FULL PAPER DOI: 10.1002/ejic.200601023 Correlation between the Stoichiometry and the Bistability of Electronic States in Valence-Tautomeric Rb x Mn[Fe(CN) 6 ] y ·zH 2 O Complexes Saioa Cobo, [a] Raquel Fernández, [a] Lionel Salmon, [a] Gábor Molnár, [a] and Azzedine Bousseksou* [a] Keywords: Prussian blue analogues / Magnetic properties / Phase transitions / Raman spectroscopy A series of Prussian blue analogues, Rb x Mn[Fe(CN) 6 ] y ·zH 2 O with different stoichiometries have been synthesized. Ele- mental analyses reveal a quasilinear decrease in x and y when z increases. Some of these samples present a first-order phase transition between the Mn II –Fe III and Mn III –Fe II elec- tronic states with large thermal hysteresis loops observed in the magnetic susceptibility. The phase-transition tempera- tures correlate with the stoichiometry, but quantitative pre- dictions cannot be made because of the disordered nature of the compounds. Contrary to previous literature reports, Introduction The bistability of electronic states in transition-metal complexes represents an important field of research in coor- dination chemistry. The bistability in these compounds gen- erally arises from strong electron-lattice couplings. Typical examples are spin crossover phenomena, [1,2] and valence- tautomerism. [3] In this latter class, valence-tautomeric Prus- sian blue analogues of transition metals with general for- mula A x M II [M' III (CN) 6 ] y ·nH 2 O (where A is an alkali metal cation, and M and M' are divalent or trivalent transition- metal cations) have attracted considerable interest because of their remarkable magnetic properties, such as high Curie temperatures [4,5] or photomagnetic phenomena. [6–8] In some of these complexes, a first-order thermal phase transition was observed and associated with metal-to-metal electron transfer. [9,10] The compound Rb x Mn[Fe(CN) 6 ] y ·zH 2 O shown in Figure 1 (noncoordinated water molecules and ru- bidium atoms are not represented) is one of the most studied members of this family. [10,11] It exhibits switching phenomena between the high-temperature (HT) Fe III (S = 1/2)–CN–Mn II (S = 5/2) and the low-temperature (LT) Fe II (S = 0)–CN–Mn III (S = 2) states that can be induced by changing the sample temperature or pressure as well as by light irradiation. [10–18] The two phases display markedly dif- ferent magnetic, optical, and electric properties. [a] Laboratoire de Chimie de Coordination, CNRS UPR-8241, 205 Route de Narbonne, 31077 Toulouse, France Fax: +33-561-553-003 E-mail: boussek@lcc-toulouse.fr Eur. J. Inorg. Chem. 2007, 1549–1555 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1549 calorimetric and Raman spectroscopic measurements reveal that the charge-transfer phase transition is accompanied by a significant entropy change, ΔS = 48–59 J K –1 mol –1 , of mainly vibrational origin. In addition to the thermal phase transition, Raman spectra also show that below ca. 120 K a photoin- duced Mn III –Fe II to Mn II –Fe III conversion occurs in certain stoichiometries. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) Figure 1. Schematic structure of Rb x Mn[Fe(CN) 6 ] y ·zH 2 O with a Fe(CN) 6 3– vacancy. (Noncoordinated water molecules and rubid- ium atoms are not represented.) Even if Rb x Mn[Fe(CN) 6 ] y ·zH 2 O has been the object of many investigations, several questions remain to be clari- fied. Notably, the control of the phase-transition tempera- ture is an important issue because these compounds present very large thermal hysteresis loops (up to 138 K). [18] Such a large bistability domain is an appealing property for the application of these materials in various devices (e.g. memo- ries). In preliminary results, Ohkoshi et al. [17] found a corre- lation between the hysteresis width and the stoichiometry of these compounds, but further measurements performed by the same group [18] and others [11] appear to escape this correlation. Another important question concerns the ori- gin of the thermal phase transition. This question has been investigated extensively in the case of spin-crossover com- plexes and it is now generally accepted that thermal spin