Patent ID: 8753743

Claim:
A process for the direct preparation of a spin transition material composed of spherical nanometric particles comprising a compound corresponding to the formula: [ ( Fe 1 - y ⁢ M y ⁢ L 3 ) w ⁢ L 3 ] [ X 2 x ⁢ ( 1 - z x ′ ) ⁢ Y 2 ⁢ ⁢ z x ′ ] w in which: L represents a 1,2,4-triazole ligand carrying an R substituent on the nitrogen in the 4 position; X is an anion having the valency x, 1≦x≦2; Y is an anion other than X having the valency x′, 1≦x′≦2; R is an alkyl group or an R 1 R 2 N— group in which R 1 and R 2 represent, each independently of the other, H or an alkyl radical; M is a metal having a 3d 4 , 3d 5 , 3d 6 or 3d 7 configuration, other than Fe; 0≦y≦1; 0≦z≦2; 3≦w≦1500; the process comprising the following steps: a) preparation of an emulsion of a water-in-oil type by addition, with vigorous stirring, of a composition of oil possessing surfactant properties to an aqueous solution of at least one iron salt and optionally a precursor of the metal M and comprising ascorbic acid; b) preparation of an emulsion of a water-in-oil type by addition, with vigorous stirring, of a composition of oil possessing surfactant properties to an aqueous solution of a ligand; c) mixing the two emulsions, followed by further vigorous stirring, for a time of 1 to 10 min; d) precipitation of the nanoparticles by addition of a solvent which does not modify the structure of the nanoparticles but which denatures the emulsion; and e) extraction of the precipitate by several washing with the solvent and centrifuging cycles followed by evaporation of the solvent, wherein the process does not comprise grounding of the spherical nanometric particles.