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Lanthanum‐ and Oxygen‐Deficient Crystal Structures of Oxide–Ion Conducting Apatite‐Type Silicates
Author(s) -
Iwata Tomoyuki,
Béchade Emilie,
Fukuda Koichiro,
Masson Olivier,
Julien Isabelle,
Champion Eric,
Thomas Philippe
Publication year - 2008
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1551-2916.2008.02731.x
Subject(s) - crystallography , lanthanum , apatite , crystal structure , ion , chemistry , anisotropy , materials science , mineralogy , inorganic chemistry , physics , organic chemistry , quantum mechanics
The structural change of La 9.33−2 x (SiO 4 ) 6 O 2−3 x with increasing x value was investigated by means of laboratory X‐ray powder diffraction (Cu K α 1 ). Crystal structures of the three types of specimens with x =0, 0.06, and 0.085 were refined by the Rietveld method, with the anisotropic displacement ( U ) parameters being assigned for all atoms. Each crystal structure was composed of three types of polyhedra: La1O 9 , La2O 7 , and SiO 4 . With increasing x value, the occupancies ( g ) of both La1 and La2 sites decreased steadily. The La vacancies were charge balanced by the deficiency of the mobile O atoms that were not covalently bonded to Si atoms. The polyhedral volume of La2O 7 decreased steadily about 0.7% with a decrease of the g (La2) value from 0.995 to 0.982. On the other hand, the polyhedral volume of La1O 9 showed an increase of about 1.0% with a decrease of the g (La1) value from 0.841 to 0.818. The marked difference in deformation behavior of the two types of polyhedra could be attributable to the distinction in site occupancies between La1 [ g (La1)<0.85] and La2 [ g (La2)>0.98]. For the mobile O atoms, the displacement magnitudes, being evaluated from the U parameters, decreased steadily with increasing x value. The positional disordering of these atoms might be related to their mobilities at high temperatures.