• optics
• ab initio
• SrNbO3

The transition metal oxide SrNbO3 has recently proven to be an efficient photocatalyst for hydrogen production via water splitting. The most fundamental feature is the strong absorption of visible light of energy higher than 2 eV. A model capable of explaining this property, and the related red color, has been the focus of several works, both experimental and theoretical. In particular, three different scenarios have been proposed: in the first two models two different interband transitions are believed to be responsible for the absorption while in the third model it would be due to a plasmon mode.
In this thesis work we have investigated the electronic structure of SrNbO3 by means of state of the art ab initio electronic calculation methods, namely Density Functional Theory (DFT) and many body perturbation techniques in the so-called ”GW approximation”. Three different structures have been considered: (i) a perfect structure, which is the natural starting point, (ii) a structure with Sr-vacancies, because this kind of defects has been reported in the literature and (iii) a structure with O-vacancies, a typical defect of transition metal oxides. From each electronic structure we have obtained, we have derived the corresponding optical properties. The proposed models have been investigated and checked out as well. With our results we have been able to exclude two out of the three scenarios proposed and we finally propose a fourth different explanation for the cause of the observed absorption based on O-vacancies. In fact, we have found strong evidences that these defects can have dramatic consequences on the optical spectra and give rise to an absorption peak in the right energy range.
We have structured the work as follows. The first chapter is an introductory one and is meant to give a general background. The Chapters 2-4 are brief theoretical description of the methods we have employed. Chapter 2 is a general explanation of DFT and the interpretation of DFT results in connection with practical calculations is discussed. In Chapter 3 we set the basis for the many body perturbation theory in the form of the Hedin’s equation and the GW approximation derived from them. Since the optical proper- ties are a central subject of our work we have decided to devote a chapter, Chapter 4, to them. All the methods are discussed in relation to the actual calculation that are done. In Chapter 5 we show and discuss our results for SrNbO3. For the O-vacancy case we give a simple picture which help us in rationalizing the emerging peak in the absorption spectrum. Moreover we give similar results for SrVO3, a similar compound, which strengthen our model. Finally, in Chapter 6 we summarize our results.