Gas solid techniques for preparation of pure lanthanum hexaboride

Abstract The processes reported for the preparation of lanthanum hexaboride (LaB 6 ) from lanthanum oxide involve the use of carbon either elemental or in the form of boron carbide or elemental boron itself as reducing agents, fused salts at high temperatures or reactions which require the product boride to be leached out. Each of these techniques either increases the process costs and/or increases chances of contamination in final product. Pure LaB 6 can best be prepared by a reaction which produces a gaseous byproduct. In the present study, such a reaction was successfully used to yield pure lanthanum hexaboride. The process involved mixing of anhydrous lanthanum chloride with aluminium and boron and heating the charges under dynamic argon flow. Lanthanum chloride is known to be highly hygroscopic; hence the process using improperly dehydrated LaCl 3 led to the formation of lanthanum oxychloride which does not convert to LaB 6 under conditions wherein LaCl 3 converts. Not only the formation of AlCl 3 but also its continuous removal from the reaction zone is necessary for the success of the process.