Nature of the Defects Induced by Photoinjection of Hydrogen in Transition Metal Oxides

Abstract

The nature of colour centres arising in WO₃ — a prominent example of perovskite-like oxide — upon implantment of hydrogen atoms is one of the most important issues for the investigation of electrochromism and photochromism in this material [1]. Although several physical models have been proposed to describe the optical absorption arising at insertion of hydrogen atoms (small polaron model, intervalence-charge transfer model, F-centre model), no one has been able to satisfy the variety of experimental data. Recently one more attempt was made to describe the optical absorption band arising at introduction of hydrogen atoms in amorphous MoO₃ and WO₃ films as a superposition of three absorption bands belonging to different colour centres, each being described by a Gaussian [2–4], since, according to the model, the absorption band each of the centres, at first approximation, can be assumed to have a Gaussian [5] or close to Gaussian [6] shape. One of the Gaussian bands was assigned to the centre of a hydrogen atom localised inside the oxide bulk (the bulk states), whereas two others — to surface states.