Method for growing absorption-free alkali metal halide single crystals

Abstract

Large ultra-pure, prism-quality essentially single crystal boules and ingots of alkali metal chlorides and alkali metal bromides are grown by the methods of Kyropoulos and Stockbarger. Optically single crystals of these alkali metal halides are clear optical bodies free of haze throughout. A typical Kyropoulos grown boule of KBr of this invention is also absorption-free, and has cleavage or crystallographic planes which deviate from parallel by 1.degree. to 3.degree. per inch. By `absorption-free` is meant freedom of absorption at 7.2 microns (.mu.) due to nitrate, at 9.5-11 .mu. due to silicate, and at between 8 and 9 .mu. due to sulfate, which are the most difficult to control but as the term implies the bodies are also free of infrared absorption for impurities such as CO.sub.3.sup.-2, PO.sub.4.sup.-3, OH.sup.-, BO.sub.2.sup.-, SH.sup.-, CNO.sup.-, HCO.sub.3.sup.-, etc., which are easily avoided by use of good commercially available growth stock. A particular process for growth for either of the foregoing methods in a silica or quartz crucible, comprises growing an ultrapure ingot of KBr from a relatively impure melt containing silicate, sulfate, nitrate and nitrite ions, and includes maintaining the sodium content of the melt during growth to less than about 20 ppm (parts per million) and the barium content less than 10 ppm, adding barium bromide to the melt if necessary, and, contacting the melt with a trace of free bromine in a covered, but not air-tight, pot.