Presentation on theme: "Metal-to-Metal Electron Transfer and Magnetic Interactions in a Mixed-Valence Prussian Blue Analogue A. Bhattacharjee, P. Gütlich et al. Department of."— Presentation transcript:
Metal-to-Metal Electron Transfer and Magnetic Interactions in a Mixed-Valence Prussian Blue Analogue A. Bhattacharjee, P. Gütlich et al. Department of Physics, Visva-Bharati University, Santiniketan , India, Department of Chemistry, University of Mainz, 55099, Mainz, Germany,
Prussian Blue (PB) Analogue The hexacyanometalate [B(CN) 6 ] x- ions are well known building blocks used for fabrication of the hetero-metal assemblies exhibiting bulk magnetization, where reaction of the [B(CN) 6 ] x- ions with metal ions gives rise to the so-called Prussian Blue (PB) analogues - MA[B(CN) 6 ].(solvent) (M = monovalent alkali metal ion, and A, B = di- and trivalent transition metal ions). These materials exhibit various magnetic properties depending on their transition metal combinations, e.g., high T C magnet, magnetic pole reversal, spin glass behavior and photo-induced magnetic transition. The alkali-doped analogues are among the most extensively studied recent materials of the Prussian Blue family in regard to photo-induced and pressure-induced metal-to-metal electron transfer and magnetism. For further details see the follwoing references: - M. Verdaguer et al., Coord. Chem. Rev (1999) 1023; - T. Yokoyama, H. Tokoro, S.-i. Ohkoshi, and K. Hashimoto Phys. Rev. B 2002, 66, ; - A. Goujon, F. Varret, V. Escax, A. Bleuzen, M. Verdaguer Polyhedron, 2001, 20 (11-14), ; - V. Ksenofontov, G. Levchenko, S. Reiman, P. Gütlich, A. Bleuzen, V. Escax, M. Verdaguer, Phys. Rev. B 2003, 68, ; - A. Bhattacharjee, S. Saha, S. Koner, V. Ksenofontov, S. Reiman, P. Gütlich, J. Magn. Magn. Mater. 2006, 302, ; - A. Bhattacharjee, S. Saha, S. Koner, Y. Miyazaki J. Magn. Magn. Mater. 2007, 312,
K 0.2 Mn II.66 Mn III 1.44 [Fe II 0.2 Fe III 0.8 (CN) 6 ]O 0.66 (CH 3 COO) 1.32 ]·7.6H 2 O Calorimetric study under magnetic field and field dependent magnetization studies of a new PB analogue - K 0.2 Mn II.66 Mn III 1.44 [Fe II 0.2 Fe III 0.8 (CN) 6 ]O 0.66 (CH 3 COO) 1.32 ]·7.6H 2 O have indicated a ferrimagnetic phase transition around 8 K along with a ferromagnetic phase transition around 2 K. The compound exhibits metamagnetic transition around 3 K observed in the magnetic measurements. Furthermore, the compound exhibits a thermal anomaly around 185 K arising due to a glass transition. Magnetic TransitionsMetamagnetic Transition
Mössbauer Spectroscopy Mössbauer spectroscopic studies of this compound were done at various temperatures. The Mössbauer spectra obtained at all the measuring temperatures exhibited the existence of both Fe III and Fe II in low spin states. Thus, the compound exists in Fe III (low spin, t 2g 5, S = ½), Fe II (low spin, t 2g 6, S = 0), Mn III (high spin, t 2g 3 e g 1, S = 2) and Mn II (high spin, t 2g 3 e g 2, S = 5/2) mixed valence states. The onset of magnetic ordering of the Fe III low spin species around 5 K is clearly seen by the broadening of the blue signal, which develops to a reasonably well resolved magnetic sextet at 4.2 K.
Metal-to-Metal Electron Transfer Mössbauer spectroscopy successfully detects the phenomenon of metal to metal electron transfer between Mn and Fe ions possibly through the [Fe III (t 2g 5, S = ½) –CN- Mn II (t 2g 3 e g 2, S =5/2)] to [Fe II (t 2g 6, S = 0)–CN- Mn III (t 2g 3 e g 1, S = 2)] process. At temperatures above the magnetic transition the compound exists as a mixture of [Fe III (S = ½) –CN- Mn II (S = 5/2)] and [Fe II (S = 0) –CN- Mn III (S = 2)] states, whereas below the magnetic transition the former state predominates. Temperature dependence of the population ratio of Fe III and Fe II low spin species obtained from Mössbauer spectroscopy
Glass Transition A glass transition at 194 K has been observed in the heat capacity study due to freezing of the orientational motion of the H 2 O molecules present. This phenomenon is reflected in the temperature dependence of the estimated Fe III and Fe II concentrations in the present material obtained through Mössbauer spectroscopy. Mössbauer spectroscopy being extremely sensitive to lattice dynamics is able to detect the effect of the glass transition due to the freezing of the orientational motion of the H 2 O molecules inducing non-rigid / dynamic character in the lattice on and around the glass transition temperature. From CalorimetryFrom Mössbauer spectroscopy Bhattacharjee, et al., J. Magn. Magn. Mater. 302 (2006) 173; J. Magn. Magn. Mater. 312 (2007) 435