Spin transition material

A spin transition material composed of at least one compound corresponding to the formula

• Fe1-mMm(R-Trz)3XbYc (I)
• in which:
• M is a metal, other than Fe, having a 3d4, 3d5, 3d6 or 3d7 configuration;
• 0≦m<1;
• R-Trz represents a 1,2,4-triazole ligand carrying an R substituent on the nitrogen in the 4 position;
• R is an alkyl group or an R1R2N— group in which R1 and R2 represent, each independently of the other, H or an alkyl radical;
• X represents at least one noncoloring monovalent or divalent anion selected from the group consisting of BF4−, PF6−, ClO4−, Br−, Cl−, NO3−, CF3SO3−, CH3SO3−, SO42− and CO32−;
• Y represents at least one coloring anion which has at least two aromatic rings and at least one SO3− group;
• b>0 and c>0, b and c being chosen so that the electrical neutrality of the compound (I) is adhered to and b and c are such that 10−5≦c/b≦0.1 (in moles), so as to provide a spin transition associated with a change in color being other than pink or white.