT) and (Ba0.92A0.08)TiO3 [where A=Ca (BCT), Sr (BST), and Pb (BPT)]. In BT and BPT, mode characteristic of the tetragonal phase at 303 and 710cm−1 persisted until >50°C above Tc, indicating the presence of local dynamic polar clusters well above the displacive phase transitions, ∼120 and ∼149°C, respectively. However, in BCT and BST, the Raman spectra exhibited only broad second order bands above Tc, suggesting that Ca and Sr, A-site dopants supress local dynamic polar clusters.

The pyrochlore to defect fluorite phase transition in Y2Sn2−xZrxO7

The system Y2Sn2−xZrxO7 (0.0 ≤ x ≤ 2.0) undergoes a phase transformation from ordered pyrochlore (Fdm) to defect fluorite (Fmm) actuated by the substitution of Zr for Sn. X-ray diffraction patterns map the retention of the pyrochlore structure up to x = 1.2. For samples with x = 1.4–2.0 the structure can be described as defect fluorite in broad terms. Electron diffraction patterns are consistent with this interpretation; however, they also demonstrate that the defect fluorite phase exhibits a strain driven compositional/displacive modulation that changes gradually with increasing Zr content. Raman spectra are consistent with gradual anion disorder up to x = 1.0 and highly disordered anion distributions inferred for x > 1.4, but the spectra also suggest the presence of residual order due to the modulated structure. The phase transformation in this system occurs at a higher Zr content than predicted by classical radius ratio models, consistent with the covalent character of Sn–O bonding. An unusual finding of this work comes from 119Sn MAS NMR and Sn L3-edge XANES analyses, indicating that Sn4+ prefers to occupy lattice sites with a 6-fold local coordination environment throughout the series. These results suggest that the incorporation of Sn or other metal cations having significant covalent bonding or a strong preference for octahedral coordination in pyrochlore-based materials may have a detrimental effect on ionic conductivity.

Synthesis of the ferroelectric solid solution, Pb(Zr1−xTix)O3 on a single substrate using a modified molecular beam epitaxy technique

High-throughput synthesis of the ferroelectric solid solution Pb(Zr1−xTix)O3 (PZT) on single Pt∕Ti∕SiO2∕Si substrates was demonstrated using a modified molecular beam epitaxy (MBE) system. The PZT films exhibited a phase transition from rhombohehdral to tetragonal symmetry as a function of Zr:Ti ratio, across the substrate diagonal. This was consistent with the presence of a morphotropic phase boundary at Zr:Ti ratio of 0.64:0.36, different from the value of 0.53:0.47 observed for bulk ceramics. All points on the films exhibited ferroelectric hysteresis. Results demonstrate the feasibility of high-throughput MBE for deposition of complex ferroelectric oxides, and pave the way for further materials discovery.

Ion irradiation of the TiO2 polymorphs and cassiterite

Abstract Thin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50-300 K. Synthetic rutile and natural cassiterite, with 0.1-0.2 wt% impurities, remain crystalline up to a fluence of 5 × 1015 ions cm-2 without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3-0.5 wt% impurities, become amorphous at fluences of 8.1 × 1014 and 2.3 × 1014 ions cm-2, respectively. We have also studied two natural rutile samples containing ~1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 × 1014 and 8.6 × 1014 ions cm-2 at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 ± 11, 209 ± 8, and 242 ± 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration.

Ion irradiation of the TiO{sub 2} polymorphs and cassiterite.

Abstract Thin crystals of rutile, brookite, anatase, and cassiterite were irradiated in situ in the transmission electron microscope using 1.0 MeV Kr ions at 50-300 K. Synthetic rutile and natural cassiterite, with 0.1-0.2 wt% impurities, remain crystalline up to a fluence of 5 × 1015 ions cm-2 without evidence for amorphization at 50 K. Natural brookite and anatase, with 0.3-0.5 wt% impurities, become amorphous at fluences of 8.1 × 1014 and 2.3 × 1014 ions cm-2, respectively. We have also studied two natural rutile samples containing ~1.7 and 1.2 wt% impurities. These samples became amorphous at 9.2 × 1014 and 8.6 × 1014 ions cm-2 at 50 K, respectively. Further analyses of the fluence-temperature data for natural brookite, rutile, and anatase give critical amorphization temperatures of 168 ± 11, 209 ± 8, and 242 ± 6 K, respectively. Results are briefly discussed with respect to several criteria for radiation resistance, including aspects of the structure, bonding, and energetics of defect formation and migration.

Density and structural effects in the radiation tolerance of TiO2 polymorphs

The radiation response of TiO₂ has been studied using molecular dynamics. The simulations are motivated by experimental observations that the three low-pressure polymorphs, rutile, brookite and anatase, exhibit vastly different tolerances to amorphization under ion-beam irradiation. To understand the role of structure we perform large numbers of simulations using the small thermal spike method. We quantify to high statistical accuracy the number of defects created as a function of temperature and structure type, and reproduce all the main trends observed experimentally. To evaluate a hypothesis that volumetric strain relative to the amorphous phase is an important driving force for defect recovery, we perform spike simulations in which the crystalline density is varied over a wide range. Remarkably, the large differences between the polymorphs disappear once the density difference is taken into account. This finding demonstrates that density is an important factor which controls radiation tolerance in TiO₂.

Phase transitions in BaTiO3: a high-pressure neutron diffraction study

Abstract The sequence of ferroelectric phase transitions (Pm3̅m—P4mm—Amm2) in BaTiO3 has been studied as a function of temperature (298 K–130 K) at a fixed pressure (3.2 GPa). The same sequence of transitions is observed as for ambient pressures, but the transition temperatures are reduced. The spontaneous strains of the non-cubic phases are determined. Whilst the structure of the tetragonal phase at high pressure is qualitatively similar to the ambient structure, both the spontaneous strain and the Ti displacement are smaller in the high pressure structure than the ambient structure.

Synthesis and Characterisation of Ln 2 TiO 5 Compounds

Bulk samples of six Ln 2 TiO 5 compounds with Ln = La, Pr, Nd, Eu, Gd and Tb were prepared and characterised. Most of the samples have a phase purity of ∼95% (based on BEI and EDS) with the predominant secondary phase primarily being Ln 2 Ti 2 O 7 . Using XRD, TEM selected area diffraction and high resolution imaging techniques, we have confirmed the results of previous studies which showed that at room temperature Pr 2 TiO 5 , Nd 2 TiO 5 , Eu 2 TiO 5 and Tb 2 TiO 5 have orthorhombic structures with Pnma symmetry. The structure of Tb 2 TiO 5 was further monitored as a function of temperature. The relevance of Ln 2 TiO 5 compounds to advanced nuclear fuel cycles is discussed.

A determination of the structure of liquid Ga2Te3 using combined X-ray diffraction and neutron diffraction with isotopic substitution

The partial structure factors of liquid Ga2Te3 have been determined using a combination of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction around the Te K edge. The inclusion of X-ray diffraction data significantly improves the statistical accuracy of the partial structure factors obtained compared with those obtained by the NDIS method alone. The results show that the liquid is hetero-coordinated in a way similar to that of typical two-component glasses or ionic liquids. There is no evidence, within the accuracy of this experiment, for substantial numbers of homopolar bonds as suggested by recent ab initio molecular dynamics simulations. The bond distances and coordination numbers show strong similarities to the tetrahedrally bonded defective zinc blende structure of the crystalline solid before melting.