Phase transitions and incommensurate structures in the brownmillerite system Ca2(Fe1−xAlx)2O5

Abstract

The solid solution series is reported to exist from x = 0 up to x = 0.69 (e.g. [1] and references therein) at ordinary pressures. A phase boundary between structures adopting space group Pnma and I2mb was found close to x = 0.28 [2]. Crystals with higher Al-contents can be synthesised using high pressure [3, 4]. HT phase transitions are known for the iron end-member and the structures crystallising in Pnma [2]. The HT phases were considered to conform to I2mb or Imma space group symmetry. Recent single crystal diffraction studies have found that Ca2Fe2O5 transforms to a modulated structure in the range of 960–980K. This incommensurate phase can be described in superspace group Imma(00γ)s00 [5]. The modulated structure exhibits an aperiodic sequence of tetrahedral (FeO4) chains. The temperature range of the transition is characterised by phase-coexistence, which can be observed by in situ HT single crystal X-ray diffraction experiments Diffraction data collected in this region can be used to perform a two-phase refinement using Jana2006 [6]. Furthermore, domains of the two phases were observed by HT high-resolution transmission electron microscopy (TEM) and dark field TEM. The aluminium end-member Ca2Al2O5 shows an isotypic incommensurate structure above 1075(10)K.