Preparation of Anhydrous Rare-Earth Trihalides

The hydrated trihalides of the rare earths are easily obtained by reacting the oxides with appropriate HX acid solution. Anhydrous halides are, however, difficult to prepare. Attempts to dehydrate the hydrated halides usually result in oxyhalides. In the case of the chlorides and bromides... 

Various methods [282] have been used to prepare anhydrous chlorides of the rare earths. Taylor and Carter [283] describe a general method for the preparation of high purity anhydrous halides in good yield. This method involves heating in vacuo, a molecularly dispersed mixture of hydrated rare earth halide with proper ammonium halide until the water and ammonium halide are expelled. All the trihalides except the iodides of Sm and Eu can be obtained using this proceedure. In the case of Sm and Eu the divalent iodides, Sml2 and Eul2 are obtained. 

More care has to be taken for the preparation of binary chlorides and bromides, RCI3 and RBr3. The anhydrous salts are moisture-sensitive and they are obtained as (hepta- or hexa-) hydrates upon crystallization from hydrochloric or hydrobromic acid solutions. Simple dehydration of the hydrates in an HCl or HBr gas flow, respectively, appears to be possible (Seifert et al. 1985) but the less skfllliil chemist produces halide products more or less contaminated with oxyhahde, ROX. Sublimation of the raw product in an all-tantalum apparatus at higher temperatures (650 C to 950 C) and low pressures (10 bar) produces pure trihalides (with the exception of the rare-earth elements europium and 3dterbium where mixed-valent (+2/+3) or even dihalides are obtained under these conditions). 

---