The Crystal Forms of Metallic Selenium and Some of Their Physical Properties

W HETHER light-sensitive selenium is made up of homogeneous crystal structure or of complex units is of importance in explaining the nature of light action in selenium.Thus the author has been led into a study of the crystal forms of metallic selenium, which study has been particularly fruitful in that new crystal forms of very large size have been isolated.These crystals are of such a size that the optical and photoelectric properties of individual crystals can be studied.Thus we are able to eliminate some of possible causes of complexity in selenium, such as polymorphic crystal mixtures, unknown condition of crystal contacts, irregular placing of the crystals, and impurities.The fact that these new crystal forms are light-sensitive opens up a large field of investigation of which this paper is only preliminary.Muthman 1 described only one crystal form of metallic selenium of the hexagonal rhombohedral system.These crystals were produced by the sublimation of the vapor of selenium in air and were never larger than 0.2 X 0.5 mm.Saunders 2 also describes only one form.His conclusions were based upon the changes in volume accompanying temperature changes, as determined by dilatometric measurements.The difficulty with Saunders's method is that different crystal forms having identical or only slightly varying density could not be detected.Marc 3 following the experiments of Uljanin 4 who observed two distinct crystallographic structures under the microscope, and following his own 1 Zeit.f.