Anisotropic thermal expansion of Laₙ(Ti,Fe)ₙO₃ₙ₊₂ (n = 5 and 6)

Titelangaben

Wölfel, Alexander ; Dorscht, Philipp ; Lichtenberg, Frank ; van Smaalen, Sander:
Anisotropic thermal expansion of Laₙ(Ti,Fe)ₙO₃ₙ₊₂ (n = 5 and 6).
In: Acta Crystallographica Section B. Bd. 69 (2013) Heft 2 . - S. 137-144.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S2052519213003126

Volltext

Link zum Volltext (externe URL):

Abstract

Crystal structures are reported for two perovskite-related compounds with nominal compositions La\sb 5(Ti\sb 0.8Fe\sb 0.2)\sb 5O\sb 17 and La\sb 6(Ti\sb 0.67Fe\sb 0.33)\sb 6O\sb 20 at seven different temperatures between 90 and 350K. 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\sb 6 groups. It is likely that the same mechanism will determine similar anisotropic thermal behaviour of other compounds \it A\sb nB\sb nO\sb 3\it n+2. The crystal structures have revealed partial chemical order of Ti/Fe over the \it 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\sp 4+ for sites fully occupied by Ti, and of Fe\sp 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\sb 6 and FeO\sb 6 octahedral groups.