The mechanism of color center production in iron doped photochromic sodalites

Bromide sodalites doped with iron (0.75 mg/cm³) have been studied by the Mossbauer technique, before and after heat treatment in hydrogen atmosphere and after coloration by u.v. light and by electron bombardment. The measurements show that u.v. or electron irradiation causes Fe²⁺ ions to convert to Fe³⁺. The number of converted ions (? 10¹⁷/cm³ for u.v. irradiation, and 5 × 10¹⁷/cm³ for electron irradiation) agrees with the expected number of F centers estimated from optical measurements. The thermal recovery time at room temperature of the Fe³⁺ ions back to their Fe²⁺ state was found to be several months.