Synthesis of the Giant Dielectric Constant Material CaCu3Ti4O12 by Wet-Chemistry Methods

CaCu3Ti4O12 (CCTO) has a giant dielectric constant of up to 105 at room temperature and great potential for technological application. In this work, we report two wet-chemistry methods to synthesize CCTO at relatively low temperatures and short reaction times. The pure-phase sample was obtained at 800 °C for 0.5 h, and the grain size of a pellet sample sintered at 1030 °C has a homogeneous distribution in the range of 0.4–1.5 μm

Interface-Controlled Ferroelectricity at the Nanoscale

Recent experimental results demonstrate that in thin films ferroelectricity persists down to film thickness of a few unit cells. This finding opens an avenue for novel electronic devices based on ultathin ferroelectrics, but also raises questions about factors controlling ferroelectricity and the nature of the ferroelectric state at the nanoscale. Here we report a first-principles study of KNbO3 ferroelectric thin films placed between two metal electrodes, either SrRuO3 or Pt. We show that the bonding at the ferroelectric-metal interface imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode in thin ferroelectric films. This does not, however, quench local polarization. If the interface bonding is sufficiently strong the ground state represents a ferroelectric double-domain structure, driven by the intrinsic oppositely-oriented dipole moments at the two interfaces. Although the critical thickness for the net polarization of KNbO3 film is finite - about 1 nm for Pt and 1.8 nm for SrRuO3 electrodes - local polarization persists down to thickness of a unit cell.Comment: 5 pages, 4 figure

Electronic structures of lanthanum, samarium, and gadolinium sulfides

In this study, we report our efforts to elucidate the electronic structures of two lattice structures of lanthanide sulfides (LnS and Ln3S4) and for three lanthanides (Ln = La, Sm and Gd) using density functional theory calculations performed with the CASTEP code. A DFT+U method was used for the corrections of on-site Coulomb interactions with U = 6 eV. The calculated electronic structures show that both lanthanum and gadolinium sulfides have metallic properties, consistent with the available experimental results. However, the calculated electronic structure of Sm3S4 is considerably different from those of the La3S4 and Gd3S4 and is predicted to have semiconducting properties

Optical Properties of Antiferroelectric Cs2Nb4O11: Absorption Spectra and First-Principles Calculations

We report a joint experimental and theoretical investigation of the optical properties of Cs2Nb4O11. In room temperature optical absorption spectra, we found a direct gap about 3.5560.05 eV and charge transfer excitations at about 4.96 and 6.08 eV, which are in good agreement with the first-principles calculations. Upon passing through the 165 C antiferroelectric to paraelectric phase transition, the peak energies of two charge transfer bands display almost no temperature dependence, yet they become even broader and exhibit enhanced oscillator strength. We infer this intriguing behavior as the manifestation of Nb cation distortions due to the charge-lattice interaction

Molecular-Dynamics Simulations of Some BaXF4 Compounds

We have carried out molecular-dynamics simulations on BaXF4 compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A21am space group reported experimentally. We were able to reverse polarization in BaMgF4 at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of molecular dynamics to materials consisting of chains of F octahedra

Structure and Antiferroelectric Properties of Cesium Niobate, Cs2Nb4O11

The compound cesium niobate, Cs2Nb4O11, is an antiferroelectric, as demonstrated by double hysteresis loops in the electric-field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297 K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5 kV/cm

Interfacial Properties of Bilayer and Trilayer Graphene on Metal Substrates

One popular approach to prepare graphene is to grow them on transition metal substrates via chemical vapor deposition. By using the density functional theory with dispersion correction, we systematically investigate for the first time the interfacial properties of bilayer (BLG) and trilayer graphene (TLG) on metal substrates. Three categories of interfacial structures are revealed. The adsorption of B(T)LG on Al, Ag, Cu, Au, and Pt substrates is a weak physisorption, but a band gap can be opened. The adsorption of B(T)LG on Ti, Ni, and Co substrates is a strong chemisorption, and a stacking-insensitive band gap is opened for the two uncontacted layers of TLG. The adsorption of B(T)LG on Pd substrate is a weaker chemisorption, with a band gap opened for the uncontacted layers. This fundamental study also helps for B(T)LG device study due to inevitable graphene/metal contact.Comment: 1 table, 8 figure

Dielectric Properties and Maxwell-Wagner Relaxation of Compounds ACu3Ti4O12 (A=Ca,Bi2/3,Y2/3,La2/3)

We have studied the frequency and temperature dependences of permittivity and impedance of the compounds ACu3Ti4O12 A=Ca, Bi2/3, Y2/3, La2/3 in the ranges of 10−1–106 Hz and −150–200 °C. All compounds investigated display similar dielectric properties. Specifically, they all have a Debye-like relaxation and their dielectric constants are independent of frequency and temperature over a wide range. They all have two electrical responses in impedance formalism, indicating that there are two distinct contributions. We attribute them to grains and grain boundaries in the ceramic samples and explain the dielectric behaviors by Maxwell-Wagner relaxation arising at the interfaces between grains and their boundaries

The Minority Spin Surface Bands of CoS2(001)

Angle-resolved photoemission was used to study the surface electronic band structure of high quality single crystals of ferromagnetic CoS2 (below 120 K). Strongly dispersing Co t2g bands are identified along the 100 k direction, the ¯–¯X line of the surface Brillouin zone, in agreement with model calculations. The calculated surface band structure includes corrections for the previously determined surface structure of CoS2(001) and is in general agreement with the experimental photoemission spectra in the region of the Fermi level. There is evidence of the existence of several minority spin surface states, falling into a gap of the projected minority spin bulk CoS2(001) band structure

CaCu3Ti4O12: Low-Temperature Synthesis by Pyrolysis of an Organic Solution

The giant-dielectric-constant material CaCu3Ti4O12 (CCTO) has been synthesized by pyrolyzing an organic solution containing stoichiometric amounts of the metal cations, which is done at lower temperature and shorter reaction time than the conventional solid-state reaction. A stable solution was prepared by dissolving calcium nitrate, copper(II) nitrate, and titanium(IV) isopropoxide in 2-methoxyethanol. This solution was pyrolyzed and heat-treated to achieve single-phase CCTO. The phases, microstructures, and dielectric properties of intermediate and final samples were characterized by X-ray diffraction, scanning electron microscopy, and dielectric spectroscopy