Charge transfer phase transition and ferromagnetism in a mixed- valence iron complex, n-C 3 H 7 ) 4 N[Fe II Fe III dto) 3 ] dto  C 2 O 2 S 2  N. Kojima a, * , W. Aoki a , M. Itoi a , Y. Ono a , M. Seto b , Y. Kobayashi b , Yu. Maeda b a Department of Pure and Applied Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba 3-8-1, Meguro-ku, Tokyo 153-8902, Japan b Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan Received 8 May 2001; received in revised form 8 August 2001; accepted 11 August 2001 by H. Takayama Abstract We have investigated the physical properties of n-C 3 H 7 ) 4 N[Fe II Fe III dto) 3 ] dto  C 2 O 2 S 2  by means of 57 Fe Mo Èssbauer spectroscopy and magnetic susceptibility measurement. From the analysis of 57 Fe Mo Èssbauer spectra and magnetic suscepti- bility, we have discovered a new type of ®rst order phase transition around 120 K, where the charge transfer transition between Fe II and Fe III occurs reversibly. In the higher temperature phase, the Fe III S  1=2 and Fe II S  2 sites are coordinated by six S atoms and six O atoms, respectively. In the lower temperature phase, on the other hand, the Fe III S  5=2 and Fe II S  0 sites are coordinated by six O atoms and six S atoms, respectively. Moreover, we have found the ferromagnetic phase transition at 6.5 K. The ferromagnetic order is presumably induced by the charge transfer interaction between the Fe III S  5=2 and Fe II S  0 sites. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 71.30.th; 75.50.Dd; 75.90.1w Keywords: A. Magnetically ordered materials; D. Phase transition; E. Moessbauer spectroscopy 1. Introduction Transition metal complexes with d 4 ± d 7 con®guration have a possibility of spin transition between a low-spin state and a high-spin state. The spin-crossover phenomenon was observed for the ®rst time by Cambi et al. in 1931 for trisdithiocarbamato)ironIII) complexes [1]. Since then, various kinds of spin-crossover complexes have been found [2]. The spin-crossover phenomenon has recently gained renewed importance since the discovery of the photo-induced spin transition called light induced excited spin state trapping LIESST)) for [Feptz) 6 ]BF 4 ) 2 ptz  1- propyltetrazole) [3] and the thermally induced spin- crossover transition with large thermal hysteresis around room temperature for a triazoletrz) bridged ironII) complex, [FeHtrz) 323x 4-NH 2 trz) 3x ]ClO 4 ) 2 ´nH 2 O [4]. In spin-crossover system, the enthalpy term H) domi- nates the Gibbs free energy G  H 2 TS in the low-spin phase, while the entropy term 2TS) dominates the free energy in the high-spin phase. In the Fe II,III mixed-valence system whose spin states are situated in the spin-crossover region, it is expected that the spin states of Fe II and Fe III are converted by simultaneous charge transfer in order to mini- mize the free energy of the Fe II and Fe III sites. From this viewpoint, we have synthesized a new mixed-valence Fe complex, n-C 3 H 7 ) 4 N[Fe II Fe III dto) 3 ] dto  C 2 O 2 S 2  [5], and have discovered a new type of ®rst order phase transition coupled with spin-crossover and charge-transfer by means of 57 Fe Mo Èssbauer spectroscopy and magnetic susceptibility measurement. Although a lot of spin- crossover systems and mixed-valence systems have been reported, a conjugated phenomenon coupled with spin and charge has not been known so far. In the present paper, we report a new type of phase transition in n-C 3 H 7 ) 4 N[Fe II Fe III dto) 3 ] around 120 K, where the charge transfer between the Fe II and Fe III sites occurs reversibly. Moreover, we report the ferromagnetic phase transition at 6.5 K and discuss its mechanism. Solid State Communications 120 2001) 165±170 0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-109801)00366-0 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: 181-3-5454-6741; fax: 181-3- 5454-4311. E-mail address: cnori@mail.ecc.u-tokyo.ac.jp N. Kojima).