Bulk and surface switching in Mn-Fe-based Prussian blue analogues

Many Prussian blue analogues are known to show a thermally induced phase transition close to room temperature and a reversible, photoinduced phase transition at low temperatures. This work reports on magnetic measurements, X-ray photoemission, and Raman spectroscopy on a particular class of these molecular heterobimetallic systems, specifically on Rb(0.97)Mn[Fe(CN)(6)](0.98)center dot 1.03H(2)O, Rb(0.81)Mn[Fe(CN)(6)](0.95)center dot 1.24H(2)O, and Rb(0.70)Cu(0.22)Mn(0.78)[Fe(CN)(6)](0.86)center dot 2.05H(2)O, to investigate these transition phenomena both in the bulk of the material and at the sample Surface. Results indicate a high degree of charge transfer in the bulk, while a substantially reduced conversion is found at the sample surface, even in case of a near perfect (Rb:Mn:Fe = 1: 1: 1) stoichiometry. Thus, the intrinsic incompleteness of the charge transfer transition in these materials is found to be primarily due to surface reconstruction. Substitution of a large fraction of charge transfer active Mn ions by charge transfer inactive Cu ions leads to a proportional conversion reduction with respect to the maximum conversion that is still stoichiometrically possible and shows the charge transfer capability of metal centers to be quite robust upon inclusion of a neighboring impurity. Additionally, a 532 nm photoinduced metastable state, reminiscent of the high-ternperature Fe(III)Mn(II) ground state, is found at temperatures of 50-100 K. The efficiency of photoexcitation to the metastable state is found to be maximized at 90 K. The photoinduced state is observed to relax to the low temperature Fe(II)Mn(III) ground-state at a temperature of approximately 123 K