Formation enthalpies of rare earth titanate pyrochlores

Helean, Katheryn B.; Ushakov, Sergey V.; Brown, C.E.; Navrotsky, Alexandra; Lian, Jie; Ewing, Rodney C.; Farmer, J. Matt; Boatner, L. A.

doi:10.1016/j.jssc.2004.01.009

Cited by 157 publications

(101 citation statements)

References 52 publications

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“…The most prevalent lanthanide in the CS/LN waste stream is Nd, and the free energy of formation of NdAlO 3 is approximately -41 kJ/mol, 12 as compared to approximately -120 kJ/mol for Nd 2 Ti 2 O 7 . 13 Although the conditions used to fabricate these compositions were likely not at equilibrium, the lower free energy of formation of Nd 2 Ti 2 O 7 is in agreement with the XRD results.…”

Section: Crystalline Phase Assemblagessupporting

confidence: 73%

Preliminary Study of Ceramics for Immobilization of Advanced Fuel Cycle Reprocessing Wastes

Fox¹,

Billings²,

Brinkman³

et al. 2010

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SUMMARYThe Savannah River National Laboratory (SRNL) developed a series of ceramic waste forms for the immobilization of Cesium/Lanthanide (CS/LN) and Cesium/Lanthanide/Transition Metal (CS/LN/TM) waste streams anticipated to result from nuclear fuel reprocessing. Simple raw materials, including Al 2 O 3 , CaO, and TiO 2 were combined with simulated waste components to produce multiphase ceramics containing hollandite-type phases, perovskites (particularly BaTiO 3 ), pyrochlores, zirconolite, and other minor metal titanate phases. Identification of excess Al 2 O 3 via X-ray Diffraction (XRD) and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) in the first series of compositions led to a Phase II study, with significantly reduced Al 2 O 3 concentrations and increased waste loadings.Three fabrication methodologies were used, including melting and crystallizing, pressing and sintering, and Spark Plasma Sintering (SPS), with the intent of studying phase evolution under various sintering conditions. XRD and SEM/EDS results showed that the partitioning of the waste elements in the sintered materials was very similar, despite varying stoichiometry of the phases formed. The Phase II compositions generally contained a reduced amount of unreacted Al 2 O 3 as identified by XRD, and had phase assemblages that were closer to the initial targets. Chemical composition measurements showed no significant issues with meeting the target compositions. However, volatilization of Cs and Mo was identified, particularly during melting, since sintering of the pressed pellets and SPS were performed at lower temperatures. Partitioning of some of the waste components was difficult to determine via XRD. SEM/EDS mapping showed that those elements, which were generally present in small concentrations, were well distributed throughout the waste forms.Initial studies of radiation damage tolerance using ion beam irradiation at Los Alamos National Laboratory (LANL) showed little if any modification of the material after irradiation. Additional study in this area is needed. Chemical durability was briefly studied using the Product Consistency Test (PCT). Most of the elements measured were retained by the ceramic waste forms, indicating good chemical durability. Cs, Mo, and Rb were released at somewhat higher rates as compared to the matrix components, although benchmark compositions and additional characterization are needed in order to qualify the PCT results.

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Section: Crystalline Phase Assemblagessupporting

confidence: 73%

Preliminary Study of Ceramics for Immobilization of Advanced Fuel Cycle Reprocessing Wastes

Fox¹,

Billings²,

Brinkman³

et al. 2010

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“…Formation enthalpies of the two compounds could be another supportive argument. La 2 Ti 2 O 7 has a formation enthalpy of DH f = À3855.5 ± 3.5 kJ/mol, which is 33 kJ/mol lower than that of Gd 2 Ti 2 O 7 , which could possibly suggest that La 2 Ti 2 O 7 has a higher stability when crystalline [50].…”

Section: Synchrotron X-ray Diffractionmentioning

confidence: 96%

Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing

et al. 2015

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Swift heavy ion (2 GeV 181 Ta) irradiation-induced amorphization and temperature-induced recrystallization of cubic pyrochlore Gd 2 Ti 2 O 7 (Fd 3m) are compared with the response of a compositionally-similar material with a monoclinic-layered perovskite-type structure, La 2 Ti 2 O 7 (P2 1 ). The averaged electronic energy loss, dE/dx, was 37 keV/nm and 35 keV/nm in Gd 2 Ti 2 O 7 and La 2 Ti 2 O 7 , respectively. Systematic analysis of the structural modifications was completed using transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy, and small-angle X-ray scattering. Increasing ion-induced amorphization with increasing ion fluence was evident in the X-ray diffraction patterns of both compositions by a reduction in the intensity of the diffraction maxima concurrent with the growth in intensity of a broad diffuse scattering halo. Transmission electron microscopy analysis showed complete amorphization within ion tracks (diameter: $10 nm) for the perovskite-type material; whereas a concentric, core-shell morphology was evident in the ion tracks of the pyrochlore, with an outer shell of disordered yet still crystalline material with the fluorite structure surrounding an amorphous track core (diameter: $9 nm). The radiation response of both titanate oxides with the same stoichiometry can be understood in terms of differences in their structures and compositions. While the radiation damage susceptibility of pyrochlore A 2 B 2 O 7 materials decreases as a function of the cation radius ratio r A /r B , the current study correlates this behavior with the stability field of monoclinic structures, where r La /r Ti > r Gd /r Ti . Isochronal annealing experiments of the irradiated materials showed complete recrystallization of La 2 Ti 2 O 7 at 775°C and of Gd 2 Ti 2 O 7 at 850°C. The annealing behavior is discussed in terms of enhanced damage recovery in La 2 Ti 2 O 7 , compared to the pyrochlore compounds Gd 2 Ti 2 O 7 . The difference in the recrystallization behavior may be related to structural constraints, i.e., reconstructing a low symmetry versus a high symmetry phase.

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“…Rare-earth titanates (RE 2 Ti 2 O 7 ; RE (rare-earth) = La-Lu, and Y) have been investigated for the sequestration of nuclear waste due to their resistance to radiation-induced structural damage, which depends on the rare-earth cation present [7,43,44]. RE 2 Ti 2 O 7 crystallites have been examined previously using transmission electron microscopy (TEM) before and after implantation, and this previous study has shown that the resistance to radiationinduced structural damage increases with decreasing radius of the RE cation [7].…”

Section: Introductionmentioning

confidence: 99%

An investigation of the electronic structure and structural stability of RE2Ti2O7 by glancing angle and total electron yield XANES

Aluri

Grosvenor

2014

Journal of Alloys and Compounds

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a b s t r a c tPyrochlore-type oxides (A 2 B 2 O 7 ) have been investigated for nuclear waste sequestration applications due to their overall resistance to radiation-induced structural damage. This damage can be simulated by implanting the materials with high-energy ions to aid in understanding how the structure of a crystalline material responds to radioactive decay of incorporated nuclear waste elements. RE 2 Ti 2 O 7 (RE = La-Lu), Yb 1.85 Ca 0.15 Ti 2 O 7Àd , and Yb 2 Ti 1.85 Fe 0.15 O 7Àd were synthesized by the ceramic method and investigated by Ti K-edge X-ray absorption near edge spectroscopy (XANES) and powder X-ray diffraction before and after being implanted by 2 MeV Au À ions. Surface sensitive total electron yield (TEY) XANES spectra and glancing-angle (GA) XANES spectra were collected to study the ion-implanted materials. It was necessary to use these surface sensitive detection methods because the 2 MeV Au À ions only implant to $450 nm into the surface of the materials. An effective sample mounting procedure has been developed that uses conductive silver paint to collect high quality TEY-XANES spectra from the damaged surface layer of the implanted pellets so as to minimize charging effects. This investigation has shown that the local coordination number of Ti decreased after implantation, and that the degree of damage observed decreases with decreasing radius of the RE cation. The study has also shown that the oxygen vacancies generated by a slight substitution of Yb 3+ for Ca 2+ or Ti 4+ for Fe 3+ does not considerably affect the resistance of Yb 2 Ti 2 O 7 to radiation-induced damage.

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Formation enthalpies of rare earth titanate pyrochlores

Cited by 157 publications

References 52 publications

Preliminary Study of Ceramics for Immobilization of Advanced Fuel Cycle Reprocessing Wastes

Preliminary Study of Ceramics for Immobilization of Advanced Fuel Cycle Reprocessing Wastes

Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing

An investigation of the electronic structure and structural stability of RE2Ti2O7 by glancing angle and total electron yield XANES

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