Charge‐Transfer Phase Transition and Ferromagnetism of Iron Mixed‐Valence Complexes (n‐C_nH_2n+1)₄N[Fe^IIFe^III(dto)₃] (n = 3–6; dto = C₂O₂S₂)

Abstract

The iron mixed‐valence complex, (n‐C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato) shows a charge‐transfer (CT) phase transition at TCT = 122.4 K. In the vicinity of TCT, the spin state changes from FeII (S = 2) – FeIII (S = 1/2) (high‐temperature phase: HTP) to FeII (S = 0) – FeIII (S = 5/2) (low‐temperature phase: LTP) accompanied by a charge transfer between FeII and FeIII. This complex also undergoes a ferromagnetic transition at 7 K in the LTP. In order to investigate the mechanism of the CT phase transition and the ferromagnetism, we have systematically synthesized (n‐CnH2n+1)4N[FeIIFeIII(dto)3] (n = 3–6), and have investigated their physical properties by magnetic susceptibility, powder X‐ray diffraction measurements, and ESR spectroscopy. The compounds (n‐CnH2n+1)4N[FeIIFeIII(dto)3] (n = 3–6) display ferromagnetic phase transitions at 7 K, 7 K (& 13 K), 19.5 K, and 22 K, respectively. For n = 3 and 4, the CT phase transitions take place at TCT ≈ 120 K and TCT ≈ 140 K, respectively. For n = 5 and 6, on the other hand, the CT phase transition does not occur, and the spin configuration of FeII (S = 2) and FeIII (S = 1/2) corresponding to the HTP for n = 3 and 4 is stable between 2 K and 300 K. The cation size of (n‐CnH2n+1)4N+ (n = 3–6) acts as an effective internal pressure which induces the CT phase transition and the ferromagnetic ordering in the [FeIIFeIII(dto)3]–∞ layer. We also discuss the mechanism of the CT phase transition and the ferromagnetism induced by the charge‐transfer interaction between FeII and FeIII. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Study on the ferromagnetic state in iron mixed-valence complexes, A[FeIIFeIII(dto)3] (A = (n-CnH2n + 1)4N; dto = C2S2O2) by means of Mössbauer spectroscopy

“…On the assumption of positive and negative V zz , θ are estimated at 90 • and 40 • , respectively. Here, θ can be recognized as 90 • , because the principal axis of V zz lies along the D 3 axis of the [Fe II Fe III (dto) 3 ] ∞ layer and H n lies in Table 1 The selected Mössbauer spectral hyperfine parameters in the ferromagnetic ordered states at 4 or 4.5 K for (n- 3 ] ∞ layer according to the anisotropic magnetization measurement [7].…”

“…The spin states of the iron sites change between Fe III S 6 (S = 1/2)-Fe II O 6 (S = 2) and Fe II S 6 (S = 0)-Fe III O 6 (S = 5/2) corresponding to the high-temperature phase (HTP) and low-temperature phase (LTP), respectively, due to the charge transfer phase transition (CTPT). The CTPT also occurs in (n-C 4 H 9 ) 4 N[Fe II Fe III (dto) 3 ], while the CTPT does not occur in (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 5 and 6) [3,4]. Moreover, (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 3-6) show the ferromagnetic phase transitions [3].…”

“…The CTPT also occurs in (n-C 4 H 9 ) 4 N[Fe II Fe III (dto) 3 ], while the CTPT does not occur in (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 5 and 6) [3,4]. Moreover, (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 3-6) show the ferromagnetic phase transitions [3]. The Curie temperatures, T C , for n = 3 and 4 are 7 K and 7 & 13 K, respectively, while those for n = 5 and 6 are 19.5 K and 22 K, respectively [3].…”

“…(n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] were prepared according to a similar way of the previous report [3] at the synthesis temperature of 10 • C. For 57 Fe Mössbauer measurement, 57 Co in Rh was used as a Mössbauer source. The spectra were calibrated by the six lines of α-Fe, the center of which was taken as zero isomer shift.…”

“…Moreover, (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 3-6) show the ferromagnetic phase transitions [3]. The Curie temperatures, T C , for n = 3 and 4 are 7 K and 7 & 13 K, respectively, while those for n = 5 and 6 are 19.5 K and 22 K, respectively [3]. In this paper, we report the detailed analysis of the Mössbauer spectra in the ferromagnetic states of (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ] (n = 3-6).…”