Computational study of iron hexacyanide in silver halide

Classical and quantum mechanical atomistic calculations are presented for the structure and energy levels of iron hexacyanide complexes in silver halide crystals. The classical calculations employ a shell-model interatomic potential for the silver halide host and a force field developed from the vibrational properties of Fe(CN)(6)(4-) and Fe(CN)(6)(3-). In addition to the normal solid state treatments it was necessary to include polarizability of the CN- ligands by means of the shell model and a bonding Ag+-N interatomic potential derived quantum mechanically in order to predict single and double silver ion vacancy orientations near the hexacyanide that could be rationalized with experimental interpretations. In AgCl, single vacancies were predicted to occupy one of the six equivalent (200) positions near Fe(CN)(6)(4-), and near Fe(CN)(6)(3-) the divacancy configurations (200) (<(2)over bar 00>); (<(2)over bar 00>) (020); and (<(11)over bar 0>) (200) were predicted to be favored. The electronic properties of the dopant-vacancy complexes in silver halide were calculated with embedded Hartree-Fock and local density methods. It is shown that Fe(CN)(6)(4-) is not a deep electron trap since its calculated electron affinity is less than that calculated for the AgCl host. Thus, this complex can only function as a shallow ionized donor center. There was a perturbation of the charge distribution calculated for the complex caused by the presence of associated cation vacancies. These complexes have an ionization potential calculated to be less than that of AgCl, which permits them to trap holes.