Detailed Investigation of the Interplay Between the Thermal Decay of the Low Temperature Metastable HS State and the Thermal Hysteresis of Spin-Crossover Solids

Abstract

International audienceWe investigated the interplay between the thermal decay of the low temperature metastable HS state and the thermal hysteresis of the metal diluted spin-crossover complexes, [FexMn1–x(dpp)2(NCS)2].py (dpp = dipyrido[3,2-a:2′,3′-c]phenazine and py = pyridine). The thermal decay temperature T(LIESST) could be shifted into the quasi-static hysteresis range by several means: increase of the Mn content, increase of the temperature scan rate of the heating experiment, and irradiation using visible light. A detailed investigation of the isothermal relaxation of the metastable HS state at various temperatures was made and compared to simulations. The theoretical investigation based on the usual macroscopic master equation was focused on the regime change generated by the meeting of T(LIESST) and the thermal hysteresis range, and provided a qualitative agreement with the experimental data. The search of quantitative agreement led to introduction of parameter distributions and allowed illustration of the already-known drawbacks of the mean-field Ising-like model

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

“…Based on the structural distortion point of view, it corresponds to zone 1 in Figure 3. It therefore induces spin state freezing also called spin crossover inhibition, with particular features depending on the investigated case [23,42]. The typical observations are reported on Figure 4.…”

“…The measurement of T(LIESST) therefore gives a number to discuss the HS*→LS relaxation and reflects the temperature above which the SCO information cannot be kept for more than a few minutes. Figure 1 shows an archetype example of the T(LIESST) measurement done on the mononuclear compound [Fe(dpp) 2 (NCS) 2 ] (dpp = dipyrido[3,2-a:2',3'-c]phenazine) using a SQUID magnetometer [23]. The magnetic response is expressed as the χ M T product (χ M stands for the molar magnetic susceptibility and T the temperature).…”

“…values as function of this rate for the [Fe(dpp) 2 (NCS) 2 ] compound (adapted from reference. [23]). (c) Simulations of the thermal relaxation curves mimicking a measurement in settle mode (the relaxation is calculated every 1 K and the delay between two consecutive temperatures is tuned to change the mean temperature scan rate) and (d) the corresponding experimental (squares) and simulated (line) T (LIESST) values.…”

Metal Substitution Effect on a Three-Dimensional Cyanido-Bridged Fe Spin-Crossover Network

“…In addition, with an increasing x value, spin crossovers become more gradual. According to the reported observations [16][17][18][19][20][21][22][23][24][25], these results are explained as follows. Since the ionic radii of Co(II) HS (0.75 Å) is closer to Fe(II) HS (0.78 Å) than Fe(II) LS (0.61 Å) [55], the spin-crossover from Fe(II) HS to Fe(II) LS becomes unfavorable, leading to a decrease of the spin transition temperature.…”

“…This phenomenon can be modulated by various physical and chemical stimulations (e.g., light, pressure, temperature, vapor molecule, and metal substitution), and it has potential for sensor and memory applications [14,15]. To control the spin-crossover behavior, the metal substitution effect on the spin-crossover behavior for some Fe II spin-crossover materials has been investigated [16][17][18][19][20][21][22][23][24][25].…”