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Anisotropic thermal expansion of La n (Ti,Fe) n O 3 n + 2 ( n = 5 and 6)
Author(s) -
Wölfel Alexander,
Dorscht Philipp,
Lichtenberg Frank,
van Smaalen Sander
Publication year - 2013
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
eISSN - 2052-5206
pISSN - 2052-5192
DOI - 10.1107/s2052519213003126
Subject(s) - anisotropy , valence (chemistry) , octahedron , thermal expansion , crystallography , perovskite (structure) , crystal structure , materials science , oxygen , chemical bond , condensed matter physics , atmospheric temperature range , crystal (programming language) , chemistry , thermodynamics , physics , metallurgy , organic chemistry , quantum mechanics , computer science , programming language
Crystal structures are reported for two perovskite‐related compounds with nominal compositions La 5 (Ti 0.8 Fe 0.2 ) 5 O 17 and La 6 (Ti 0.67 Fe 0.33 ) 6 O 20 at seven different temperatures between 90 and 350 K. For both compounds no evidence of a structural phase transition in the investigated range of temperatures was found. The thermal expansions are found to be anisotropic, with the largest thermal expansion along a direction parallel to the slabs of these layered compounds. The origin of this anisotropy is proposed to be a temperature dependence of tilts of the octahedral (Ti,Fe)O 6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds A n B n O 3 n + 2 . The crystal structures have revealed partial chemical order of Ti/Fe over the B sites, with iron concentrated towards the centers of the slabs. Local charge compensation is proposed as the driving force for the chemical order, where the highest‐valent cation moves to sites near the oxygen‐rich borders of the slabs. A linear dependence on the site occupation fraction by Fe of the computed valences leads to extrapolated valence values close to the formal valence of Ti 4+ for sites fully occupied by Ti, and of Fe 3+ for sites fully occupied by Fe. These results demonstrate the power of the bond‐valence method, and they show that refined oxygen positions are the weighted average of oxygen positions in TiO 6 and FeO 6 octahedral groups.