Polymorphism of the iron doped strontium aluminate SrAl1.5Fe0.5O4

The partial substitution of Al by Fe atoms in SrAl₂O₄ allowed to stabilize four stuffed tridymite derivative structures SrAl_1.5Fe_0.5O₄. The different phases have been characterized by TEM and XRPD techniques. Two are isotypes of those observed for the undoped oxides, namely the hexagonal phase with A×A×C (with A and C being the tridymite unit cell parameters) and space group P6₃ and the monoclinic one with A×A×C, β≈93° and space group P2₁, with a synthesis temperature lower than the one required for SrAl₂O₄. By annealing, two original phases, denoted O₁ and O₂, are obtained; they are metrically similar (3A×A×C and β≈90°) and only differ by their space groups. The TEM study showed that the transitions between the different phases follow topotactic mechanisms, through the formation of twinning boundaries, antiphase boundaries and planar defects. The annealed sample exhibits a mesomorph state and a reversible transition from this semi-ordered state to a crystalline phase. This dynamic transition takes place over a very wide temperature range from 620 to 1120 °C. The reversibility of the transition has been studied by DSC measurements. The crystallization energy of the orthorhombic phases is of the order of 10 J/g, at T≈622 °C as T decreases. The variation of the peak height observed as the annealing temperature increases is explained by the complex microstructures, which create an ill-defined energy barrier. Structural models related to the stuffed tridymite derivative structures are proposed for the new forms of the ferri-aluminate.