Pyrochlores. X. Madelung energies of pyrochlores and defect fluorites

1976 ◽  
Vol 54 (14) ◽  
pp. 2316-2334 ◽  
Author(s):  
William W. Barker ◽  
Peter S. White ◽  
Osvald Knop
Keyword(s):  
Pyrochlore Phase ◽  
Fluorite Structure ◽  
Type Structure ◽  
Correct Sign ◽  
Partial Disorder ◽  
Wide Range ◽  
Pyrochlore Type ◽  
Lattice Energies ◽  
Positive Ion

Electrostatic (Madelung) lattice energies have been calculated for a wide range of compounds A2B2X6Y with the pyrochlore-type structure. The cases of disorder and partial disorder on the non-metal sites have been examined, as well as the inverse case, where Y is a positive ion. The results of the calculations are compared with those for defect fluorite structure and an attempt is made to relate them to the conditions under which the pyrochlore phase is found. The effect of the site potential in restricting the existence boundaries of the pyrochlore phase is examined, as the potential must have the correct sign relative to the charge occupying the site. Alternative pyrochlore arrangements in which the 8(b) or 32(e) equipoints are occupied have also been explored.

Study of Natural Minerals of U-Ryrochlore-Type Structure as Analogues of Plutonium Ceramic Waste-form

2002 ◽  
Vol 713 ◽  
Author(s):  
Roman V. Bogdanov ◽  
Yuri F. Batrakov ◽  
Elena V. Puchkova ◽  
Andrey S. Sergeev ◽  
Boris E. Burakov
Keyword(s):  
Chemical Shifts ◽  
Pyrochlore Structure ◽  
Type Structure ◽  
Waste Form ◽  
X Ray ◽  
Ceramic Waste ◽  
Natural Minerals ◽  
The Us ◽  
Pyrochlore Type

ABSTRACTAt present, crystalline ceramic based on titanate pyrochlore, (Ca,Gd,Hf,Pu,U)2Ti2O7, is considered as the US candidate waste form for the immobilization of weapons grade plutonium. Naturally occuring U-bearing minerals with pyrochlore-type structure: hatchettolite, betafite, and ellsworthite, were studied in orders to understand long-term radiation damage effects in Pu ceramic waste forms. Chemical shifts (δ) of U(Lδ1)– and U(Lβ1) – X-ray emission lines were measured by X-ray spectrometry. Calculations were performed on the basis of a two-dimensional δLá1- and δLδ1- correlation diagram. It was shown that 100% of uranium in hatchettolite and, probably, 95-100% of uranium in betafite are in the form of (UO2)2+. formal calculation shows that in ellsworthite only 20% of uranium is in the form of U4+ and 80% of the rest is in the forms of U5+ and U6+. The conversion of the initial U4+ ion originally occurring in the pyrochlore structure of natural minerals to (UO2)2+ due to metamict decay causes a significant increase in uranium mobility.

TUNGSTEN OXIDES AND BRONZES: SYNTHESIS, DIFFUSION AND REACTIVITY

1993 ◽  
Vol 07 (23n24) ◽  
pp. 4145-4164 ◽  
Author(s):  
JING-DONG GUO ◽  
M. STANLEY WHITTINGHAM
Keyword(s):  
Tungsten Bronze ◽  
Pyrochlore Phase ◽  
Reaction Medium ◽  
Pyrochlore Structure ◽  
Ionic Mobility ◽  
Soft Chemistry ◽  
Quarter Century ◽  
Tungsten Oxides ◽  
Hydronium Ion ◽  
Wide Range

The tungsten oxides and bronzes have been extensively studied since their discovery in the last century, because of their brilliant colors and high electrical conductivity. More recently the driving interest resulted from their potential use in electrochromic displays and other electrochemical systems. Their crystalline structures are generally based on the corner sharing of WO 6 octahedra giving tunnels of variable size and shape leading to exciting intercalation chemistry. These structures readily undergo redox reactions, and in the last quarter century these reactions have often involved soft chemistry. Most recently hydrothermal techniques have been used to prepare new sodium tungstates with the hexagonal tungsten bronze and the pyrochlore structures. The phase formed is a function of the pH of the reaction medium. The pyrochlore phase readily undergoes ion-exchange with a wide range of monovalent cations giving the compounds, M x W 2 O 6+x/2 · y H 2 O ; the value of y is strongly dependent on the identity of the cation, M. WO 3 with the pyrochlore structure could be formed from the hydronium and ammonium complexes. Lithium can be readily intercalated either chemically and electrochemically into both these phases, just as in the previously-known bronze phases. Surprisingly more lithium is incorporated in most cases in the hexagonal than in the pyrochlore phase. The ions in the pyrochlore structure show rapid ionic mobility, with the hydronium ion showing the greatest mobility.

scholarly journals Metal-insulator Crossover in Pb-Ru Based Oxides with Pyrochlore-type Structure

2018 ◽  
Vol 65 (5) ◽  
pp. 249-254
Author(s):  
Masaki KATO ◽  
Kazuya IWAMOTO ◽  
Yuki IWAKURA ◽  
Ken HIROTA ◽  
Angel AREVALO-LOPEZ ◽  
...  
Keyword(s):  
Type Structure ◽  
Metal Insulator ◽  
Pyrochlore Type

New Family of Dielectric Materials in Ca–Y–Ti–Nb–O System Having Pyrochlore Type Structure

2007 ◽  
Vol 90 (11) ◽  
pp. 3656-3659 ◽  
Author(s):  
K. Ravindran Nair ◽  
P. Prabhakar Rao ◽  
Anupama V. Raj ◽  
Smila K. Joseph ◽  
Marottikunnathu Raman Chandran ◽  
...  
Keyword(s):  
Dielectric Materials ◽  
Type Structure ◽  
New Family ◽  
Pyrochlore Type

scholarly journals Evidence of ferroelectricity in La2Zr2O7thin films with a frustrated pyrochlore-type structure

2015 ◽  
Vol 71 (a1) ◽  
pp. s513-s513
Author(s):  
Pascal Roussel ◽  
Alexandre Bayart ◽  
Sébastien Saitzek ◽  
Zhen-Mian Shao ◽  
Anthony Ferri ◽  
...  
Keyword(s):  
Type Structure ◽  
Pyrochlore Type

Ion-beam irradiation of Gd2Sn2O7 and Gd2Hf2O7 pyrochlore: Bond-type effect

2004 ◽  
Vol 19 (5) ◽  
pp. 1575-1580 ◽  
Author(s):  
Jie Lian ◽  
Rodney C. Ewing ◽  
L.M. Wang ◽  
K.B. Helean
Keyword(s):  
Room Temperature ◽  
Ion Beam ◽  
Pyrochlore Structure ◽  
Fluorite Structure ◽  
Structure Type ◽  
Radiation Response ◽  
Beam Irradiation ◽  
Ion Beam Irradiation ◽  
Bond Type ◽  
Wide Range

Ceramics with III-IV pyrochlore compositions, A3+2B4+2O7 (A = Y and rare earth elements; B = Ti, Zr, Sn, or Hf), show a wide range of responses to ion-beam irradiation. To evaluate the role of the B-site cations on the radiation stability ofthe pyrochlore structure-type, Gd2Sn2O7 and Gd2Hf2O7 have been irradiated by1 MeV Kr+. The results are discussed in terms of the ionic size and type ofbonding of Sn4+ and Hf4+ and compared to previous results for titanate andzirconate pyrochlores. Gd2Sn2O7 is sensitive to ion beam–induced amorphizationwith a critical amorphization dose of approximately 3.4 displacements per atom(dpa) (2.62 × 1015 ions/cm2) at room temperature and a critical amorphization temperature of approximately 350 K. Gd2Hf2O7 does not become amorphous at adose of approximately 4.54 displacement per [lattice] atom (3.13 × 1015 ions/cm2) at room temperature, but instead is transformed to a disordered fluorite structure upon ion-beam irradiation. Although the radius ratio of the A- to B-site cations provides a general indication of the type of radiation response of different pyrochlore compositions, the results for Gd2Sn2O7 emphasize the importance of bond type, particularly the covalency of the 〈Sn–O〉 bond in determining the radiation response.

scholarly journals A Durable, Lightweight Structure for Conducting Field Shading Experiments

HortScience ◽  
1992 ◽  
Vol 27 (12) ◽  
pp. 1274-1275 ◽  
Author(s):  
D.S. NeSmith ◽  
P.L. Raymer ◽  
M.S.S. Rao ◽  
D.C. Bridges
Keyword(s):  
Polyvinyl Chloride ◽  
Low Cost ◽  
Weather Conditions ◽  
Type Structure ◽  
Reinforcing Bars ◽  
Total Cost ◽  
Air Movement ◽  
Wide Range ◽  
Lightweight Structure

A low-cost field shading structure was developed that offers durability and simplicity. The quonset-type structure uses readily available materials, including polyvinyl chloride pipe, construction-grade reinforcing bars, nylon rope, and commercial shade fabric. The total cost for a 3.0 × 6.0 × 2.4-m (width/length/height) structure that provided 47% shade was $88.00. The structure offers substantial flexibility in terms of size and degree of shading without significantly altering design. The structure was durable under a wide range of weather conditions, and the design allowed sufficient air movement to prevent a stagnant air layer from developing over the crop.

Sign in / Sign up

Export Citation Format

Share Document