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Register a new userOptical and electronic properties in ferroelectric barium titanate-based compounds
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Ingrid Canero Infante
Publication date
2019
fields
Physics
and research's language is
English
Authors
Ingrid C. Infante
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The bandgap energy values for the ferroelectric BaTiO3-based solid solutions with isovalent substitution Ba1-x SrxTiO3, BaZrxTi1-xO3 and BaSnxTi1-xO3 were determined using diffuse reflectance spectra. While the corresponding unit cell volume follows Vegards law in accordance with the different ionic radii of the ionic substitutions, the bandgap values depict non-linear compositional dependences for all the solid solutions. The effect is considerably large for BaZrxTi1-xO3 and BaSnxTi1-xO3 solutions, depicting a bandgap linear compositional dependence up to x=0.6, for x>0.6 BaZrxTi1-xO3 compounds present much larger bandgap values than BaSnxTi1-xO3 counterparts. Electronic properties have been investigated through X-ray photoelectron spectroscopy in BaSnxTi1-xO3 compounds, indicating that the Sn 3d and Ti 2p core levels shift against the Ba 3d ones within the whole compositional range with the same energy trend as that observed for the optical bandgap. Since for Ba1-x SrxTiO3 compounds no major bandgap variation is observed, we conclude that the bandgap compositional dependences observed for BaSnxTi1-xO3 compounds and BaZrxTi1-xO3 ones are originated from the structural sensitivity of the O, Ti and Sn or Zr electronic bands involved in the bandgap transition of these compounds. With this work, we underline the reliability of the bandgap determined from diffuse reflectance spectrometry experiments, as a means to non-invasively evaluate the electronic properties of powder materials.
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The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiO3 strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C-C dimer pair accompanied by stronger C-C and weaker C-Ti hybridizations as the C-C distance becomes smaller. As the C-C distance increases, C-doped SrTiO3 changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO3, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.
Hexagonal BaTiO_3 undergoes a structural phase transition to an orthorhombic C222_1 phase at T_0 = 222 K. The transition is driven by a soft optical mode with E_2u symmetry whose couplings force the appearance of a spontaneous E_2g strain (improper ferroelastic character). Staying within the same E_2u subspace, the system could in principle settle into a second (not observed) orthorhombic phase (Cmc2_1). We have carried out a first-principles investigation of these questions, studying the structure of the existing C222_1 and the virtual Cmc2_1 phases, and describing the spontaneous E_2g strain in accord with the experimental observations. In addition, we show that the occurrence of C222_1 instead of Cmc2_1 cannot be explained by the E_2u soft modes themselves and, therefore, must be related to their couplings with secondary order parameters. A more detailed analysis proves that the E_2g strains do not account for the experimental preference.
Ferroelectric domain walls exhibit a range of interesting electrical properties and are now widely recognized as functional two-dimensional systems for the development of next-generation nanoelectronics. A major achievement in the field was the development of a fundamental framework that explains the emergence of enhanced electronic direct-current (DC) conduction at the domain walls. In this Review, we discuss the much less explored behavior of ferroelectric domain walls under applied alternating-current (AC) voltages. We provide an overview of the recent advances in the nanoscale characterization that allow for resolving the dynamic responses of individual domain walls to AC fields. In addition, different examples are presented, showing the unusual AC electronic properties that arise at neutral and charged domain walls in the kilo- to gigahertz regime. We conclude with a discussion about the future direction of the field and novel application opportunities, expanding domain-wall based nanoelectronics into the realm of AC technologies.
The calculated results of FeCl3 graphite intercalation compounds show the detailed features. The stage-1 FeCl3-graphite intercalation compounds present diversified electronic properties due to the intercalant. The first-principles calculations on VASP are utilized to analyze the essential properties, such as the geometric structures, spatial charge distributions, charge variations, band structures and density of states. The density of states displays full information for an explanation of the hybridizations with the special structures van Hove singularities on it. The van Hove singularities in graphite-related systems are very important and can provide full information for examining the intercalation effects. The orbital-decomposed density of states for C atoms shows that the {pi} bondings are orthogonal to the sp2 bondings and the C-C bondings retain in the intralayer C atoms. The Fe atoms and Cl atoms form the Fe-Cl bondings at some unique energy range, presenting the multi-orbital hybridizations of [4s, 3dxy, 3dyz, 3dxz, 3dx2-y2, 3dz2]-[3px, 3py, 3pz]. For C-Cl and Cl-Cl bonds, the unique van Hove singularities exhibit their coupling interactions, revealing the multi-orbital hybridizations of [3px, 3py, 3pz]-[ 3px, 3py, 3pz] and [3s, 3px, 3py, 3pz]-[3s, 3px, 3py, 3pz], respectively. The Fe-Cl bondings arise from multi-orbital hybridizations of [4s, 3dxy, 3dyz, 3dxz, 3dx2-y2, 3dz2]-[ 3px, 3py, 3pz]. Due to the band structures and density of states, the multi-orbital interactions between intercalants and graphene layers dominate in the low-lying energy range. The charge transfers per atom (electrons/atom) for C, Fe, Cl are -0.02 e/atom, -0.28 e/atom and +0.46 e/atom, respectively. Thus, the C atoms in graphene layers present as positive ones after intercalation, i.e., the graphite system exhibit p-type doping features in agreement with previous study.
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