On the Crystal Structure of Ln
The crystal structures of La{sub 2}O{sub 2}CO{sub 3} II and Nd{sub 2}O{sub 2}CO{sub 3} II have been shown by means of high-resolution powder neutron (PND) and synchrotron X-ray diffraction (SXRD) combined with selected area electron diffraction (SAED) studies to be far more complex than earlier anticipated, owing to ordering of carbonate groups between (Ln{sub 2}O{sub 2}{sup +2}){sub n} layers. In contrast to earlier descriptions, the carbonate groups appear to be rather regular. Relative to an average model, the SAED patterns show additional scattering in the form of closely distributed, but essentially discrete, spots along < 1/3, 1/3, 1 >. Most of the observed scattering, H, can be described as H=G{+-}m q{sub 1}+n q{sub 2}, where G is the Bragg reflections of the underlying average P6{sub 3}/mmc lattice, q1=[1/3, 1/3, {+-}1/2]*, q2=[1/3, 1/3, {+-}2/3]*, and m and n are integers. The additional scattering reflects ordering of the carbonate groups into trigonal layers between the (Ln{sub 2}O{sub 2}{sup +2}){sub n} layers, but it remains open whether q{sub 1} and q{sub 2} represent two separate structures with different stacking sequences of such layers or whether they correspond to an even more complex stacking sequence. In any case, some disorder and rotational domain twinning are present. Two structure models, one for each modulation wave vector, were constructed. Rietveld-type refinements of PND data of La{sub 2}O{sub 2}CO{sub 3} II were performed, approximating the complex, and at least partly disordered, stacking sequence as a two-phase mixture of the two modulated phases. Satisfactory convergence was achieved with R{sub p}=6.4%, R{sub wp}=8.3%, and {chi}{sup 2}=3.32. The isothermal expansivities, {alpha}{sub p}, for La{sub 2}O{sub 2}CO{sub 3} II and Nd{sub 2}O{sub 2}CO{sub 3} II between 298 and 893 K were determined as 2.92x10{sup {minus}5} and 2.70x10{sup {minus}5} K{sup {minus}1}, respectively.
- Research Organization:
- Department of Chemistry, University of Oslo, Oslo (NO); Inorganic Chemistry, Arrhenius Laboratories, University of Stockholm, Stockholm (SE); Institute for Energy Technology, Kjeller (NO)
- Sponsoring Organization:
- Swiss Norwegian Beam Lines at European Synchrotron Radiation Facility, Grenoble (FR); Swedish National Science Research Council (NFR) (US)
- OSTI ID:
- 784364
- Journal Information:
- Journal of Solid State Chemistry, Vol. 158, Issue 1; Other Information: PBD: 1 Apr 2001; ISSN 0022-4596
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
LANTHANUM OXIDES
NEODYMIUM OXIDES
NEODYMIUM CARBONATES
BRAGG REFLECTION
LANTHANUM CARBONATES
CRYSTAL STRUCTURE
ELECTRON DIFFRACTION
MIXTURES
MODULATION
NEUTRON DIFFRACTION
TWINNING
X-RAY DIFFRACTION
HCP LATTICES
TRIGONAL LATTICES
SPACE GROUPS
LATTICE PARAMETERS
THERMODYNAMIC PROPERTIES
TEMPERATURE RANGE 0273-0400 K
TEMPERATURE RANGE 0400-1000 K
EXPERIMENTAL DATA