The effect of pressure on the absorption edge in silver halides

Abstract

The absorption edge of AgCl, AgBr and AgI has been measured to over 200,000 atmospheres. The behavior of AgCl and AgBr is quite similar with pressure. There is a small red shift in the low pressure (f.c.c.) phase, a transition at 87,000 (83,000) atmospheres, probably to the simple cubic phase, accompanied by a large red shift, and a further small red shift, at higher pressures. The behavior of AgI is quite different. The atmospheric (wurtzite) phase transforms to the f.c.c. at 2900 atmospheres with a large red shift. The f.c.c. shifts red very strongly with pressure to 112,000 atmospheres, where there is a second phase change accompanied by a large blue shift. There is a further small red shift at high pressures. A peak which appears at 423 mμ (428 mμ) in AgI at atmospheric pressure has been attributed by Seitz to a forbidden internal transition in the Ag + ion, which is permitted by the symmetry of the wurtzite lattice. The peak disappears in the high pressure phase, which is consistent with Seitz' theory.