Thulium(III) chloride or thulium trichloride is as an inorganic salt composed of thulium and chlorine with the formula TmCl<sub>3</sub>. It forms yellow crystals. Thulium(III) chloride has the YCl<sub>3</sub> (AlCl<sub>3</sub>) layer structure with octahedral thulium ions. It has been used as a starting material for some exotic nanostructures prepared for NIR photocatalysis.
Thulium(III) chloride can be obtained by reacting thulium(III) oxide or thulium(III) carbonate and ammonium chloride:
The hexahydrate of thulium(III) chloride can be obtained by adding thulium(III) oxide to concentrated hydrochloric acid.
Thulium(III) chloride can also be obtained by directly reacting thulium and chlorine:
Thulium(III) chloride is a light yellow powder. Its hexahydrate is a light green hygroscopic solid. Both are soluble in water. Thulium(III) chloride has a monoclinic crystal structure with the space group C2/m (No. 12) corresponding to that of aluminum(III) chloride.
Thulium(III) chloride reacts with strong bases to make thulium(III) oxide.