Observation of the Fixed Fe–CN–Mn Cluster in Cesium Manganese Hexacyanoferrate Kotaro ISHIJI, Masataka DEGUCHI 1 , Kazutoshi KAWAKAMI 1 , Nobuo NAKAJIMA 1 , Tomoyuki MATSUDA 2 , Hiroko TOKORO 2 , Shin-ichi OHKOSHI 2 , and Toshiaki IWAZUMI 3 Kyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu, Saga 841-0005, Japan 1 Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8526, Japan 2 Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan 3 Department of Mathematical Sciences, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan (Received September 18, 2009; accepted May 10, 2010; published June 25, 2010) The temperature-induced phase transition of cesium manganese hexacyanoferrate is based on the charge transfer between transition metal ions, and the behavior is complex. To clarify the mechanism for the phase transition, X-ray absorption and magnetic susceptibility were measured. The magnetic susceptibility behavior was different from that of rubidium manganese hexacyanoferrate. X-ray data analyses suggested that the phase transition of cesium manganese hexacyanoferrate is the result of a local charge transfer in the fixed Fe–CN–Mn cluster and the interaction among the clusters. Calculation of the Gibbs free energy of the mean-field model indicates that the cluster interaction influences the phase transition behavior of cesium manganese hexacyanoferrate. KEYWORDS: cesium manganese hexacyanoferrate, temperature-induced phase transition, X-ray absorption spectroscopy, magnetic susceptibility, localized charge transfer DOI: 10.1143/JPSJ.79.074801 1. Introduction Prussian blue analogues exhibit an interesting temper- ature- and photo-induced first-order phase transition. 1–13) These analogues are composed of a three-dimensional (3D) network structure in which the transition-metal ions are strongly bridged by cyano groups. 14,15) The temperature- induced phase transition of rubidium manganese hexacya- noferrate was observed by Ohkoshi and collaborators, 1–5) in which they described the charge transfer from Fe III –CN– Mn II for the high-temperature (HT) phase to Fe II –CN–Mn III for the low-temperature (LT) phase as the origin of the phase transition. Matsuda et al. 7) indicated that Fe II –CN–Mn II and Fe III –CN–Mn II are predominant in the HT phase, and the LT phase is composed of Fe II –CN–Mn II and Fe II –CN–Mn III in cesium manganese hexacyanoferrate; complicated phase transition behavior that is more complex than rubidium manganese hexacyanoferrate is assumed. Recently, we investigated the electronic states of the transition-metal ions in cesium manganese hexacyanoferrate using X-ray absorp- tion spectroscopy, 8) and reported no contribution of Fe II – CN–Mn II toward the phase transition, but speculated local interaction among ions. However, more data are required in order to understand the cause of the phase transition. For this purpose, we have investigated the phase transition behavior by the magnetic susceptibility and X-ray absorption of six samples, and attempted to construct a phase transition model. 2. Sample Preparation and Experimental Method The target cesium manganese hexacyanoferrate (CMF) samples were prepared by reacting an aqueous solution of MnCl 2 (0.025 mol dm 3 ) and CsCl (a mol dm 3 ) with a mixed aqueous solution of K 3 [Fe(CN) 6 ] (0.025 mol dm 3 ) and CsCl (b mol dm 3 ): ða; bÞ¼ð1:0; 3:0Þ for CMF-1 and CMF-2, ð0:5; 0:5Þ for CMF-3 and CMF-4, and ð0:1; 0:1Þ for CMF-5 and CMF-6. CMF-1, CMF-3, and CMF-5 were prepared at room temperature, while the others were synthesized at 320 K to accelerate the reaction. The resultant precipitates were filtered and dried prior to analysis. In addition, cesium(I)–manganese(II)–hexacyanoferrate(II), whose bond structure was occupied by Fe II –CN–Mn II , was synthesized as a reference by reacting an aqueous solution of MnCl 2 (0.025 mol dm 3 ) and CsCl (1 mol dm 3 ) with a mixed solution of K 4 [Fe(CN) 6 ] (0.025 mol dm 3 ) and CsCl (1 mol dm 3 ) at room temperature. After the sample prep- aration, X-ray absorption spectroscopy and the magnetic susceptibility measurements were conducted. The X-ray absorption spectra around the Fe and Mn K edges were collected using transmitted geometry of the beamline 9A and 10B at the Photon Factory, Institute of Materials Structure Science, Japan. The magnetic susceptibilities were measured with a superconducting quantum interference device magne- tometer (SQUID). 3. Results Determination of the composition formulae is significant, because there is the correlation between the phase transition and the elemental composition. 4,7) We determined the formulae of the prepared samples from the room temperature X-ray absorption spectra. Figure 1 shows the observed spectra (open circles) around the (a) Fe K and (b) Mn K edges at room temperature. In each of the six spectra in Figs. 1(a) and 1(b), the profile varies slightly with each sample, due to the different elemental compositions. Determination of the formulae from the spectra was Journal of the Physical Society of Japan Vol. 79, No. 7, July, 2010, 074801 #2010 The Physical Society of Japan 074801-1