Structure of BaTiO<Subscript>3</Subscript> phases is studied by comparing neutron diffraction and Raman spectroscopy data

Considering the structural unit of the rhombohedral BaTiO₃ phase in the form of Ti(-O...Ti)₆ which was constructed by neutron diffraction data, we revealed an atomic group in the form of a pyramidal complex anion TiO₃²⁻. Inspection of the Raman spectra of this phase showed symmetry group C _s for this complex anion. From the similarity of the Raman spectra of all ferroelectric BaTiO₃ phases, we inferred that the orthorhombic phase and tetragonal phase are also built of TiO₃²⁻ complex anions. In the paraelectric cubic phase, TiO₃²⁻ complex anions decompose to oxygen ions O²⁻ and Ti²⁺(O⁻)₂ molecules, which are randomly oriented over 12 possible positions. Average Ti-O distances derived from neutron diffraction data were used to calculate Ti-O⁻ and Ti=O bond lengths in TiO₃²⁻ complex anions of the ferroelectric BaTiO₃ phases. Two types of Ti-O...Ti bridges were found to exist; they belong to weak and strong intermolecular bonds, which are similar to weak and strong hydrogen bonds (H-bonds) in ferroelectrics. Weak bonds exist in all of the three ferroelectric phases; strong bonds are only in the orthorhombic phase and the tetragonal phase. As a result of a low potential barrier height in strong bonds, an oxygen atom persistently hops from one potential well to an adjacent one, which is responsible for bands appearing below 100 cm⁻¹ in the Raman spectra of the tetragonal phase and orthorhombic phase. The data obtained on the structure of the ferroelectric BaTiO₃ phases were used to interpret their spontaneous polarization, repolarization, and permittivity.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

A device type photocatalyst using oppositely polarized ferroelectric substrates

A device type photocatalyst with a semiconductor-metal-ferroelectric (TiO₂/Pt/LiNbO₃, PZT, BaTiO₃) structure was fabricated by utilizing either the positive or negative polar surfaces of the ferroelectric as substrate and was examined for the photoassisted decomposition of water. TiO₂ on the positive polar ferroelectric exhibited higher efficiency for the formation of hydrogen, in contrast to negligibly small activity of TiO₂ on the negative polar ferroelectric.     

First‐principles calculations on the four phases of BaTiO3

The calculations based on linear combination of atomic orbitals basis functions as implemented in CRYSTAL09 computer code have been performed for cubic, tetragonal, orthorhombic, and rhombohedral modifications of BaTiO₃ crystal. Structural and electronic properties as well as phonon frequencies were obtained using local density approximation, generalized gradient approximation, and hybrid exchange‐correlation density functional theory (DFT) functionals for four stable phases of BaTiO₃. A comparison was made between the results of different DFT techniques. It is concluded that the hybrid PBE0 [J. P. Perdew, K. Burke, M. Ernzerhof, J. Chem. Phys. 1996, 105, 9982.] functional is able to predict correctly the structural stability and phonon properties both for cubic and ferroelectric phases of BaTiO₃. The comparative phonon symmetry analysis in BaTiO₃ four phases has been made basing on the site symmetry and irreducible representation indexes for the first time. © 2012 Wiley Periodicals, Inc.     

Physics and Chemistry of Creating New Titanates with Perovskite Structure

The phase formation, structural features, and dielectric, ferroelectric, and piezoelectric properties of ceramics with compositions from the region of the morphotropic phase boundary in the (Na_0.5Bi_0.5)TiO₃–BaTiO₃–Bi(Mg_0.5Ti_0.5)O₃ system modified by different low-melting additives (Bi₂O₃, V₂O₅, KCl, NaCl–LiF, LiF, CuO, and MnO₂) are studied. First-order phase transitions are detected near 700–800 and 400 K that display relaxor behavior and are indicative of the presence of polar regions in a nonpolar matrix. Prospects for improving the piezoelectric properties of the modified ceramic samples are confirmed.     

Investigations of the local structures and spin Hamiltonian parameters for the trigonal Rh and Ir centers in rhombohedral BaTiO3

The local structures and spin Hamiltonian parameters (g factors and the hyperfine structure constants) of the Rh⁴⁺(4d⁵) and Ir⁴⁺(5d⁵) centers in rhombohedral BaTiO₃ are theoretically investigated from the formulas of these parameters for a nd⁵ (n = 4 and 5) ion with low spin (S = 1/2) in a trigonally distorted octahedron. From the calculations, the impurity ions are found not to occupy exactly the host Ti⁴⁺ site in BaTiO₃ but to suffer a slight inward shift (0.13 Å) towards the center of the oxygen octahedron along the C₃ axis, yielding much smaller trigonal distortion as compared with that of the host Ti⁴⁺ site. The theoretical spin Hamiltonian parameters based on the above impurity axial shifts are in good agreement with the observed values.     

Classical or relaxor ferroelectric ceramics in the BaTiO3-KNbO3-CaTiO3 system

In perovskites, the relaxor behavior occurs dominantly in lead based compositions. The study of the BaTiO₃-KNbO₃ binary diagram provided previously evidence for new free from lead ferroelectric relaxor properties but unfortunately at low temperature. The present dielectric studies of ceramics with composition inside the BaTiO₃-KNbO₃-CaTiO₃ showed some of them to be ferroelectric relaxor at temperature close to 300K. Such a favourable effect is due to the Ca²⁺-Ba²⁺ cationic substitution. These new compositions are of interest for applications (dielectric for capacitors, actuators) in the way of environmental protection.     

[Cu32(H)20{S2P(OiPr)2}12]: The Largest Number of Hydrides Recorded in a Molecular Nanocluster by Neutron Diffraction

An air‐ and moisture‐stable nanoscale polyhydrido copper cluster [Cu₃₂(H)₂₀{S₂P(OiPr)₂}₁₂] ( 1_H ) was synthesized and structurally characterized. The molecular structure of 1_H exhibits a hexacapped pseudo‐rhombohedral core of 14 Cu atoms sandwiched between two nestlike triangular cupola fragments of (2×9) Cu atoms in an elongated triangular gyrobicupola polyhedron. The discrete Cu₃₂ cluster is stabilized by 12 dithiophosphate ligands and a record number of 20 hydride ligands, which were found by high‐resolution neutron diffraction to exhibit tri‐, tetra‐, and pentacoordinated hydrides in capping and interstitial modes. This result was further supported by a density functional theory investigation on the simplified model [Cu₃₂(H)₂₀(S₂PH₂)₁₂].     

Identification of molecules in inorganic compounds from X-ray or neutron diffraction data and correction of their structures on the basis of vibrational spectroscopy data

A method of identification of molecules in inorganic crystals with asymmetric M-X...M bridges has been suggested on the basis of analysis of interatomic distances in the structure unit M (−X...M) _n , which includes the atoms of the first and second coordination spheres. This analysis makes it possible to discern molecules (complex anions or cations) as a groups of atoms linked with each other by short M-X bonds, whereas the atoms of neighboring groups are linked by long M...X bonds. The symmetry of such a group is often lower than it follows from X-ray or neutron diffraction data. Studying vibrational spectra affords information on the true symmetry of a molecule. The use of the method is exemplified by the rhombohedral BaTiO₃ phase.     

Effects of Nonpolar and Polar Solvents on the Qx and QY Energies of Bacteriochlorophyll a and Bacteriopheophytin a

The effects of nonpolar and polar solvents on the Q_x and Q_y energies of bacteriochlorophyll (BChl) a and bacteriopheophytin (BPhe) a were examined by electronic absorption spectroscopy. All of the four different energies exhibited a linear dependence on R(n) = (n² - l)/(n²+ 2), where n is the refractive index of the solvent, in both nonpolar and polar solvents. The energy of each state of both pigments could be expressed as v = -dR(n) + e (in cm^-1) where coefficient d was related to the dispersive interaction between the solute and the solvent molecules. A theory developed by Nagae showed that coefficient d originates from the quantum-mechanical fluctuation of the multipole moments of the solute, in terms of which the following characteristics of the observed d values were explained: (1) In all of the four cases of the Q_x, and Q_y energies of both BChl a and BPhe a, the d values for the polar solvents were smaller than those for the non-polar solvents. (2) In both nonpolar and polar solvents, the d value of BChl a was larger than that of BPhe a in the Q_y energy, whereas the d value of BPhe a was larger than that of BChl a in the Q_x energy. (3) The d value of the Q_x energy was larger than that of the Q_y, energy for either case of BChl a or BPhe a.     

Identification of complex anions in La1 − x A x MnO3 manganites (A = Ca, Sr) from neutron diffraction data and refinement of their structures on the basis of raman spectroscopy data

The consideration of the Mn(-O…Mn)₆ structure unit of the LaMnO₃ orthorhombic phase constructed on the basis of neutron diffraction data revealed the presence of atomic groups in the form of (MnO₂)^? complex anions and separate O^2? ions. The Raman spectra have demonstrated that complex anions have a nearly linear O=Mn-O^? structure. Comparison of the Raman spectra of the rhombohedral CaMnO₃ and BaTiO₃ phases and pyramidal molecules of the ZXY₂ type has shown that the CaMnO₃ phase is composed of pyramidal MnO ₃ ^2? molecules with one Mn=O double bond and two Mn-O^? ordinary bonds. The complex anions are linked by O=Mn-O^? intermolecular bonds to form planar zigzag Mn=O…Mn chains similar to C=C-C planar chains in some organic compounds exhibiting high electrical conductivity. With decreasing temperature, the Mn…O intermolecular distances decrease, and the Mn=O…Mn chains become even more similar to the C=C-C chains and exhibit high electrical conductivity.     

New lead-free relaxor ceramics derived from BaTiO3 by cationic heterovalent substitutions in the 12 C.N. crystallographic site

Crystallographic and low frequency dielectric studies were performed on new lead-free ceramics derived from BaTiO₃ by cationic substitutions in the 12 C.N. crystallographic site and having the composition Ba_1−xK_x/2La_x/2TiO₃ or Ba_1−xA_2x/3TiO₃ (A=La, Bi). A classical ferroelectric behaviour was shown for compositions close to BaTiO₃, while a relaxor one occured when the composition deviated from BaTiO₃. In any case, the value of T_C (or T_m) and the relaxor characteristics are higher as x increases. Concerning potassium-free ceramics, bismuth compositions compared with lanthanum ones favoured both high ferroelectric and relaxor characteristics due to the lone pair of Bi³⁺. Such new materials are of interest for environmental friendly applications.     

Electron Capture by the Thiyl Radical and Disulfide Bond: Ligand Effects on the Reduction Potential

The effect of non‐polar and polar ligands and of monovalent cations on the one‐electron reduction potential of the thiyl radical and the disulfide bond was evaluated. The reduction potentials E° for the CH₃S^.–n L/CH₃S^?–n L and CH₃SSCH₃–L/CH₃SSCH₃^.?–L redox couples were calculated at the B3LYP, M06‐2X and MP2 levels of theory, with n=1, 2 and L=CH₄, C₂H₄, H₂O, CH₃OH, NH₃, CH₃COOH, CH₃CONH₂, NH₄⁺, Na⁺, K⁺ and Li⁺. Non‐polar ligands decrease the E° value of the thiyl radical and disulfide bond, while neutral polar ligands favour electron uptake. Charged polar ligands and cations favour electron capture by the thiyl radical while disfavouring electron uptake by the disulfide bond. Thus, the same type of ligand can have a different effect on E° depending on the redox couple. Therefore, properties of an isolated ligand cannot uniquely determine E°. The ligand effects on E° are discussed in terms of the vertical electron affinity and reorganization energy, as well as molecular orbital theory. For a given redox couple, the ligand type influences the nature of the anion formed upon electron capture and the corresponding reorganization process towards the reduced geometry.     

Alloying n‐Butylamine into CsPbBr3 To Give a Two‐Dimensional Bilayered Perovskite Ferroelectric Material

Cesium‐lead halide perovskites (e.g. CsPbBr₃) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr₃, we design the first cesium‐based two‐dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C₄H₉NH₃)₂CsPb₂Br₇ ( 1 ). Strikingly, 1 shows a high Curie temperature (T_c=412 K) above that of BaTiO₃ (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm^?2), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs⁺ ions. To our knowledge, such a 2D bilayered Cs⁺‐based metal–halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large “on/off” ratios of photoconductivity (>10³).     

The influence of polyvinylpyrrolidone on thick and optical properties of BaTiO<Subscript>3</Subscript>:Er<Superscript>3+</Superscript> thin films prepared by sol–gel method

Erbium (Er³⁺) 0.5% mol doped barium titanate (BaTiO₃) thin films were elaborated via sol–gel method and dip-coating technique using titanium alkoxide and barium pentanedionate. Two syntheses were performed [with and without polyvinylpyrrolidone (PVP)] in order to obtain thick films. The BaTiO₃:Er³⁺ thin films prepared from the sol with PVP were elaborated with 1 layer and those without PVP and were elaborated with 17 layers. In both cases, the films were deposited on silica quartz substrates. Both BaTiO₃:Er³⁺ films presented a cubic phase, as determined by X-ray diffraction. BaTiO₃:Er³⁺ films elaborated with PVP via single-step dip coating produced crack-free films with thicknesses of ~800 nm. SEM micrographs for the obtained BaTiO₃:Er³⁺ films revealed high homogeneity and density. Mapping images obtained from BaTiO₃:Er³⁺ revealed homogeneous distribution of Er³⁺ ions on the surface. XPS analyses of the chemical state and chemical environment of the constituent elements in the compositions showed that Er³⁺ ions in (Ba_1−x Er _x )TiO₃ are in mixed valence of Er³⁺/Er²⁺. The BaTiO₃:Er³⁺-PVP film exhibited luminescent properties under near-infrared excitation, as revealed by green emissions. The BaTiO₃:Er³⁺-PVP film has good potential for optical applications.     

Impact of ferroelectric nanoparticles on the dielectric constant of nematic liquid crystals

ABSTRACT

The influence of ferroelectric BaTiO₃ nanoparticles on the dielectric constant in the isotropic phase of the isotropic-nematic phase transition is studied within Landau phenomenological theory. The ferroelectric BaTiO₃ nanoparticles dependence of transition temperature and dielectric constant in the isotropic phase is calculated. We compare our theoretical results with experimental data available in the literature.     

Influence of Ferroelectric Materials and Catalysts on the Performance of Non-Thermal Plasma (NTP) for the Removal of Air Pollutants

The introduction of ferroelectric and catalytically active materials into the discharge zone of NTP reactors is a promising way to improve their performance for the removal of hazardous substances, especially those appearing in low concentrations. In this study, several coaxial barrier-discharge plasma reactors varying in size and barrier material (glass, Al₂ O₃, and TiO₂) were used. The oxidation of methyl tert-butyl ether (MTBE), toluene and acetone was studied in a gas-phase plasma and in various packed-bed reactors (filled with ferroelectric and catalytically active materials). In the ferroelectric packed-bed reactors, better energy efficiency and CO₂ selectivity were found for the oxidation of the model substances. Studies on the oxidation of a toluene/acetone mixture in air showed an enhanced oxidation of the less reactive acetone related to toluene in the ferroelectric packed-bed reactors. It can be concluded that the change of the electrical discharge behaviour was caused by a larger number of non-selective and highly reactive plasma species formed within the ferroelectric bed. When combining ferroelectric (BaTiO₃) and catalytically active materials (LaCoO₃), only a layered implementation led to synergistic effects utilising both highly energetic species formed in the ferroelectric packed-bed and the potential for total oxidation provided by the catalytically active material in the second part of the packed bed.     

Thin poly(ionic liquid) and poly(vinylidene fluoride) blend films with ferro‐ and piezo‐electric polar γ‐crystals

Ferro‐ and piezo‐electric poly(vinylidene fluoride) (PVDF) thin film is reported to be obtained by using a poly(ionic liquid) (PIL) [poly(2‐(dimethylamino)ethyl methacrylate) methyl chloride quaternary salt] through solution route. The short range interactions between localized cationic ions of PIL and polar >CF₂ of PVDF are responsible for modified polar γ‐PVDF (T₃GT₃Ḡ) formation. Modification in chain conformation of PVDF is confirmed by FTIR, XRD, and DSC studies suggesting the miscible PVDF–PIL (PPIL) blend. Up to 40 wt % loading of PIL in PVDF matrix enhances relative intensity of γ‐phase up to 50% in the entire crystalline phase. The P‐E hysteresis loop of PVDF‐PIL blends at 25 wt % PIL loading (PPIL‐25) thin film at sweep voltage of ±50 V shows excellent ferroelectric property with nearly saturated high remnant polarization ∼6.0 µC cm⁻² owing to large proportion of γ‐PVDF. However, non‐polar pure PVDF thin film shows unsaturated hysteresis loop with 1.4 µC cm⁻² remnant polarization. The operation voltage decreases effectively because of the polar γ‐phase formation in PPIL blended film. High‐sensitivity piezo‐response force microscopy shows electromechanical switching property at low voltages in PPIL‐25 thin films through local switching measurements, making them potentially suitable as ferroelectric tunnel barriers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 795–802     

Structured diffuse scattering and polar nano-regions in the Ba(Ti1−xSnx)O3 relaxor ferroelectric system

The observation via electron diffraction of relatively sharp G±{001}* sheets of diffuse intensity arising from the large amplitude excitation of inherently polar, transverse optical modes of distortion in Ba(Ti_1−xSn_x)O₃ (BTS), 0.1?x?0.5, samples, both at room temperature as well as liquid nitrogen temperature, shows that the polar nano-regions (PNRs) in these relaxor ferroelectric materials correspond to the same highly anisotropic 〈001〉 chain dipoles as are characteristic of the normal ferroelectric end member BaTiO₃ itself. The correlation length along the chain of these 1-d PNRs can, in principle, be determined from the width of the observed {001}* diffuse sheets in reciprocal space and is estimated to be at least 5 nm even for the higher x samples. The distribution of the substitutional Sn ions thus appears to have only a minor effect upon the correlation length along the 〈001〉 chain dipole directions. It is suggested that the role of the dopant Sn ions is not to directly induce PNRs but rather to set up random local strain fields preventing the condensation of long wavelength homogeneous strain distortions of the unit cell thereby suppressing transverse correlations of the 〈001〉 chain dipoles and the development of long-range ordered ferroelectric state/s.     

Ba Titanate and Barium/Strontium Titanate Thin Films from Hydroxide Precursors: Preparation and Ferroelectric Behavior

Randomly oriented ferroelectric BaTiO₃ and (Ba_0.6Sr_0.4) TiO₃ thin films on platinum coated Si (100) were prepared by a sol-gel method. The precursor solutions were derived from barium hydroxide or a mixture of barium/strontium hydroxides dissolved in acetic acid and titanium butoxide. Polarization versus applied voltage hysteresis studies indicated a remanent polarization of 3 µC/cm² and a coercive field of 43.4 kV/cm for BaTiO₃ films annealed at 800°C for 1 h. Corresponding parameters for (Ba_0.6Sr_0.4)TiO₃ films annealed at 800°C were found to be 7.2 µC/cm² and 102.7 kV/cm, respectively. Microstructural study of the surface morphology of these films indicated grains of less than 0.1 µm in size. The leakage current for (Ba_0.6Sr_0.4)TiO₃ films was found to be two orders of magnitude lower than that for BaTiO₃ films.