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 Rb0.97Mn[Fe(CN)(6)](0.98)center dot 1.03H(2)O, Rb0.81Mn[Fe(CN)(6)](0.95)center dot 1.24H(2)O, and Rb0.70Cu0.22Mn0.78[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 (FeMnII)-Mn-III 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 (FeMnIII)-Mn-II ground-state at a temperature of approximately 123 K.