Structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3: First-principles density-functional studies

Pb Stabilization by a New Chemically Durable Orthophosphate Phase: Insights into the Molecular Mechanism with X-ray Structural Analysis

The rapid progression of piezoelectric technology and the upgradation of electronic devices have resulted in a global increase in Pb-based piezoelectric ceramic materials. In this study, the feasibility of incorporating Pb into a PbZr(PO₄)₂ double orthophosphate structure was evaluated by investigating the interaction mechanism of the perovskite with phosphate. The unique combination of X-ray absorption spectroscopy, selected area electronic diffraction, and Pawley refinement revealed that Pb was incorporated into a hexagonal structure and tetra-coordinated with oxygen in the phosphate-treated product. The chemical durability was enhanced through the structural alterations via Zr-O-P and Pb-O-P bond linkages. The stable phase encapsulating both Pb and phosphate showed effectiveness not only in stabilizing Pb but also in inhibiting P release as a secondary pollution risk within a wide pH range (1 ≤ pH ≤ 13). Despite the excellent chemical durability of the robust PbZr(PO₄)₂ crystalline phase, the increased Ti doping amounts at the Zr site resulted in a slight decrease in the lattice parameters and further enhanced the Pb stabilization effect through the formation of PbZr_xTi_(1-x)(PO₄)₂ solid solutions. This study demonstrates that the newly robust crystalline structure, developed through a well-designed thermal treatment scheme, provides an effective strategy for the treatment of Pb frequently encountered in electronic wastes.

An Unconventional Transient Phase with Cycloidal Order of Polarization in Energy-Storage Antiferroelectric PbZrO3

Antiferroelectric-based dielectric capacitors are receiving tremendous attention for their outstanding energy-storage performance and extraordinary flexibility in collecting pulsed powers. Nevertheless, the in situ atomic-scale structural-evolution pathway, inherently coupling to the energy storage process, has not been elucidated for the ultimate mechanistic understanding so far. Here, time- and atomic-resolution structural phase evolution in antiferroelectric PbZrO₃ during storage of energy from the electron-beam illumination is reported. By employing state-of-the-art negative-spherical-aberration imaging technique, the quantitative transmission electron microscopy study presented herein clarifies that the hierarchical evolution of polar oxygen octahedra associated with the unit-cell volume change and polarization rotation accounts for the stepwise antiferroelectric-to-ferroelectric phase transition. In particular, an unconventional ferroelectric category-the ferrodistortive phase characteristic of a unique cycloidal polarization order-is established during the dynamic structure investigation. Through clarifying the atomic-scale phase transformation pathway, findings of this work unveil a new territory to explore novel ferrodistortive phases in energy-storage materials with the nonpolar-to-polar phase transitions.

Ab initio study for the IR spectroscopy of PbTiO3 and PbZrO3, primary blocks of PbZr1-x Ti x O3

PbTiO₃ (PT) and PbZrO₃ (PZ) are the two primary blocks of the solid solution PbZr_1-x Ti _x O₃ (PZT). They can be modelled in different ways; but, in order to do comparable DFT calculations on PZT, with different values of x, one must find a unique method that can be used for both PT and PZ. In particular, we want to evaluate their vibrational properties to compare them with experimental data. Density functional theory (DFT) is used to perform structure geometry optimizations and electronic structure calculations, both on low- and high-temperature phase. Then, harmonic vibrational frequencies of their low-temperature phase are determined for transverse and longitudinal optical (TO & LO) phonons. Moreover, a detailed study of the eigenvectors shows that accurate calculations are necessary to correctly interpret and understand the IR spectra. In the end, the comparison of our theoretical results with previous experimental and theoretical data confirm the strong potential of the SOGGA (second-order generalized gradient approximation) functional to correctly describe PT, PZ and, hopefully, PZT; especially their structural and vibrational properties.

Ferroelectric triggering of carbon monoxide adsorption on lead zirco-titanate (001) surfaces

Atomically clean lead zirco-titanate PbZr_0.2Ti_0.8O₃ (001) layers exhibit a polarization oriented inwards P⁽⁻⁾, visible by a band bending of all core levels towards lower binding energies, whereas as introduced layers exhibit P⁽⁺⁾ polarization under air or in ultrahigh vacuum. The magnitude of the inwards polarization decreases when the temperature is increased at 700 K. CO adsorption on P⁽⁻⁾ polarized surfaces saturates at about one quarter of a monolayer of carbon, and occurs in both molecular (oxidized) and dissociated (reduced) states of carbon, with a large majority of reduced state. The sticking of CO on the surface in ultrahigh vacuum is found to be directly related to the P⁽⁻⁾ polarization state of the surface. A simple electrostatic mechanism is proposed to explain these dissociation processes and the sticking of carbon on P⁽⁻⁾ polarized areas. Carbon desorbs also when the surface is irradiated with soft X-rays. Carbon desorption when the polarization is lost proceeds most probably in form of CO₂. Upon carbon desorption cycles, the ferroelectric surface is depleted in oxygen and at some point reverses its polarization, owing to electrons provided by oxygen vacancies which are able to screen the depolarization field produced by positive fixed charges at the surface.

Local ordering in lead-based relaxor ferroelectrics

Lead-based ferroelectric materials are both well-studied and widely used and have a wide range of applications from ultrasonics to energy harvesting and beyond. However, the use of Pb-containing materials is environmentally undesirable, due to the toxicity of lead. This is particularly highlighted by the disposal of Pb-based devices when their lifespan is through. Because of this large drawback, chemists have been searching for Pb-free ferroic materials that can replace PZN (PbZn1/3Nb2/3O3), PMN (PbMg1/3Nb2/3O3), PZT (PbZr1-xTixO3), and all their derivatives. Underlying much of materials chemistry is the idea that function arises from structure, so if we can determine the structure of a material, we can understand how its useful properties arise. This understanding can then lead to the tuning of these properties and the development of new materials. However, the question arises: What is meant by structure? Conventionally, structure is determined by X-ray or neutron diffraction, in which the Bragg peak intensities are measured and a unit cell is determined. In many materials, local ordering, order that persists only for few unit cells or nanometers, is important in determining the physical properties. This is very much the case in the relaxor ferroelectrics, an important class of functional oxides. Indeed, disorder, randomness, and short-range order (SRO) are all invoked to help explain many of the key properties. The local order in Pb-based ferroelectrics has been extensively studied, with the most definitive probe being single-crystal diffuse scattering. In this Account, I outline the current debate on the nature of the local order and explore how this information can inform the search for lead-free materials. Local order, as distinct from the overall average order revealed by conventional techniques, relates more closely to the crystal chemistry of the individual ions and so appears to give a better insight into how the crystal chemistry leads to the ferroelectric properties.

First-principles model potentials for lattice-dynamical studies: general methodology and example of application to ferroic perovskite oxides

We present a scheme to construct model potentials, with parameters computed from first principles, for large-scale lattice-dynamical simulations of materials. We mimic the traditional solid-state approach to the investigation of vibrational spectra, i.e., we start from a suitably chosen reference configuration of the compound and describe its energy as a function of arbitrary atomic distortions by means of a Taylor series. Such a form of the potential-energy surface is general, trivial to formulate for any material, and physically transparent. Further, such models involve clear-cut approximations, their precision can be improved in a systematic fashion, and their simplicity allows for convenient and practical strategies to compute/fit the potential parameters. We illustrate our scheme with two challenging cases in which the model potential is strongly anharmonic, namely, the ferroic perovskite oxides PbTiO3 and SrTiO3. Studying these compounds allows us to better describe the connection between the so-called effective-Hamiltonian method and ours (which may be seen as an extension of the former), and to show the physical insight and predictive power provided by our approach-e.g., we present new results regarding the factors controlling phase-transition temperatures, novel phase transitions under elastic constraints, an improved treatment of thermal expansion, etc.

Gas-phase ion chemistry of Ti(O-i-Pr)4

The positive and negative gas-phase ion chemistry of Ti(O-i-Pr)4 was investigated at low pressures by FT-ICR. The fragment ion, (i-PrO)3Ti-O+=C(H)Me, reacts with the parent neutral by proton transfer and by a nucleophilic addition–elimination reaction. The nature of the fragment ion and the ensuing ion–molecule reactions clearly indicate that Ti(O-i-Pr)4 exists as a monomer in the gas phase. In the negative ion mode, F– was found to react easily with Ti(O-i-Pr)4 to yield the pentacoordinated complex FTi(O-i-Pr)4– ion. This hypervalent Ti species undergoes a series of sequential fragmentations induced by IR multiphoton excitation. The first step is unusual because two channels are observed by IRMPD: one involves loss of HF, and the other loss of i-PrOH. The subsequent dissociation processes are characterized by progressive elimination of propene giving rise to a number of different titanaoxirane-containing anions with the general formula [(η2-CMe2O)Ti(OH)3–n(i-PrO)n]–. FTi(O-i-Pr)4– was also observed to undergo multiple alkoxide–fluoride exchanges with BF3 leading to the eventual formation of TiF5–.Key words: titanium tetraisoproxide, gas-phase ion chemistry, hypervalent Ti, ion–molecule reactions, IRMPD.

Unusual Physical and Chemical Properties of Cu in Ce1-xCuxO2 Oxides

The structural and electronic properties of Ce(1-x)Cu(x)O(2) nano systems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Cu atoms embedded in ceria had an oxidation state higher than those of the cations in Cu(2)O or CuO. The lattice of the Ce(1)(-x)Cu(x)O(2) systems still adopted a fluorite-type structure, but it was highly distorted with multiple cation-oxygen distances with respect to the single cation-oxygen bond distance seen in pure ceria. The doping of CeO(2) with copper introduced a large strain into the oxide lattice and favored the formation of O vacancies, leading to a Ce(1-x)Cu(x)O(2-y) stoichiometry for our materials. Cu approached the planar geometry characteristic of Cu(II) oxides, but with a strongly perturbed local order. The chemical activities of the Ce(1-x)Cu(x)O(2) nanoparticles were tested using the reactions with H(2) and O(2) as probes. During the reduction in hydrogen, an induction time was observed and became shorter after raising the reaction temperature. The fraction of copper that could be reduced in the Ce(1-x)Cu(x)O(2) oxides also depended strongly on the reaction temperature. A comparison with data for the reduction of pure copper oxides indicated that the copper embedded in ceria was much more difficult to reduce. The reduction of the Ce(1-x)Cu(x)O(2) nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu(2)O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials.

The structural and electronic properties of nanostructured Ce1−x−yZrxTbyO2 ternary oxides: Unusual concentration of Tb3+ and metal↔oxygen↔metal interactions

Ceria-based ternary oxides are widely used in many areas of chemistry, physics, and materials science. Synchrotron-based time-resolved x-ray diffraction, x-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy, and density-functional calculations were used to study the structural and electronic properties of Ce-Zr-Tb oxide nanoparticles. The nanoparticles were synthesized following a novel microemulsion method and had sizes in the range of 4-7 nm. The Ce1-x-yZrxTbyO2 ternary systems exhibit a complex behavior that cannot be predicted as a simple extrapolation of the properties of Ce1-xZrxO2, Ce1-xTbxO2, or the individual oxides (CeO2, ZrO2, and TbO2). The doping of ceria with Zr and Tb induces a decrease in the unit cell, but there are large positive deviations with respect to the cell parameters predicted by Vegard's rule for ideal solid solutions. The presence of Zr and Tb generates strain in the ceria lattice through the creation of crystal imperfections and O vacancies. The O K-edge and Tb LIII-edge XANES spectra for the Ce1-x-yZrxTbyO2 nanoparticles point to the existence of distinctive electronic properties. In Ce1-x-yZrxTbyO2 there is an unexpected high concentration of Tb3+, which is not seen in TbO2 or Ce1-xTbxO2 and enhances the chemical reactivity of the ternary oxide. Tb<-->O<-->Zr interactions produce a stabilization of the Tb(4f,5d) states that is responsible for the high concentration of Tb(3+) cations. The behavior of Ce1-x-yZrxTbyO2 illustrates how important can be metal<-->oxygen<-->metal interactions for determining the structural, electronic, and chemical properties of a ternary oxide.

Reduction of CuO and Cu2O with H2: H embedding and kinetic effects in the formation of suboxides

Time-resolved X-ray diffraction, X-ray absorption fine structure, and first-principles density functional calculations were used to investigate the reaction of CuO and Cu(2)O with H(2) in detail. The mechanism for the reduction of CuO is complex, involving an induction period and the embedding of H into the bulk of the oxide. The in-situ experiments show that, under a normal supply of hydrogen, CuO reduces directly to metallic Cu without formation of an intermediate or suboxide (i.e., no Cu(4)O(3) or Cu(2)O). The reduction of CuO is easier than the reduction of Cu(2)O. The apparent activation energy for the reduction of CuO is about 14.5 kcal/mol, while the value is 27.4 kcal/mol for Cu(2)O. During the reduction of CuO, the system can reach metastable states (MS) and react with hydrogen instead of forming Cu(2)O. To see the formation of Cu(2)O, one has to limit the flow of hydrogen, slowing the rate of reduction to allow a MS --> Cu(2)O transformation. These results show the importance of kinetic effects for the formation of well-defined suboxides during a reduction process and the activation of oxide catalysts.