Isovalent-substitution effect on the Verwey-type transition in the A-site-ordered double perovskite (Ba,Sr)RFe₂O₅

Two types of chemical-pressure effects on the Verwey-type transition in BaSmFe₂O₅ are studied, viz., ionic-size contractions at the Ba and Sm sites. Whereas the replacement of Sm by the smaller Eu and Gd atoms increases the transition temperature T_V from 232 K to 264 K, the Sr-for-Ba substitution decreases it from 232 K to 180 K for x=0.15 in (Ba_1-xSr_x)SmFe₂O₅. Of various structural parameters determined by synchrotron x-ray diffraction above T _V, the magnitude of the orthorhombic distortion (=b-a) is found to correlate with T_V in a manner that unifies both substitution schemes. It is suggested that when an increase in b-a above T_V is achieved by means of the structural substitution, it makes it easier for the d_xz Fe²⁺ orbital ordering to occur below T_V.