Resumen:
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[EN] Mössbauer and Raman spectroscopies, calorimetric measurements, the LIESST effect (light induced excited spin state trapping) and dynamics of the high-spin (HS) to low-spin (LS) relaxation have been investigated in ...[+]
[EN] Mössbauer and Raman spectroscopies, calorimetric measurements, the LIESST effect (light induced excited spin state trapping) and dynamics of the high-spin (HS) to low-spin (LS) relaxation have been investigated in order to thoroughly study the spin-crossover compounds of formula {Fe(L)[H2B(pz)2]2}, where L = 2,2`-bipyridine (1) and 1,10-phenanthroline (2). The Mössbauer studies have revealed the occurrence of an almost complete spin conversion at low temperature, while 9.5% (1) and 15% (2) of molecules remain LS at room temperature. The thermal variation of the proportion of HS molecules, nHS, obtained from the Mössbauer spectra has been used to calibrate the thermal dependence of nHS deduced from the magnetic susceptibility measurements. The thermodynamic Slichter and Drickamer model was used to fit the thermal variation of nHS. The estimated thermodynamic parameters are ¿H = 7.7 kJ mol-1, ¿S = 47.4 J mol-1 K-1 and interaction parameter ¿ = 2.1 kJ mol-1 for 1 and ¿H = 7.9 kJ mol-1, ¿S = 48.3 J mol-1 K-1 and ¿ = 3 kJ mol-1 for 2. These data agree with those obtained from heat capacity experiments:¿ ¿H = 7.7 ± 0.4 kJ mol-1 and ¿S = 48 ± 3 J mol-1 K-1 for 1, and ¿H = 8.0 ± 0.4 kJ mol-1 and ¿S = 49 ± 3 J mol-1 K-1 for 2. The transition temperatures, estimated from the heat capacity peaks, were Tc = 159.5 K for 1, and Tc(up) = 165.0 K, Tc(down) = 162.7 K for 2, in heating and cooling processes. Raman spectra have been recorded for 2 both in the HS (300 K) and LS (110 K) states. Significant changes occur in the position of several low-frequency Raman modes in the low-temperature spectrum. The entropy change associated with the 15 normal vibrational modes of the [FeN6] pseudo-octahedron has been estimated from the measured spectra as ¿Svib = 36 J K-1 mol-1, and the total entropy change is ¿S = ¿Svib + ¿Sel = 49 J K-1 mol-1. This value is comparable to that obtained from the calorimetric measurements. Photomagnetic studies revealed a complete conversion from the LS into the HS state upon irradiation at 10 K in the 647.1¿676.4 nm wavelength region. The critical LIESST temperatures are 52 K (1) and 44 K (2). The dynamics of the LS to HS conversion was studied for the two compounds in the 6¿50 K temperature range. In the thermally activated region the apparent activation energy, Ea, was estimated to be 240 cm-1 for 1 and 170 cm-1 for 2. In the tunneling region the rate constants were estimated to lie in the 9 × 10-7 to 1 × 10-6 s-1 range.
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Agradecimientos:
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We deeply thank Professor François Varret for very helpful discussions. We are grateful to the European Commision for financial support through the TMR-Network "Thermal and Optical Switching of Spin States (TOSS)", Contract ...[+]
We deeply thank Professor François Varret for very helpful discussions. We are grateful to the European Commision for financial support through the TMR-Network "Thermal and Optical Switching of Spin States (TOSS)", Contract No. ERB-FMRX-CT98-0199EEC/TMR. We thank the financial assistance from the Spanish DGICYT through Projects PB97-1397 and MAT97-0987, and the Programa de AccionesIntegradas Hispano-Francesas (HF1999-0091)
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