Structure evolution upon chemical and physical pressure in (Sr_1−xBa_x)₂FeSbO₆

Here we demonstrate the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P6₃/mmc structure upon the isovalent larger-for-smaller A-site cation substitution in the B-site ordered double-perovskite system (Sr_1−xBa_x)₂FeSbO₆. This is the same transformation sequence previously observed up to Fm-3m upon heating the parent Sr₂FeSbO₆ phase to high temperatures. High-pressure treatment, on the other hand, transforms the hexagonal P6₃/mmc structure of the other end member Ba₂FeSbO₆ back to the cubic Fm-3m structure. Hence we may conclude that chemical pressure, physical pressure and decreasing temperature all work towards the same direction in the (Sr_1−xBa_x)₂FeSbO₆ system. Also shown is that with increasing Ba-for-Sr substitution level, i.e. with decreasing chemical pressure effect, the degree-of-order among the B-site cations, Fe and Sb, decreases. - Graphical abstract: In the (Sr_1−xBa_x)₂FeSbO₆ double-perovskite system the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P6₃/mmc structure is seen upon the isovalent larger-for-smaller A-site cation substitution. High-pressure treatment under 4 GPa extends stability of the cubic Fm-3m structure within a wider substitution range of x. - Highlights: • Gradual structural transitions upon A-cation substitution in (Sr_1−xBa_x)₂FeSbO_6. • With increasing x structure changes from I2/m to I4/m, Fm-3m and P6₃/mmc. • Degree of B-site order decreases with increasing x and A-site cation radius. • High-pressure treatment extends cubic Fm-3m phase stability for wider x range. • High-pressure treatment affects bond lengths mostly around the A-cation.