EPR of Pb + ions and exchange-coupled Pb + ion pairs in U.V.-irradiated PbCl 2 and PbBr 2 crystals

Abstract

In PbCl 2 and PbBr 2 single crystals at low temperatures (< 200 K) three types of paramagnetic centres (to be called A, B and C) can be created by u.v. irradiation. The A signal in PbCl 2 consists of five equidistant (35 G) gaussian-shaped lines with a width of 15 G. The experimental g-value of the central component is 2.0665−0.0310 cos 2 θ with θ the angle in the ab plane. The A signal is ascribed to Pb +-type centres at Pb ++ lattice sites. These centres may occur isolated as well as in exchange-coupled pairs along the a axis. The positive shift of the experimental g-values is ascribed to covalent admixtures of excited charge-transfer configurations to the Pb + (6p) ground state. The unusual behaviour of the A-signal intensity as a function of time and irradiation history is explained by a model in which photoelectrons are trapped at dislocations, from which they subsequently are released by a one-phonon interaction, migrate through the lattice and are trapped at Pb ++ ions near trapped holes. The lifetime at 77 K of an electron at a dislocation trap level with depth E is approximately given by (4−0.6 E/kT) -1 10 3 s for 0< E/kT<6. The exchange coupling of the Pb + pair is shown to be isotropic and antiferromagnetic with J =(16.7 ± 2.1) cm -1. The B signal ( g=2.0024) is probably due to colloidal lead particles. The C signal in PbCl 2 ( g=2.064−0.008 cos 2 θ, width 72 G at 10 K) is only found at temperatures <50 K and is ascribed to trapped holes. In PbBr 2 only weak A- and B-type signals are found. The exchange coupling of the Pb + pairs in PbBr 2 is slightly anisotropic.