Structural transformation in fluorinated LaACuGaO5 (A=Ca, Sr) brownmillerites

Abstract

The brownmillerite type phases LaSrCuGaO5 and LaCaCuGaO5 were fluorinated using XeF2 and the resulting materials were investigated using X–ray diffraction, high resolution electron microscopy (HREM) and electron diffraction. For low content of XeF2, both materials undergo a decrease in orthorhombicity maintaining an aO≈4aP, bO≈aP2, cO≈aP2 unit cell with space group Ima2. On increasing the amount of XeF2, LaSrCuGaO5 transforms to a tetragonal structure with cell parameters, aT≈aP, cT≈2aP (space group P4/mmm). The model for the tetragonal structure was proposed on the basis of HREM observations. The anion exchange reaction results in the replacement of one oxygen anion by two fluorines in the (GaO□) layers. This reaction is accompanied by a random occupation of the anion positions in the Ga-layers by oxygen, fluorine and anion vacancies, leading to a tetragonal symmetry. Different superstructures, due to local ordering of anions and anion vacancies, were observed by electron microscopy.