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Using DFT methods we have studied structure, equation of state, and phase behavior of BaTiO3. We have identified the pressure induced phase transformations from the rhombohedral to orthorhombic structure at ca. 5 GPa and from tetragonal phase to cubic phase at ca. 7.5 GPa.
Thin films of lead zirconate titanate (PZT) are currently being used in a novel MEMS device to generate power. A piezoelectric stack consisting of platinum/PZT/gold is deposited by sputtering, spin coating, and subsequent heat treatments onto a thin silicon membrane, which is cyclically polarized by a flexing motion. The membrane must withstand strains between 0.1% and 0.5% for several billion cycles to provide a useful device. This study has examined the processing-structure-property relationships in developing the PZT film for use in this device. In the sol-gel deposition of PZT, pyrolysis and crystallization temperatures have been shown to alter both microstructure and properties of the piezoelectric film. The chemistry of the PZT film has also been tailored to increase piezoelectric output for this device. Ferroelectric properties are compared to the piezoelectric outputs, and fatigue behavior is measured on bulk silicon and on membranes.
The effects of a seeding layer, which was deposited on Pt/TiO2/SiO2/Si substrates using magnetron sputtering, on the characteristics of sol-gel-deposited strontium-bismuth-tantalate (SBT) thin films are investigated. The seeding layer serves as nucleation sites so homogeneous crystalline SBT films of bismuth-layered structure (BLS) with fine grains are successfully obtained by 750°C rapid thermal annealing in O2 ambient. The remanent polarization (2Pr) improves from 12.1 to 18.8 μC/cm2 with the addition of the seeding layer. In addition, the seeding layer also results in a lower nucleation temperature, allowing the use of 700°C annealing for 10 min to grow SBT films that are fully crystallized with BLS phase and shows good ferroelectric properties. Finally, crystallinity and microstructures of SBT films are found to be strongly dependent on the thickness of the seeding layer. Optimum Ta-seeded SBT thin film crystallized at 700°C for 10min depicts a higher 2Pr value (12.9 μC/cm2 (@5V) than that of the un-seeded films crystallized at 750°C for 1min.
Dielectric permittivities and polarization-electric-field (P-E) hysteresis loops have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)0.68(PbTiO3)0.32 (PMN-32%PT) for <110>cub and <211>cub orientations. Contrary to the pure PbMg1/3Nb2/3O3 (PMN), PMN-32%PT exhibits apparent orientation dependences of dielectric permittivities, polarizations, and phase transitions. With a prior fieldcooled treatment, a field-induced state, perhaps of orthorhombic symmetry, is evidenced and coexists with the rhombohedral symmetry in the low-temperature region. This field-induced phase is manifested by an extra dielectric peak observed near 373 K for the <211>cub orientation.
Superconductivity and ferromagnetism have been believed to be incompatible over any extended temperature range until certain specific examples - RuSr2GdCu2O8 and UGe2- have arisen in the past 2-3 years. The discovery of superconductivity above 8 K in MgCNi3, which is primarily the ferromagnetic element Ni and is strongly exchange-enhanced, provides a probable new and different example. This compound is shown here to be near ferromagmnetism, requiring only hole-doping by 12% substitution of Mg by Na or Li. This system will provide the means to prode coupling, and possible coexistence, of these two forms of collective behavior without the requirement of pressure.
Particle size dependent transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in presence and in absence of magnetic field B=0.0-1.5T (maximum). All the samples show metal-insulator transition (MIT) with a peak at the MIT temperature (Tp). Magnetic field decreases the resistivity with an increase in the peak temperature Tp. Particle size, conductivity and Tp of the sample increase with increasing annealing time. High temperature semiconducting (insulating) part of the resistivity curve is divided into two distinct regimes. Resistivity data for T>qθ/2, can be well fitted with the nearest neighbor small polaron hopping (SPH) model. Polaron hopping energy (WH) decreases with increase of particle size. The lower temperature part (Tp>T>qθ/2) of the semiconducting (insulating) regime is found to follow variable range hopping (VRH) model. With the increase of particle size, the temperature range of validity of the VRH mechanism decreases. The low temperature metallic regime (for T<Tp) of the resistivity (both in absence and in presence of field) data fit well with ρ = ρ0 +ρ2.5 T2.5 and transport mechanism in this region is mainly dominated by magnon-carrier scattering (∼T2.5). Particle size has, however, comparatively little effect on Seebeck coefficient (S). In all the samples with different particle sizes, S changes sign below Tp. In contrast to magnetoresistance, application of magnetic field increases S at low temperature (T<Tp) for these samples. Similar to the resistivity results, thermopower data in the metallic phase (both for B=0.0 and 1.5T) can also be analyzed by considering magnon-scattering term along with an additional spin-wave fluctuation term (∼T4).
The effect of changing sintering temperature on the grain boundary properties and the room temperature resistivity (ρRT) of Pb(Fe1/2Nb1/2)O3 (PFN) was investigated. Monitering the temperature dependence of resistivity showed that the ρRT's of 1050°C and 1150°C-sintered specimen were 1011ΩEcm and 104ΩEcm respectively, but the resistivity above 300°C became nearly identical. The previous model, that the low resistivity of PFN is due to the electron hopping between Fe2+ and Fe3+ driven by the reduction of PFN, couldn't explain this phenomenon, and the reconsideration of the Fe reduction revealed that the difference of electron concentration between the 1050°C and 1150°C-sintered specimen couldn't exceed one order of magnitude. The role of the grain boundary was introduced in order to account for this phenomenon.
Elastic deformations during phase transition in freestanding BaTiO3 thin films were investigated. BaTiO3 films were prepared by sol-gel technique or RF magnetron sputtering on silicon substrates, covered by randomly oriented 120 nm thick Al2O3. The as-deposited films were under tensile stress of 100-170 MPa and did not show neither pyroelectric nor piezoelectric properties. Partial substrate removal caused the freestanding films to expand laterally by 0.3-0.5% and corrugate. Dielectric constant of the freestanding films (620±10) was found to be significantly higher than that of the substrate supported films (110±20). The freestanding films showed detectable piezoelectric effect, which indicated that the lateral expansion was originated from the substrate-suppressed cubic-tetragonal phase transition.
Ion exchange of Dion-Jacobson-type double- and triple-layered perovskites with divalent transition-metal halides leads to the templated assembly of metal-anion layers. A whole series of compounds of the form (MCl)LaNb2O7 (M = V, Cr, Mn, Fe, Co, Cu) and (CuCl)Ln2Ti2NbO10 (Ln = Pr, Nd, Sm, Eu) have been prepared. Additionally, a new series of host compounds is being developed to exploit this exchange chemistry; the new oxyfluorides, RbLnTiNbO6F (Ln = La, Pr, Nd) and RbLaM'NbO6F (M' = V4+, Mn4+, Nb4+, Mo4+), have been prepared by a direct route.
The renewed interest in KTa1-xNbxO (KTN) mixed perovskite materials is connected with their remarkable dielectric properties in the dilute compositions. KTN thin films with x = 0.35 have been prepared on different substrates by sol-gel technique as well as a set of powders with x = 0, 0.05, 0.1, 0.25, 0.48, 0.65, 0.75, and 1. Properties of the material change drastically with the change of x, because of relaxation of both translational and inversion symmetry due to a static disorder in the Nb distribution and the dynamic effect of a precursor ferroelectric order above Tc. Special attention was paid to the characteristic feature of coupling of the single-phonon state to a two-acoustic-phonon feature through anharmonic terms in the potential function as well as behavior of the TO3 mode which becomes a narrow peak of the first-order scattering in the tetragonal ferroelectric phase and shows a tendency to split below Tc2 in the orthorhombic phase. The wide range of x allows better understanding of dynamic processes in the KTN bulk materials which in turn helps in the studies of thin films. The above mentioned materials were studied using Raman scattering, XRD, and thermal analysis techniques.
Barium zirconium titanate (BZT) thin films are attractive candidates for dynamic random access memories and tunable microwave devices. In the present work a wide range of Zr doped BaTiO3 thin films have been prepared by sol-gel technique. X-ray diffraction and micro-Raman scattering studies confirmed the structural phases in the powder and film of BZT and various structural transitions of BaTiO3 as a function of different Zr content compared well with the published result on ceramics and single crystalline BZT. The deposited films had smooth, crackfree and homogeneous microstructure and Zr content has strong influence on the evolution of the microstructures of the films. Some selected compositions of these films were characterized in terms of their dielectric properties and phase transition behavior. BZT film with 20 at % Zr had a ferroelectric to paraelectric transition in the vicinity of room temperature. Efforts are underway to optimize the annealing condition and grow epitaxial BZT films, with various Zr contents, on a suitable single-crystalline substrate.
We report very large changes under modest hydrostatic pressure in the nature of the relaxor ferroelectric (FE) response for a single crystal of SrTiO3 doped with 0.7 mol % CaTiO3. The Ca cation dopants generate polar clusters within the FE soft mode, readily polarized host crystal. At ambient pressure the cluster size increases with decreasing temperature, leading to a dispersive (relaxor) state below ∼18 K. However, the application of modest pressure stiffens the soft mode frequency of the host lattice and reduces cluster growth, thereby decreasing the glass-like transition temperature at roughly –35 K/kbar. Above 0.5 kbar there is no evidence for the relaxor state; rather, a temperature- and frequency-independent dielectric response reflecting quantum paraelectric behavior evolves. These results demonstrate the extreme sensitivity to pressure of relaxors with low transition temperatures, i.e., in the quantum regime where characteristic energies are small.
A first principles density funtional computational method has been used to investigate the atomic stucture and energy of various distinct forms of the σ = 5(001) twist grain boundary in SrTiO3. The study focuses on four non-stoichiometric geometries which are all found to be stable to be the most stable. The grain boundary energies are computed as a function of TiO2 chemical potential and these define the limits of stability. The computed volume expansions are consistent with experimental observation and the in plane relaxations lower the boundary symmetry.
We have performed Rietveld refinements on neutron and synchrotron diffraction patterns and density functional calculations on various ferroelectric lead perovskites and on α lead monoxide (litharge). These structural data have allowed to shed some light on lead stereochemistry in these compounds. In particular, we discuss the changing in the lead behaviour between the paraelectric cubic phases and the low temperature anti or ferroelectric phases in Pb2CoWO6 and Pb2MgTeO6 (both incommensurate), in Pb2MgWO6 (antiferroelectric) and in PbMg1/3Nb2/3O3 (relaxor). The possible phase transition mechanisms are reviewed and the bonds are compared to those in the aperiodic structure of α-lead monoxide.
In this paper, size-induced ferroelectricit yweakening, phase transformation, and anomalous lattice expansion are observed in nanocrystalline BaTiO3 (nc-BaTiO3) deriv ed b y low temperature hydrothermal methods, and they are w ellunderstood using the terms of the long-range interaction and its cooperative phenomena altered by particle size in covalen t ionic nanocrystals. In cubic nc-BaTiO3, five modes centerd at 186, 254, 308, 512 and 716 cm-1 are observed Raman active in cubic nanophase, and they are attributed to local rhombohedral distortion breaking inversion-symmetry in cubic nanophase. The254 and 308 cm-1 modes are significantly affected not only by the concentration of hydroxyl defects, but also their particular configuration. And the 806 cm-1 modes found to be closely associated with OH - absorbed on grain boundaries.
Quantum paraelectric SrTiO3 undergoes a transition to ferroelectric by the substitution of 18O for 16O. The Tc vs. x in SrTi(16O1-x18Ox)3 follows Tc = 30.4(x - 0.33)1/2. Application of the hydrostatic pressure on the SrTiO3 and SrTi(16O0.0718O0.93)3 have the effects of decreasing ε and depressing Tc, respectively. Above the critical pressure pc, ferroelectricity of SrTi(16O0.0718O0.93)3 disappears. Fitting the data to the Barrett's formula, ε , elucidated that T0 and T1 for both compounds changes linearly with pressure.
The electronic and magnetic properties of half-metallic CrO2 have been studied by using the full-potential linearized muffin-tin orbital method within the local spin-density approximation (LSDA)+U approach. It is found that the orbital magnetic moment of Cr atom is quenched while O atom exhibit relatively significant orbital moment in CrO2. For the Hubbard U of 3 eV, LSDA+U gives the orbital moment of -0.051μB/atom for Cr and -0.0025μB/atom for O, being in good agreement with the experimental orbital moments of -0.05 for Cr and -0.003μB/atom for O, respectively. In contrast, LSDA gives the orbital moment of -0.037 for Cr and -0.0011 μB/atom for O, being too small as compared with the magnetic circular dichroism measurements. For the larger U considered in this work, both spin and orbital moments almost increase linearly with respect to U.
Barium strontium titanate (BST) thin films were grown directly on Si substrates by the conventional and ultraviolet-assisted pulsed laser deposition techniques. X-ray photoelectron spectroscopy, x-ray diffraction and reflectivity, variable angle spectroscopic ellipsometry, current-voltage, capacitance-voltage, and high-resolution transmission electron microscopy were used to investigate the composition, thickness, and electrical properties of the grown structures. It has been found that at the interface between the Si substrate and the grown dielectric layer, an interfacial layer was always formed. The chemical composition of the layer consisted of SiOx partially mixed with the grown BST, without any evidence of silicate formation.
Novel perovskite compound SrRhO3 was synthesized in a polycrystalline form by high-pressure technique at 6 GPa and 1500°C, followed by measurements of magnetic susceptibility, electrical resistivity, thermopower, and specific heat. Powder x-ray diffraction study found the slightly distorted perovskite structure, GdFeO3-type, to be likely to SrRhO3; space group was Pnma and lattice parameters were α = 5.5394(2) Å, b = 7.8539 (3) Å, and c = 5.5666(2) A. Oxygen vacancies in the perovskite were quantitatively investigated by thermogravimetric analysis and then found either absent or at least insignificant. The title compound shows a Fermi-liquid behavior in its electrical resistivity. The magnetic susceptibility is large [χ(300) ∼1.1x10-3 emu/mol-Rh], and the characteristics seem to be intermediate between enhanced Pauli- and Curie-Weiss-type paramagnetism.
Optical properties and thermal properties of self-polarized thin PZT films were determined by variable angle spectral ellipsometry and by the AC hot strip method, respectively. Analyzing the temperature dependencies of the optical gap and the specific heat, evidence of two not as yet observed phase transitions in the ferroelectric was provided. The origin of these phase transitions was attributed to film stress caused by substrate/PZT thin film lattice mismatch and to the presence of a negative space charge layer in the PZT film at the bottom electrode/PZT interface.