Thermal, pressure and light induced spin transition in the two-dimensional coordination polymer {Fe(pmd)2[Cu(CN)2]2}

Abstract

A complete structural, calorimetric, and magnetic characterisation of the 2D coordination spin crossover polymer {Fe(pmd)₂[Cu(CN)₂]₂} is reported. The crystal structure has been investigated below room temperature at 180 K and 90 K, and at 30 K after irradiating the sample at low temperature with green light (λ = 532 nm). The volume cell contraction through the thermal spin transition is only 18 ų which is lower than the usually observed value of around 25–30 ų while the average Fe–N bond distances decrease by the typical value of about 0.19 Å. The structural data of the irradiated state indicate that the high spin state is well induced since the cell parameters are consistent with the data at 180 K. Calorimetric and photo-calorimetric experiments have also been performed. The entropy content for the thermal spin transition, ΔS = 35–37 J mol⁻¹K⁻¹ lies in the lowest range of the typical values and correlates with the low volume cell contraction. The combination of the crystallographic and calorimetric data predicts, in accordance with a mean-field approach, a linear pressure dependence of the critical temperature with a slope of 302 K GPa⁻¹. Magnetic measurements under pressure reveal an anomalous behaviour since the critical temperature and hysteresis do not change up to 0.22 GPa but an apparent linear dependence is obtained for higher pressures (up to 0.8 GPa) with a slope two times higher than the mean-field estimation.