Pyrochlores. X. Madelung energies of pyrochlores and defect fluorites

Barker, William; White, Peter S.; Knop, Osvald

doi:10.1139/v76-330

Cited by 24 publications

(12 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Again, with variation in x-parameter the shape of polyhedra in pyrochlore also changes. Based on Madelung energy and site potential calculations, it had been found that (3+ , 4+ ) pyrochlores with x E0.36 may also exist but in a highly disordered state [22,23]. Earlier we [24] have shown that for Dy 2 Hf 2 O 7 , the xparameter is 0.348 and it crystallizes as a disordered pyrochlore.…”

Section: Resultsmentioning

confidence: 99%

Order–disorder transition in the Nd2−yYyZr2O7 system: Probed by X-ray diffraction and Raman spectroscopy

Mandal

Krishna

Tyagi

2010

Journal of Solid State Chemistry

114

View full text Add to dashboard Cite

Section: Resultsmentioning

confidence: 99%

Order–disorder transition in the Nd2−yYyZr2O7 system: Probed by X-ray diffraction and Raman spectroscopy

Mandal

Krishna

Tyagi

2010

Journal of Solid State Chemistry

114

View full text Add to dashboard Cite

“…20 In natural pyrochlore, molecular water and hydroxyl may be part of the structure so that the general formula becomes: A 1-2 B 2 O 6 (O,OH,F) 0-1 pH 2 O. A number of structural derivatives are possible, such as ''inverse'' pyrochlores, in which the A-site is vacant and monovalent cations occupy the Y-site, ͓ ͔ VI B 2 III X 6 VI M. 55 The M-atom may be displaced from the 8a site to the 32e position, hence the rather unusual flexibility of this structure. Stanek et al 56 have simulated the energetics of vacancy formation using an energy minimization method and a Born-like description of the forces between the ions.…”

Section: Structure and Compositionmentioning

confidence: 99%

Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides

Ewing

Weber

Lian

2004

Journal of Applied Physics

998

723

View full text Add to dashboard Cite

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the ''minor'' actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A 2 B 2 O 7 , for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that many zirconate pyrochlores do not become amorphous, but remain crystalline as a defect fluorite structure-type due to disordering of the A-and B-site cations. The zirconate pyrochlores will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, analyses of natural U-and Th-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls on the response of pyrochlore to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.

show abstract

“…Combinations of A-site cations and Y-site anions may be removed to produce defect structures described by the formulas VlA2VlB2Ivx6, VIAVlBzmX6,or VIB2nX 6 (Grins 1980;Rotella et al 1982;Chakoumakos and Ewing 1985;Chakoumakos 1984Chakoumakos , 1986. The defect structure known as "inverse" pyrochlore has the formula WByX6VIM and is formed by replacing the Y anion with a large monovalent cation (Barker et al 1976). The range of possible defect structures illustrates why the BzX6 octahedral framework is considered to be the essential structural feature ofpyrochlore (Figure 2).…”

Section: Structure and Chemistry Of Pyrochlorementioning

confidence: 99%

Alpha-decay damage in minerals of the pyrochlore group

1988

View full text Add to dashboard Cite

Abstract. X-ray diffraction analysis and transmission electron microscopy have been used to study the effects of alpha-decay damage in pyrochlore group minerals, characterized by the general formula A1 ,~BzO6(O,OH,F)I -n'pH20.As defined by the XRD intensity ratio I/Io, both the saturation dose (for which I/Io =0.1-0.0) and the dose which signifies the initial loss of crystallinity (for which I/Io = 1.0-0.8) increase as a function of geologic age. The increase is attributed to annealing of isolated alpha-recoil tracks back to the original crystalline structure. The tracks have calculated mean lives, za, on the order of 108 years. In contrast, minerals which remain crystalline (e.g., uraninite, UO2) despite doses of up to 1018 alpha-events/mg have mean alpha-recoil track lives ~ 104 years (Eyal and Fleischer 1985).After correcting the calculated dose for annealing of alpha-recoil damage, I/Io is observed to decrease exponentially to zero over the dose range 0.02-1.0 x 1016 alphaevents/rag. The relationship between I/Io and "corrected" dose was used to calculate an average alpha-recoil track diameter of 4.6 nm, in which < 2600 atoms are displaced. XRD line broadening due to strain dominates the first half of the crystalline-to-metamict transition, reaching a maximum of 0.003, then decreasing to < 0.001. Line broadening due to decreasing crystallite size dominates the latter half of the transition. Estimated crystallite dimensions decrease from 450 nm to < 15 nm prior to reaching the fully metamict state. With increasing dose HRTEM images of microlites from the Harding pegmatite sequentially exhibit: 1) mottled diffraction contrast, 2) isolated 1-5 nm aperiodic areas, 3) coexisting aperiodic and crystalline areas, 4) relict" islands" of crystalline material in an aperiodic matrix, and 5) complete loss of lattice fringe periodicity. With no consideration given to alpha-recoil track fading, the transition covers a dose range of 0.04-1.7x 1017 alpha-events/mg. Using a value of za=108 years, this dose range is corrected down to 0.02-1.2 x 10 ~6 alpha-events/mg.The metamict state is characterized by a range of M-M and M-O distances which give rise to bands of diffuse scattering centered at 0.30 nm and 0.18 nm, respectively, in x-ray and electron diffraction patterns. Random image contrast shown by HRTEM is consistent with a random network type structure, an interpretation supported by EX-AFS/XANES studies (Greegor et al. 1985a(Greegor et al. , b, 1987. The structure of metamict pyrochlore consists of an aperiodic framework of corner-sharing B--O polyhedra. Compared to the crystalline precursor, the metamict state displays a reduced M-O coordination number and mean bond length, increased distortion of the B-site, and a slight increase in the average M-M distance.

show abstract

Pyrochlores. X. Madelung energies of pyrochlores and defect fluorites

Cited by 24 publications

References 9 publications

Order–disorder transition in the Nd2−yYyZr2O7 system: Probed by X-ray diffraction and Raman spectroscopy

Order–disorder transition in the Nd2−yYyZr2O7 system: Probed by X-ray diffraction and Raman spectroscopy

Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides

Alpha-decay damage in minerals of the pyrochlore group

Contact Info

Product

Resources

About