In:
Acta Crystallographica Section B Structural Science, International Union of Crystallography (IUCr), Vol. 68, No. 2 ( 2012-04-01), p. 101-106
Abstract:
The incommensurately modulated crystal structures of Ca 0.28 Ba 0.72 Nb 2 O 6 (CBN28) and Ce 0.02 Ca 0.25 Ba 0.72 Nb 2 O 6 (Ce:CBN28) were refined in the supercentred setting X 4 bm ( AA 0,− AA 0) of the 3 + 2-dimensional superspace group P 4 bm ( aa ½,− aa ½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO 6 octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.
Type of Medium:
Online Resource
ISSN:
0108-7681
DOI:
10.1107/S0108768111054863
DOI:
10.1107/S0108768111054863/ck5044sup1.cif
DOI:
10.1107/S0108768111054863/ck5044cbnsup2.hkl
DOI:
10.1107/S0108768111054863/ck5044ce-cbnsup3.hkl
DOI:
10.1107/S0108768111054863/ck5044cbnsup4.hkl
DOI:
10.1107/S0108768111054863/ck5044ce-cbnsup5.hkl
DOI:
10.1107/S0108768111054863/ck5044sup6.pdf
Language:
Unknown
Publisher:
International Union of Crystallography (IUCr)
Publication Date:
2012
detail.hit.zdb_id:
2020841-8
SSG:
13
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