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Keywords:

  • spin-crossover;
  • polymorphism;
  • phase transition

We have identified two polymorphs of the molecular complex [(TPA)Fe(III)(TCC)]PF6 [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF6 anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented.