Rb_xBa_yMn_[3-(x+2y)]/2[Fe(CN)₆]·zH₂O prussian blue analogues: controlling magnetic ordering by alkaline earth metal cation substitution and magnetic field

Abstract

We have controlled the nature of magnetic ordering in Rb_xBa_yMn_[3-(x+2y)]/2[Fe(CN)₆]·zH₂O prussian blue analogues by alkaline earth metal cation (Ba²⁺) substitution and application of an external magnetic field. Structural analysis, by employing Rietveld refinement of X-ray and neutron diffraction patterns, reveals a tetragonal crystal structure for all these compounds. The presence of Fe³⁺-C ≡ N-Mn²⁺ chains in the structure, and absence of both cyanide flipping (Mn²⁺-C ≡ N-Fe³⁺) and charge transfer from Mn²⁺ to Fe³⁺ (Fe²⁺-C ≡ N-Mn³⁺) have been confirmed by infrared and Mössbauer studies. The analysis of the low temperature neutron powder diffraction data reveals a long-range antiferromagnetic ordering in these compounds. The magnetic structure is layered, consisting of an antiparallel stacking of ferromagnetic sheets along the crystallographic c-direction. Within each sheet in the ab plane, the ordered moments of Mn²⁺ [5.04(1) μ_B for (x = 0.84, y = 0) and 4.99(7) μ_B for (x = 0.19, y = 0.3) sample at 1.5 K] and Fe³⁺ [0.98(3) μ_B for (x = 0.84, y = 0) and 0.99(3) μ_B for (x = 0.19, y = 0.3) sample at 1.5 K] are aligned ferromagnetically along the c-axis. Interestingly, dc magnetization study indicates that on application of an external magnetic field, an antiferromagnetic to ferrimagnetic-like phase transition occurs below ∼5 K. This phase transition becomes more and more prominent with increasing magnetic field as well as for higher Ba substitution. The role of a number of factors (such as ionic radius of Ba²⁺ ion and change in net ligand strength around Mn due to chemical insertion of Ba²⁺ ions) on the affecting nature of magnetic ordering has been discussed. The observed magnetism in these compounds is seen in light of the orbital model of superexchange interaction between magnetic ions (Mn²⁺ and Fe³⁺) mediated by cyanide ligands.