Synthetic photochromic sodalite was prepared (1) by solid-state reaction, (2) from zeolite X-SO2 or zeolite X-Has, and (3) hydrothermally ; the preparations are compared and discussed. X-ray diffraction, ultraviolet and visible light absorption (transmission and reflection), and ESR data served to follow the reactions and characterize the products. The content of sulfur and chlorine and the S :C1 ratio affect the depth of color and the bleaching rate. The partial substitution of other halogens for C1 may speed up or slow down the coloring rate a s a function of electronegativity. Fluorine may shift the visible absorption maximum from 5300 to 5100 A. Stoichiometry and ESR data indicate that the photochromism is effected by the transfer of a n electron between sulfur a s St = and a C1 vacancy between four sodium atoms.

Synthetic sodalites containing sulfur and showing considerable photochromic activity have been investigated by ESR. The center responsible for the color has been shown to be an electron trapped at a chlorine vacancy. The origin of the electron which is reversibly transferred during the processes of coloration and bleaching is believed to be the ion S2−.

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