Structural and electronic properties of vacancy complexes in cubic SrTiO3

Abstract

Research on imperfection of solid systems has been a continuing subject in condensed matter physics. Oxygen vacancies in oxide materials induce intriguing phenomena that are sometimes dramatically different in physical properties from its parent material. Vacancy-induced properties provide a way to design new functional materials with unique properties that can even be utilized in real applications. Therefore, a systematic study of the possible vacancies in oxides provides a valid perspective and ability to tailor such systems.

SrTiO3 is one of the systems that is most frequently studied in reference to the role of vacancies as many interesting properties were observed when vacancies are introduced. Only several kinds of vacancies, however, have been examined in theory but also their detailed electronic structures show different features according to individual computational details. Moreover, recent experimental observations cannot be explained by single vacancy. This implies that more complex structure is involved in reality. To make these points clear and understand the experimental results, we took notice of various possible new vacancy models.

In this work, we analyze specific electronic properties of both single and complex vacancies in SrTiO3 such as Sr, Ti, O, O-O, Sr-O, and Sr-O-O vacancies (V_Sr, V_Ti, V_O, V_OO, V_SO, V_SOO) using first-principle calculations. Further, we survey the formation energies of these vacancies to search for structure that can actually stabilize in reality with ab-initio thermodynamics. The detailed electronic structure is closely connected with the local strain around the vacancy. Thus, we perform the atomic relaxation up to 0.01 eV/A of Hellmann-Feynman force. In addition, we employ the local spin density approximation plus U (LSDA + U) method to describe a sizable Ti d-orbital occupation induced by the oxygen vacancy.

We investigate all possible representative vacancies and clarified their electronic structure and compared them to previous works. We notice that the V_O leads to a singly occupied defect states below conduction bands which has a spin-polarized ground state in contrast to LDA results. The on-site Coulomb energy on Ti-d orbital plays an essential part to describe the defect states including strong electron correlation. The minimum structure for V_OO induce a shallow defect states below conduction bands where two electrons occupying two separate vacancy site with anti-parallel spin alignment.

Our systematic analyses suggest that a vacancy configuration of Sr and two O vacancies (Sr-O-O vacancy) show good agreement with experiments which observed deep-level defect states in SrTiO3 with vacancies. Such defect states cannot be explained with single O vacancy in SrTiO3, and that is mainly composed of Ti-d e_g(3z^2-r^2) orbital. Formation energy of this type of vacancy is comparable to a single oxygen vacancy system in oxygen poor environment due to its relatively high binding energy between Sr and O vacancy. Besides, a clear asymmetric structure can explain the observed ferroelectricity induced by vacancies at room-temperature.

In addition to the Sr-O-O vacancy, we also expanded V_SOO to an extended line defect. The most stable line defect has same vacancy arrangement with most stable configuration of V_SOO, and it shows a shallow defect state below conduction bands. Although the electronic state is semiconducting, we find that the electronic property of line defect is very sensitive to the vacancy contents. This results may provide good microscopic picture to understand the resistance switching behavior in SrTiO3.