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Title: Neutron diffraction and MAS NMR of Cesium Tungstate defect pyrochlores

Abstract

Defect-pyrochlores based on the formulation CsM {sub 0.5}W{sub 1.5}O{sub 6} (M=Ti, Ti/Zr, Zr and Hf) have been studied using neutron diffraction and magic-angle spinning nuclear magnetic resonance (MAS NMR). The results show that structural changes are linearly linked to the change in ionic-radius for the B-site, e.g. the unit cell changes from 10.2763 A for CsTi{sub 0.5}W{sub 1.5}O{sub 6} to 10.3820 A for CsZr{sub 0.4}W{sub 1.6}O{sub 6}. Changes in the NMR chemical shift correlate with the change in electronegativity on the B-site, and show the presence of only one Cs crystal site.

Authors:
 [1];  [2];  [2]
  1. Department of Earth Sciences, Cambridge Centre for Ceramic Immobilisation (C3i), University of Cambridge, Downing Street, Cambridge CB2 3EQ (United Kingdom). E-mail: kwhi02@esc.cam.ac.uk
  2. Department of Earth Sciences, Cambridge Centre for Ceramic Immobilisation (C3i), University of Cambridge, Downing Street, Cambridge CB2 3EQ (United Kingdom)
Publication Date:
OSTI Identifier:
20784890
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 179; Journal Issue: 2; Other Information: DOI: 10.1016/j.jssc.2005.11.011; PII: S0022-4596(05)00516-5; Copyright (c) 2005 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; CESIUM COMPOUNDS; CESIUM IONS; CHEMICAL SHIFT; ELECTRONEGATIVITY; NEUTRON DIFFRACTION; NUCLEAR MAGNETIC RESONANCE; PYROCHLORE; TUNGSTATES

Citation Formats

Whittle, K.R., Lumpkin, G.R., and Ashbrook, S.E. Neutron diffraction and MAS NMR of Cesium Tungstate defect pyrochlores. United States: N. p., 2006. Web. doi:10.1016/j.jssc.2005.11.011.
Whittle, K.R., Lumpkin, G.R., & Ashbrook, S.E. Neutron diffraction and MAS NMR of Cesium Tungstate defect pyrochlores. United States. doi:10.1016/j.jssc.2005.11.011.
Whittle, K.R., Lumpkin, G.R., and Ashbrook, S.E. Wed . "Neutron diffraction and MAS NMR of Cesium Tungstate defect pyrochlores". United States. doi:10.1016/j.jssc.2005.11.011.
@article{osti_20784890,
title = {Neutron diffraction and MAS NMR of Cesium Tungstate defect pyrochlores},
author = {Whittle, K.R. and Lumpkin, G.R. and Ashbrook, S.E.},
abstractNote = {Defect-pyrochlores based on the formulation CsM {sub 0.5}W{sub 1.5}O{sub 6} (M=Ti, Ti/Zr, Zr and Hf) have been studied using neutron diffraction and magic-angle spinning nuclear magnetic resonance (MAS NMR). The results show that structural changes are linearly linked to the change in ionic-radius for the B-site, e.g. the unit cell changes from 10.2763 A for CsTi{sub 0.5}W{sub 1.5}O{sub 6} to 10.3820 A for CsZr{sub 0.4}W{sub 1.6}O{sub 6}. Changes in the NMR chemical shift correlate with the change in electronegativity on the B-site, and show the presence of only one Cs crystal site.},
doi = {10.1016/j.jssc.2005.11.011},
journal = {Journal of Solid State Chemistry},
number = 2,
volume = 179,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}
  • The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO{sub 4}(F, OH). These compounds are ideal for studying F {leftrightarrow} OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state {sup 6}Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F {leftrightarrow} OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and Vmore » with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U{sub 3} eigenvalue, which lies between the c and c* directions. U{sub eq} values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder. At the montebrasite end of the series, there are five short Li- (= O, OH, F) distances and one very long Li-O4 bond. With increasing F content, the Li-O4 distance decreases and the Li-O3f distance increases, such that at the amblygonite end, the coordination is 4 + 2. The disorder in the Li site is obviously caused by the substitution of F for OH. The driving force is the loss of the hydrogen bond to O4, which causes the Li-O4 bond to strengthen and improves the bond valence to O4. The results show that the H atom position is imbedded within the distorted octahedral oxygen coordination of the Li atom. To represent the disorder better, we used a split Li site model. The results show that Li1 occupancy increases and the Li2 occupancy decreases with increasing F content, and that the Li1-Li2 distance is longer for intermediate compositions than for the end-members. The {sup 6}Li MAS NMR experiments provide important structural information complementary to the neutron diffraction results. The spectra of samples in the amblygonite-montebrasite series show two well-resolved peaks, confirming the presence of Li disorder over two distinct sites, and highly resolved {sup 6}Li MAS NMR spectra are obtained at the very high magnetic field strength of 18.8 T. The peaks at -0.3 ppm and -0.9 ppm were unambiguously assigned to the Li2(OH) and Li1(F) sites found in the neutron diffraction structures. The isotropic chemical shifts are consistent with the coordinations of these Li sites found in the neutron diffraction structures. The relative intensities of the two peaks across the series of samples reflect the varying F/(F + OH). In addition to confirming the assignments of the peaks, it is possible to measure H-Li2 and F-Li1 internuclear distances by {sup 6}Li({sup 1}H) and {sup 6}Li({sup 19}F) CP and REDOR NMR that are consistent with the corresponding distances from the neutron diffraction structures. The {sup 6}Li({sup 1}H) and {sup 6}Li({sup 19}F) CP and REDOR results indicate that the Li disorder is random throughout the crystals rather than over large domains, a conclusion that cannot be made from diffraction experiments. Variable temperature {sup 6}Li MAS NMR spectra confirm that the disorder is static and there is no dynamic exchange involving F, OH, or Li. Each Li ion has access to only one of the two observed sites as determined by the presence of either OH or F in its immediate environment and there is no possibility of a dynamic exchange.« less
  • The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO{sub 4}(F, OH). These compounds are ideal for studying F {leftrightarrow} OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state {sup 6}Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F {leftrightarrow} OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and Vmore » with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacement ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U{sub 3} eigenvalue, which lies between the c and c* directions. U{sub eq} values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder.« less
  • Power neutron diffraction and Rietveld analysis were used to investigate the crystal structures of the defective pyrochlores Pb/sub 1.5/Nb/sub 2/O/sub 6.5/ and Pb/sub 1.5/Ta/sub 2/O/sub 6.5/. Both materials crystallize with the symmetry of space group Fd3m, with lattice parameters a = 10.5647(2) and a = 10.5558(2) /Angstrom/, respectively. No evidence has been observed of oxygen or lead vacancy ordering in these compounds. This result is interpreted in terms of a model in which all lead present in the structure has sevenfold pyramidal coordination and forms domain separated by regions of lead vacancies with hexagonal or bipyramidal configurations of the oxygenmore » atoms. This model, built on the assumption that the driving force in the formation of this type of defect pyrochlore is the coordination of lead, leads us to conclude that the system Pb/sub 1+x/M/sub 2/O/sub 6+x/ (M = Nb, Ta) may exist over a range of compositions with 0.33 less than or equal to x less than or equal to 0.6, and may also explain results obtained in other studies of related materials. The electric measurements show that both compounds are predominantly electronic conductors and that the ionic contribution to the total conductivity is very small even at the highest temperatures used in the study.« less
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