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Title: A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates

Abstract

Lanthanum-aluminates of the composition (1 {minus} x)Al{sub 2}O{sub 3}{center_dot}xLa{sub 2}O{sub 3} (0 < x < 0.7) were prepared by sol-gel synthesis. Subsequent heat-treatment temperatures ranging from 200 to 1,200 C were applied. The resulting samples were characterized by X-ray powder diffraction and {sup 27}Al MAS NMR. The most complex spectra were analyzed by MQMAS NMR in order to get insight in the number of sites and the possible distribution of the NMR parameters in the amorphous samples. Analysis of the MQMAS spectra, taking the efficiency of this experiment for different sites into account, helped to resolve ambiguities in the MAS spectra, which could otherwise not be deconvoluted in a unique way. The analyses show that, except for the sample with the lowest La (x = 0.085) concentration, hardly any pentacoordinated aluminum is observed over the whole temperature treatment range. For all samples the concentration of six-coordinated aluminum decreases as the heating temperature increases until the point where crystallization occurs. The concentration of 4-fold coordinated aluminum shows the opposite behavior. When crystallization occurs, a remarkable transformation between four- and six-coordinated aluminum takes place. Crystallization of the samples occurs at lower temperatures for samples with high amounts of lanthanum. At low Lamore » content crystallization is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. Storage of the samples in a water-saturated atmosphere leads to a conversion of four- to six-coordinated aluminum. This shows that the low-coordinated Al atoms are accessible to water molecules and must therefore be situated at the surface.« less

Authors:
;  [1]; ;  [2]
  1. Babes-Bolyai Univ., Cluj-Napoca (Romania)
  2. Univ. of Nijmegen (Netherlands)
Publication Date:
OSTI Identifier:
691296
Resource Type:
Journal Article
Journal Name:
Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical
Additional Journal Information:
Journal Volume: 103; Journal Issue: 36; Other Information: PBD: 9 Sep 1999
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; NUCLEAR MAGNETIC RESONANCE; AMORPHOUS STATE; CRYSTALS; LANTHANUM; ALUMINATES; X-RAY DIFFRACTION; CRYSTAL STRUCTURE

Citation Formats

Iuga, D., Simon, S., Boer, E. de, and Kentgens, A.P.M. A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates. United States: N. p., 1999. Web. doi:10.1021/jp991257q.
Iuga, D., Simon, S., Boer, E. de, & Kentgens, A.P.M. A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates. United States. doi:10.1021/jp991257q.
Iuga, D., Simon, S., Boer, E. de, and Kentgens, A.P.M. Thu . "A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates". United States. doi:10.1021/jp991257q.
@article{osti_691296,
title = {A nuclear magnetic resonance study of amorphous and crystalline lanthanum-aluminates},
author = {Iuga, D. and Simon, S. and Boer, E. de and Kentgens, A.P.M.},
abstractNote = {Lanthanum-aluminates of the composition (1 {minus} x)Al{sub 2}O{sub 3}{center_dot}xLa{sub 2}O{sub 3} (0 < x < 0.7) were prepared by sol-gel synthesis. Subsequent heat-treatment temperatures ranging from 200 to 1,200 C were applied. The resulting samples were characterized by X-ray powder diffraction and {sup 27}Al MAS NMR. The most complex spectra were analyzed by MQMAS NMR in order to get insight in the number of sites and the possible distribution of the NMR parameters in the amorphous samples. Analysis of the MQMAS spectra, taking the efficiency of this experiment for different sites into account, helped to resolve ambiguities in the MAS spectra, which could otherwise not be deconvoluted in a unique way. The analyses show that, except for the sample with the lowest La (x = 0.085) concentration, hardly any pentacoordinated aluminum is observed over the whole temperature treatment range. For all samples the concentration of six-coordinated aluminum decreases as the heating temperature increases until the point where crystallization occurs. The concentration of 4-fold coordinated aluminum shows the opposite behavior. When crystallization occurs, a remarkable transformation between four- and six-coordinated aluminum takes place. Crystallization of the samples occurs at lower temperatures for samples with high amounts of lanthanum. At low La content crystallization is shifted to higher temperatures and thus the lower Al (four and five) coordinations are maintained over a larger temperature range. Storage of the samples in a water-saturated atmosphere leads to a conversion of four- to six-coordinated aluminum. This shows that the low-coordinated Al atoms are accessible to water molecules and must therefore be situated at the surface.},
doi = {10.1021/jp991257q},
journal = {Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical},
number = 36,
volume = 103,
place = {United States},
year = {1999},
month = {9}
}