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

We have controlled the nature of magnetic ordering in RbxBayMn[3-(x + 2y)]/2[Fe(CN)(6)]center dot zH(2)O 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 Mossbauer 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) mu(B) for (x = 0.84, y = 0) and 4.99(7) mu(B) for (x = 0.19, y = 0.3) sample at 1.5 K] and Fe3+ [0.98(3) mu(B) for (x = 0.84, y = 0) and 0.99(3) mu(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 similar to 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3695453]