A TEM, XRD, and crystal chemical investigation of oxygen/vacancy ordering in (Ba 1− xLa x) 2In 2O 5+ x, 0⩽ x⩽0.6

Abstract

A careful TEM and XRD study of the (Ba 1− x La x ) 2In 2O 5+ x , 0⩽ x⩽0.6, ‘defect-perovskite’-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1×1×2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the G ± 1 2 〈001〉 p * regions of reciprocal space was found to be typical of all (Ba 1− x La x ) 2In 2O 5+ x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the G ± 1 2 〈001〉 p * -type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward.