Tetragonal to cubic transition of Sr0.8Dy0.2CoO3-delta and oxygen mobility: TG-DSC-XRD study

Processes of ordering (d-o) and disordering (o-d) of Sr2+/Dy(3+ )cations in a single-phase Sr0.8Dy0.2CoO3-delta was investigated by TG-DSC and XRD as a function of heating/cooling rate (beta= 2,10,20,50,99 K min(-1) and similar to 50 K/s) in 20% O-2-Ar flow. According to DSC data the interconversion of disordered cubic (c) and ordered tetragonal (t) structure appears at 1276-1328 K as a first-order phase transition; the temperature and enthalpy of o-d transformation have only slight dependence on beta whereas the characteristics of reversed d-o process vary greatly with cooling rate. XRD powder patterns of all samples showed no indications of a simultaneous presence of c+t domains, pointing to a single phase composition (c or t). It was suggested that the observed behavior is a consequence of two simultaneous interconnected processes of A-sublattice melting and cation/ (anion vacancy) ordering. A rarely used novel TG-DSC method based on variable gas phase composition was utilized to study properties of mobile oxygen over Sr0.8Dy0.2CoO3-delta samples. It was shown that the appearance of tetragonal phase reduces both oxygen mobility and its bonding energy, the latter decreasing substantially only at high degree of Sr2+/Dy3+ ordering. (C) 2020 Elsevier B.V. All rights reserved.