Spin Transition Molecular Alloys: An Attempt of Fine Tuning of the Transition Temperatures

The goal of this work has been to propose a novel approach to fine tune the transition temperatures of spin transition materials, based on the concept of spin transition molecular alloys. It has concerned the alloys of composition [Fe(R1trz)3-3x(R2trz)3x]A2, where R1trz and R2trz stand for two different 4-substituted-1,2,4-triazole ligands, and A- stands for a counteranion. It has been shown that for any composition defined by x the alloy gives a unique spin transition regime, while the mixture (1 − x)[Fe(R1trz)3]A2 + x[Fe(R2trz)3]A2 of the same chemical composition shows two spin transition regimes with the relative intensities 1 − x and x, respectively. In the case of the alloys [Fe(Htrz)3-3x(NH2trz)3x](ClO4)2·H2O, the inversion temperatures T1/2↑ and T1/2↓ in the warming and cooling modes, respectively, have been found to vary almost linearly as a function of x. For x = 0.15, the alloy is bistable at room temperature. A theoretical treatment of this molecular alloy effect in the mean-field approximatio...