Zone Refining of the Silver Halides

Abstract

The techniques of zone melting have been applied to AgCl and AgBr in an attempt to obtain large crystals of extremely high purity. By measuring distributions in ingots with deliberate impurity additions, both optimum conditions for zoning and distribution coefficients for several impurities were determined. For AgCl, zone melting in a chlorine atmosphere led to near‐ultimate distributions for Cu, Pb, Ni, and Fe after passage of 70 zones at a rate of 3 in./hr. The distribution coefficients determined were as follows: Cu, 0.4; Pb, 0.4; Ni, 1.4; and Fe, 0.7. Zone melting in vacuum resulted in similar distributions for Cu, Pb, and Ni, but Fe separated with an effective distribution coefficient slightly greater than one. Under these conditions, Mn and Cd separated in a direction opposite to that of zone travel, and Sn, Al, and Sr separated in the direction of zone travel. Zone refining of nominally pure AgCl resulted in crystals which probably contain less than one part in 10⁹ of Cu and Ni, less than one part in 10⁸ of Pb, and less than five parts in 10⁸ of Fe. Limited data on AgBr indicate that in this case, too, useful purification can be obtained. The dark electrical conductivity of the zoned crystals was found to be intrinsic above 315°K for AgCl and 300°K for AgBr.