First Principles Studies On The Magnetism Modulation And Photovoltaic Effect Of ABX₃-type Metal Compounds

Due to various combinations of A,B and X site cations,the ABX3-type metal compounds form a huge family.The research objects of this thesis are the transition metal oxides and the organic-inorganic hybrid halide perovskites.Both systems show abundant physical and chemical properties and have wide application prospects,which make them become hotspots in both the communities of condensed matter physics and material science.From the viewpoint of fundamental physical research:Transition metal oxides ABO3 are typical strongly correlated electronic materials.They usually contain partly-filled-d orbital electrons and the correlations between these d electrons can not be ignored any longer.In transition metal oxides,multi-degrees of freedom(include charge,spin,orbital,and lattice)compete and couple with each other,which may lead to a lot of fascinating properties,such as ferroelectricity,complex magnetism,orbital ordering,multiferroicity et al.All these phenomena are important issues for strongly correlated electronic systems and relate to a series of basic problems in the condensed matter physics.Besides,the introduction of additional dimensions,such as strain,interface,and doping,could further tune the physical properties of transition metal oxides and lead to new behaviors.Therefore,transition metal oxides have stimulated intensive research activities in recent years.For the organic-inorganic hybrid compounds ABX3,they offer an important opportunity to combine useful properties from respective organic and inorganic compounds.In general,organic components can offer a number of useful properties,including structural diversity,ease of processing and efficient luminescence.Inorganic materials have a distinct set of advantages,including good thermal stability,interesting magnetic or dielectric transitions,and band gap tunability.Hence,the organized combinations may lead to composites with superior properties compared with either independent components.Regarding the perspective of application potentials,transition metal oxides ABO3 show good application prospect in broad fields,such as storage,sensor,and detector.And the solar cells based on the organic-inorganic hybrid halide perovskites have stunned the photovoltaic community with their remarkable performance and rapid progress in the recent years,which have attracted extraordinary research attentions.Compared with the conventional semiconductor solar cells,these perovskite solar cells are low cost and simple to manufacture,and thus have huge commercial potential.Although in the past decades great progress has been made on the above two systems,due to their complexity,there are still many problems worth to explore.Using the first principles method based on density functional theory,three particular issues are studied in this thesis,which are strain engineered magnetism in thin perovskite films,photovoltaic effect of ferroelectric transition metal oxide,and surface properties of organometal halide perovskite.The whole thesis is organized as follow:In the first chapter,the research background is introduced.First,we outline the structure characteristics of the ABX3-type metal compounds.Then the magnetism engineering and photovoltaic effect in ABX3-type metal compounds is reviewed.Finally,the motivation of our research will be briefly described.In the second chapter,the theoretical basic knowledges involved in this thesis are introduced.First,we briefly introduce the magnetic materials,light absorption in solid,and the basic theory of solar cells.Then the development of first principles method based on density functional theory is also outlined.In the third chapter,the epitaxial strain effects on the electronic structure and magnetic ground state of rare-earth titanates(RTiO3)are studied.Different oriented substrates are used to tune the degree of epitaxial strain.Our research predicts a new magnetic ground state which has never been found in any other RTiO3 system before.In the fourth chapter,the photovoltaic properties of ferroelectric hexagonal rare-earth manganites(RMnO3)are studied in detail.The hexagonal TbMnO3 is chosen as a typical representative.Through exploring the electronic structure and optical properties of hexagonal TbMnO3,we predict its solar-to-electricity conversion efficiency.Our results show that the hexagonal RMnO3 system is a kind of potential ferroelectric photovoltaic material with high solar-to-electricity conversion efficiency.In the fifth chapter,the properties of organometal halide perovskite surface are studied.By using the slab model,we construct the CH3NH3PbBr3(001)surface and investigate its thermodynamic stability,electronic structure,and surface ionization potential energy.Meanwhile,collaborative experiment also explores the electronic structure of CH3NH3PbBr3 surface.The sixth chapter is devoted to the conclusion and perspective.