An e. s. r. study of AgCl-NaCl binary mixtures

Abstract

E. s. r. spectroscopy was used to study the microscopic structure and electronic properties of AgCl–NaCl solid solutions. After γ- or u. v. irradia­tion at or below 77 K, Ag 2+ was observed as the hole-trapped centre, and depending on concentration, either localized or extended electron states were observed. At silver concentrations under 10%, the localized electron states were Ag atoms and at higher concentrations, they were conduction electrons confined to AgCl clusters. At the classical percolation limit (30 mol % Ag), a narrow conduction electron c. e. s. r. signal at g = 1.888 was observed; this was assigned to electrons in a AgCl-like band. The elec­trons giving rise to this signal sampled extended regions of the crystal and had a lifetime greatly extended over that observed for conduction elec­trons in pure AgCl. The c. e. s. r. lineshape was examined as a function of electron concentration and the observed asymmetry at low electron concentrations is discussed in terms of wavefunction localization in band tails. The exceptionally long conduction electron lifetime is explained in terms of the inability of the conduction electrons to move into zones containing the stationary Ag 2+ centres. We suggest that Ag 2+ centres formed in AgCl like regions rapidly recombine with electrons, while those in more NaCl-like regions are insulated therefrom. The AgCl-NaCl solid solutions were metastable, and the Ag 2+ and c. e. s. r. parameters could be used to follow incipient phase separation.

Observation by ESR of electrons localized at intrinsic shallow traps in silver halide systems

“…In the Pb2+-doped material, photoelectrons were either localized at, or hopping between, Pb2+-associated effective mass states located about 50 meV below the conduction band edge. Localisation of the electrons occurred a t temperatures less than 60 K. The measured g-value was 1.8785 f 0.0005 [lo, 111, Similar behaviour was observed in X-irradiated Agl -,Na,Cl mixed crystals, where electrons, excited from shallow, localized impurity states into the bottom of an amalgamation type band which was derived principally from AgCl conduction band states, had a g-value of 1.888 [12]. I n AgBr, the assignment of the STC signal is also based on g-value measurements for electrons localized at Pb2+ states in Pb2+-doped powders [ll].…”