Infrared characterization of natural and cultured quartz: The effect of electrodiffusion and irradiation

A study of OH−-related stretching vibrations showing infrared absorption bands in natural and cultured quartz has been made to include the effect of exchange of monovalent ions and irradiation at 77 and 300 K using 1.75-MeV electrons from a Van de Graaff accelerator. A variety of natural quartz crystals of different origins from Brazil and Arkansas used in the present investigation is found to contain a large number of absorption bands of uncorrelated strength in the 3200–3600 cm−1 region in the near infrared. Most of these bands are found to be associated with the presence of Li+ ions. Studies on samples of matching thickness from different cultured quartz specimens show that the bands present in the natural quartz do not get transferred to cultured quartz during their growth. While cultured quartz has always a set of bands at 3350, 3400, 3437, and 3581 cm−1, called s1, s2, s3, and s4, the set of major bands found to be present in all natural quartz are 3476 cm−1 (designated in the present work as the N1-Li+ band), 3367 cm−1 (e2), and 3306 cm−1 (e1) bands. Other bands in natural quartz may or may not be present in conjunction with these bands. Electrodiffusion (sweeping) of unswept natural quartz with Na+ and H+ shifts the N1-Li+ band to N1-Na+ at 3451 cm−1 and N1-H+ at 3468 cm−1; e1 and e2 bands are unaffected by alkali sweeping. Other bands are also affected by sweeping. Alkali sweeping in cultured quartz does not affect the position of any of the s bands. On the other hand, H sweeping carried out introduces e1 and e2 bands in cultured quartz and saturates them while their strength is augmented in natural quartz to saturate all the substitutional aluminum. Irradiation at 77 K suppresses all the bands except e1. N1-H+ centers deplete much faster in relation to N1-M+ centers, where M+ is either Li+ or Na+. Subsequent irradiation at 300 K further suppresses the N1-M+ centers in about the same proportion as on 77-K irradiation. The N1-H+ centers do not show any depletion at 300-K irradiation. Isochronal annealing on N1-M+ and N1-H+ centers shows that they all recover during annealing. The recovery is essentially complete at about 760 K.