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AgNbO3 is one of the prominent lead-free antiferroelectric (AFE) oxides, which readily exhibits a field-induced AFE to ferroelectric phase transition and thus a high energy storage density. The solid-statesynthesis of AgNbO3 is considered difficult and an oxidizing atmosphere is typically employed during AgNbO3 processing,on the premise that oxygen can prevent possible decomposition of the silver oxide at high temperatures. However, details about the influence of processing parameters on the functional properties of AFE AgNbO3 are insufficiently understood. In this work, the solid-state reaction of a stoichiometric AgO and Nb2O5 mixture was investigated. We found that ball milling can convert AgO into metallic Ag, which is beneficial for lowering the reaction temperature for the formation of the perovskite phase to 500‒600℃. Moreover, the influence of the processing atmosphere (air, O2, and N2) was investigated by thermal analysis and in situ X-ray diffraction. Since the reaction between Ag and Nb2O5 to form AgNbO3 requires oxygen uptake, AgNbO3 was only found to form in air and O2, whereby the kinetics were faster in the latter case. All the sintered AgNbO3 samples exhibited a similar crystallographic structure, although the samples processed in O2 had a lower oxygen vacancy concentration. Despite this, well-defined AFE double polarization loops were obtained in all cases. Our results indicate that decomposition of sliver oxide during ball milling is beneficial for the solid-state reaction, while a pure O2 atmosphere is not essential for the synthesis of high-quality AgNbO3. These findings may simplify the processing and facilitate further research of AgNbO3-based antiferroelectrics.

The original version of this Article contained an error where Figures 3 and 4 were interchanged. The original Figures 3 and 4 and accompanying legends appear below. The original Article has been corrected. (Figure presented.). CA Gökce

We present a detailed study of solid state dewetting choosing epitaxial bismuth films on silicon as a model system. Exploiting both diffraction and imaging methods, we determine atomistic parameters like unit cell coverage, lattice spacings and gradients thereof through the analysis of x-ray diffraction crystal truncation rods. A different dewetting behavior of samples with and without strain gradient is revealed. Additionally, we discuss possible reasons for the impeding influence of a strain gradient, such as a reduced interface energy or increased step edge diffusion energy barrier, which we quantify by using a Johnson-Mehl-Avrami-Kolmogorov model. A model for the step edge self diffusion of Bismuth is presented.

Chemical ordering in NiMn-based Heusler alloys with magnetostructural phase transition is crucial for understanding the physics of the phase transition. In the new field of all-d-metal Ni(Co)MnTi Heusler alloys, the experimental determination of chemical order is challenging due to the low difference in scattering power of the different elements. Here, we report a combined approach of neutron and x-ray diffraction for an analysis of chemical order in Ni(Co)MnTi alloys and show that no Heusler-typical L 2 1 order between Ti and Mn is present. Furthermore, Co and Ni atoms do not exhibit order among them; however, the phase transition of Co containing samples can be shifted significantly by changing the degree of B 2 order with proper heat treatment. Using first-principles calculations, we reveal how the structural and magnetic sub-systems depend on the degree of B 2 disorder.

Competition between antiferromagnetic (AFM), paramagnetic (PM), and ferromagnetic (FM) states in ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ compound is investigated by means of temperature- and magnetic field-dependent x-ray diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. It is shown that both AFM and PM phases get converted into the FM phase via a first-order metamagnetic transition, which is accompanied by a huge forced-volume magnetostriction $\mathrm{\ensuremath{\Delta}}V/V(25\phantom{\rule{0.16em}{0ex}}\mathrm{K},\phantom{\rule{0.16em}{0ex}}6\phantom{\rule{0.16em}{0ex}}\mathrm{T})=1.15%$. X-ray powder diffraction reveals a magnetic field-induced crystallographic phase transition from a $R\overline{3}m$ rhombohedral (AFM, PM) to a $C2/m$ monoclinic (FM) structure. A peculiarly anisotropic lattice expansion as well as giant negative thermal expansion with a volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}193\ifmmode\times\else\texttimes\fi{}{10}^{--6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{--1}$ are observed. These findings point to the significance of magnetoelastic effects in this metamagnet and illustrate the strength of the coupling between lattice and spin degrees of freedom in the ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ intermetallic compound.

Magnetoelastic coupling, structural, magnetic, electronic transport, and magnetotransport properties of ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ have been studied by a combination of macroscopic [magnetization, electrical resistivity, and magnetoresistance (MR)] and microscopic temperature- and magnetic-field-dependent x-ray powder diffraction measurements. The itinerant-electron system ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_{6}$ exhibits an antiferromagnetic (AFM) ground state and multiple magnetic transitions, AFM-ferromagnetic (FM) and FM-paramagnetic (PM), triggered by changes in both temperature and magnetic fields. At low temperatures, the field-induced first-order AFM-FM metamagnetic phase transition is discontinuous, manifesting itself by extremely sharp steps in magnetization as well as in MR and is accompanied by large magnetic hysteresis. A remarkably large negative MR of \ensuremath{-}73% was discovered. In addition, the time evolution of the electrical resistivity displays a colossal spontaneous jump when both the applied magnetic field and temperature are constant. Diffraction data reveal a magnetic-field-induced structural phase transition associated with the AFM-FM and PM-FM transformations. The lattice distortion is driven by magnetoelastic coupling and converts the crystal structure from rhombohedral ($R\overline{3}m$) to monoclinic $(C2/m)$. The AFM and PM states are related to the rhombohedral structure, whereas the FM order develops in the monoclinic symmetry. A huge volume magnetostriction of \ensuremath{\sim}0.9% accompanies this symmetry-lowering lattice distortion. Meanwhile, a highly anisotropic thermal expansion involving giant negative thermal expansion with an average volumetric thermal expansion coefficient ${\ensuremath{\alpha}}_{V}=\ensuremath{-}195\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ was observed. The consistency seen in these different experimental data constitutes direct evidence of the strong correlations between charge, magnetic, and crystallographic degrees of freedom in this material.

The development of magnetocaloric materials represents an approach to enable efficient and environmentally friendly refrigeration. It is envisioned as a key technology to reduce CO2 emissions of air conditioning and cooling systems. Fe-Rh has been shown to be one of the best-suited materials in terms of heat exchange per material volume. However, the Fe-Rh magnetocaloric response depends on its composition. Hence, the adaptation of material processing routes that preserve the Fe-Rh magnetocaloric response in the generated structures is a fundamental step towards the industrial development of this cooling technology. To address this challenge, the temperature-dependent properties of laser synthesized Fe-Rh nanoparticles and the laser printing of Fe-Rh nanoparticle inks are studied to generate 2D magnetocaloric structures that are potentially interesting for applications such as waste heat management of compact electrical appliances or thermal diodes, switches, and printable magnetocaloric media. The magnetization and temperature dependence of the ink’s γ-FeRh to B2-FeRh magnetic transition is analyzed throughout the complete process, finding a linear increase of the magnetization M (0.8 T, 300 K) up to 96 Am2/kg with ca. 90% of the γ-FeRh being transformed permanently into the B2-phase. In 2D structures, magnetization values of M (0.8 T, 300 K) ≈ 11 Am2/kg could be reached by laser sintering, yielding partial conversion to the B2-phase equivalent to long-time heating temperature of app. 600 K, via this treatment. Thus, the proposed procedure constitutes a robust route to achieve the generation of magnetocaloric structures.

Ferromagnetic Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were synthesized using the reactive flux technique. Derived from single crystal x-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals - the threefold twinning of an orthorhombic unit cell with (3 + 1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and a modified Sucksmith-Thompson technique. Ms and K1 were also evaluated in the framework of Density Functional Theory (DFT) and an overall good agreement was observed between the calculated and experimental results. Furthermore, a critical evaluation of different analytical models for the assessment of magnetocrystalline anisotropy was performed, which are restricted to the analysis of uniaxial magnetic domain patterns, and it is shown that such high-throughput techniques can lead to unrealistic results. Finally, a DFT high-throughput screening of the Fe-Sn phase diagram was used to identify Fe-Sn based phases with potential to be stabilized upon alloying, and their magnetization and magnetocrystalline anisotropy were evaluated. The results show that a similar strong anisotropy as observed in Fe3Sn may also be found in other Fe-Sn based phases, having higher potential to be used as hard magnetic material.

Materials with strongly coupled magnetic and structural transitions can display a giant magnetocaloric effect, which is of interest in the design of energy-efficient and environmentally friendly refrigerators, heat pumps, and thermomagnetic generators. There also exist, however, a class of materials with no known magnetostructural transition that nevertheless show remarkable magnetocaloric effects. MnB has been recently suggested as such a compound, displaying a large magnetocaloric effect at its Curie temperature (570 K) showing promise in recovering low-grade waste heat using thermomagnetic generation. In contrast, we show that isostructural FeB displays very similar magnetic ordering characteristics, but is not an effective magnetocaloric. Temperature- and field-dependent diffraction studies reveal dramatic magnetoelastic coupling in MnB, which exists without a magnetostructural transition. No such behavior is seen in FeB. Furthermore, the magnetic transition in MnB is shown to be subtly first-order, a...

We present a study of the impeding influence of a capping, native oxide layer on the solid state dewetting of thin bismuth films on silicon(111) in vacuum. We study the temperature dependence of the film thickness and strain of the thin films through the analysis of crystal truncation rods of clean and capped bismuth films. This analysis reveals a dewetting temperature difference of 40 ° C between capped and uncapped films. The results are supported by scanning electron microscopy and x-ray photoelectron spectroscopy experiments. Furthermore, a model for the retarding effect of the oxide layer and the final shape of the thin film is presented.

The aim of the acousto-elastic theory was to measure ultrasonic velocity changes which characterize the mechanical nonlinearity of a prestressed material. In this context, our purpose is to tabulate the invariant third-order elastic coefficients including the piezoelectric, electrostrictive, and dielectric corrections. The investigation is limited to trigonal and hexagonal crystalline structures, which represent the most often encountered symmetry classes for the piezoelectric materials. In fact, the enumeration includes the high-order tensors involved in the analysis of nonlinear behaviors associated with various electromechanical coupling forms. The obtained results are extensions to previous calculations in this area which bring some corrections to certain published combinations related to the invariance rules. The numerical procedure built using the software MATLAB is based on coordinate system transformations performed on the eigenbasis of their corresponding symmetry axes three- and sixfold. In this purpose, we found some contradictions between our results and a former paper published in Journal of Applied Physics. To the authors’ knowledge, rechecking of the relationships between the invariant third-order constants and comparison with this last reference has not been discussed yet. The relationships between the invariant third-order coefficients presented in this work provide a number of attractive properties for use in mechanical and physical applications.

The influence of germanium on microstructure, crystallography, and thermal stability of the eutectic Cr-Cr3Si alloy was investigated. Fine eutectic microstructures could be preserved but slightly degenerated when up to 2 at.% Si was substituted by Ge. A transition from eutectic to peritectic microstructure composed of primary dendrites and A15 Cr3Si matrix was observed when higher amounts of Ge were added. X-ray diffraction results showed no limit for substitution of Si and Ge in the studied composition range. The lattice parameter of both Cr solid-solution and A15 silicide increased with increasing the Ge/Si ratio. A macro was developed for the ImageJ program to automate the quantitative microstructural image analysis. Results showed that Ge addition increases the A15 phase fraction of the two-phase alloys. Microstructural investigations on annealed alloys at 1350 °C demonstrated that the coarsening of both binary Cr-Cr3Si and ternary Cr-Cr3(Si, Ge) is interface-controlled. Although Ge addition to the eutectic alloy offered an isotropic microstructure, it decreased the thermal stability by degeneration of the lamellar structure.

Pt-decorated $$\hbox {TiO}_{2}$$ nanotubes Pt@TiO2 are prepared only by applying a set of facile wet-chemical redox reactions to ion track-etched polycarbonate templates. First, a homogeneous layer of Pt nanoparticles is deposited onto the complex template surface by reducing potassium tetrachloroplatinate with absorbed dimethylaminoborane. Second, the template is coated with a conformal $$\hbox {TiO}_{2}$$ layer, using a chemical bath deposition reaction based on titanium(III) chloride. After the removal of the template, the rutile-type $$\hbox {TiO}_{2}$$ nanotubes remain decorated with Pt nanoparticles and nanoparticle-clusters on their outside. During the process, neither vacuum techniques nor external current sources or addition of heat are employed. The crystallinity, composition, and morphology of the composite nanotubes are analysed by X-ray diffraction, scanning and transmission electron microscopy as well as by energy-dispersive X-ray spectroscopy. Finally, the obtained materials are examplarily applied in the electrooxidation of ethanol and formic acid, and their performances have been evaluated. Compared to conventional carbon black-supported Pt nanoparticles, the Pt@TiO2 nanotubes show higher reaction rates. Mass activities of 2.36 $$\hbox {A}\hbox { mg}_{\rm Pt}^{-1}\hbox { cm}^{-2}$$ are reached in ethanol oxidation and 7.56 $$\hbox {A}\hbox { mg}_{\rm Pt}^{-1}\hbox { cm}^{-2}$$ in the formic acid oxidation. The present structures are able to exploit the synergy of Pt and $$\hbox {TiO}_{2}$$ with a bifunctional mechanism to result in powerful but easy-to-fabricate catalyst structures. They represent an easily producible type of composite nanostructures which can be applied in various fields such as in catalytics and sensor technology.

This study evaluates the change of flow stress as related to dislocation density in SrTiO3 single crystals in order to provide guidance for later electrical studies. The key parameters varied are temperature and loading rate during the deformation. It is found that in -oriented SrTiO3 single crystals, the dislocation density is enhanced by plastic deformation, more so at higher temperature as compared to room temperature. The experimental approach of quantifying the dislocation density through a determination of ex situ X-ray diffraction rocking curves was successfully applied over the upper temperatures region of the lower temperature ductility zone for strontium titanate, i.e., in the so-called “A-regime”. For 1.0% deformed samples deformed at 300°C, a fourfold increase in dislocation density to 1.4 3 1013 m – 1 was found as compared to the nondeformed state (3.7 3 1012 m – 1). Cross-section techniques confirmed that the observed dislocation densities measured at the surfaces were identical to those seen in the core of the crystals. The use of rapid changes in loading rate provided an estimate for activation volume of the dislocation core for both 25°C and 300°C.

The design of core–shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core–shell materials has focused on dielectrics, piezoelectric core–shell ceramics may gain similar interest. Generalities of core–shell functional ceramics features are addressed in this work. A model system, Bi1/2Na1/2TiO3–SrTiO3, is introduced to discuss structure–property relationships. We demonstrate that this system features a core–shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core–shell with field and temperature determines its functional properties. The high strain of the system, ~0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell.

The local dynamics of the lead-free relaxor $0.964{\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}\ensuremath{-}0.036{\mathrm{BaTiO}}_{3}$ (NBT-3.6BT) have been investigated by a combination of quasielastic neutron-scattering (QENS) and ab initio molecular dynamics simulations. In a previous paper, we were able to show that the tetragonal platelets in the microstructure are crucial for understanding the dielectric properties of NBT-3.6BT [Pforr et al., Phys. Rev. B 94, 014105 (2016)]. To investigate their dynamics, ab initio molecular dynamics simulations were carried out using ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}$ with 001 cation order as a simple model system for the tetragonal platelets in NBT-3.6BT. Similarly, 111-ordered ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}$ was used as a model for the rhombohedral matrix. The measured single-crystal QENS spectra could be reproduced by a linear combination of calculated spectra. We find that the relaxational dynamics of NBT-3.6BT are concentrated in the tetragonal platelets. Chaotic stages, during which the local tilt order changes incessantly on the time scale of several picoseconds, cause the most significant contribution to the quasielastic intensity. They can be regarded as an excited state of tetragonal platelets, whose relaxation back into a quasistable state might explain the frequency dependence of the dielectric properties of NBT-3.6BT in the 100 GHz to THz range. This substantiates the assumption that the relaxor properties of NBT-3.6BT originate from the tetragonal platelets.

Spin fluctuations are a crucial driving force for magnetic phase transitions, but their presence usually is indirectly deduced from macroscopic variables like volume, magnetization or electrical resistivity. Here we report on the direct observation of spin fluctuations in the paramagnetic regime of the magnetocaloric model system LaFe11.6Si1.4 in the form of neutron diffuse scattering. To confirm the magnetic origin of the diffuse scattering, we correlate the temperature dependence of the diffuse intensity with ac magnetic susceptibility and x-ray diffraction experiments under magnetic field. Strong spin fluctuations are already observable at 295 K and their presence alters the thermal contraction behavior of LaFe11.6Si1.4 down to the Curie temperature of the first-order magneto-structural transition at 190 K. We explain the influence of the spin fluctuation amplitude on the lattice parameter in the framework of the internal magnetic pressure model and find that the critical forced magnetostriction follows Takashi’s spin fluctuation theory for itinerant electron systems.

We study the temperature-dependent evolution of the octahedral tilt order in a lead-free relaxor ferroelectric and its impact on the ferroelectric properties. Using diffuse neutron scattering on a $0.964{\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}0.036{\mathrm{BaTiO}}_{3}$ single crystal, we suggest a model for the temperature-dependent nanostructure of this perovskite that features chemically pinned tetragonal platelets embedded in the rhombohedral matrix, often separated by a cubic intermediate phase. Our results show a clear correlation between the squared thickness of the tetragonal platelets and the dielectric permittivity. This is interpreted as a sign for increased polarizability of the strained and distorted lattice at the center of the tetragonal platelets.

Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, ⟨h00⟩, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For ⟨hhh⟩ oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed “rotopolarization”. These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0

High-energy synchrotron x-ray scattering offers a powerful technique for investigation of single-crystal material structures. Large, mm-sized crystals can be used, allowing complex in situ sample environments to be employed. Here, we demonstrate how this technique can be applied for the collection of single-crystal diffuse scattering volumes from the electro-active material 96%Bi0.5Na0.5TiO3-4%BaTiO3 while electric fields are applied in situ. The data obtained allow correlation of the atomic and nanoscale structures with the observed macroscopic electro-active properties of interest. This article presents a recent study relating the nanoscale stacking fault structure in BNT-BT to the relaxor-ferroelectric nature of the material [Daniels et al. in Appl. Phys. Lett. 98, 252904 (2011)], and extends this study with further experimental description and analysis.

The order-disorder transition in a 170 A thick Cu3Au(111) epitaxial alloy film is examined using x-ray diffraction techniques. Below the transition temperature T0, a disordered surface layer of thickness l is detected. This layer exhibits wetting behavior, i.e., l increases, obeying a logarithmic growth law on approach to T0. We show by a detailed analysis of the order parameter profiles that the stiffness of the order-disorder interface is temperature dependent, resulting in a nonuniversality of the wetting transition.

Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating at an argon gas pressure of 8 × 10 −4 mbar. The content of cubic boron nitride (c-BN) is very high, as determined by infrared absorption spectroscopy (IR, 84% c-BN), factor analysis of Auger electron spectroscopy (AES, approximately 100% sp 3 ) and X-ray reflectivity (approximately 100% sp 3 ). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM). The film-forming particles (fluxΦ n ) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (fluxΦ i ) are argon ions. The current density is about 2.25 mA cm −2 , as determined from energy and mass analysis. The energy dependence due to r.f. substrate bias shows a maximum in the c-BN content at 137 eV at an arrival ratioΦ i /Φ n = 13 where AFM investigations show a minimum of the area roughness of 0.2 nm. Increasing the arrival ratio (to 20 and 66), the optimal c-BN formation is shifted to lower energies (87 eV and 62 eV respectively) which is in agreement with the subplantation model. Stress reduction experiments, such as UV-assisted deposition and post-annealing, are discussed.

Despite the great potential for flexible large-area applications in organic electronics, electrical and structural instabilities have been widely regarded as an unavoidable weakness of organic semiconductors. Electronic components made of organic molecules are considered more fragile and sensitive to external conditions than their inorganic counterparts and the lifetime of them is shorter than that of conventional devices. Despite these shortcuts it is even more surprising that organic field-effect transistors (FETs) can undergo a self-healing process, i.e. their electrical characteristics can counter-intuitively improve over time, due to the unstable nature of weakly bound molecules. This remarkable phenomenon has never been understood completely. In this work, we demonstrate that the self-healing in pentacene FETs is induced by a structural phase transformation and can be triggered by various extrinsic or intrinsic influences. We evaluate in detail the necessary conditions for electrical self-healing in pentacene FETs and reveal the structure-property relation at the heart of this phenomenon: a structural phase transformation from the metastable monolayer phase to the so called “thin film phase”. The transformation basically involves a tilt of the molecules against the surface normal near both interfaces of pentacene thin films (pentacene-dielectric and metal-pentacene) and has a profound influence on the electronic properties of pentacene FETs, which we demonstrate by way of electronic structure calculations using DFT and semi-classical transport theory. Finally, possible external triggers able to initiate the self-healing process are discussed. A case study adopting various fabrication procedures reveals that external triggers such as nitrogen gas or other organic compounds seem to be necessary to initiate the self-healing process.

The preferred adsorption sites of xenon in the recently synthesized metal–organic framework MFU-4l(arge) possessing a bimodal pore structure (with pore sizes of 12 A and 18.6 A) were studied via the combination of low temperature thermal desorption spectroscopy and in situ X-ray powder diffraction. The diffraction patterns were collected at 110 K and 150 K according to the temperature of the desorption maxima. The maximum entropy method was used to reconstruct the electron density distribution of the structure and to localize the adsorbed xenon using refined data of the Xe-filled and empty sample. First principles calculations revealed that Xe atoms exclusively occupy the Wyckoff 32f position at approximately 2/3 2/3 2/3 along the body diagonal of the cubic crystal structure. At 110 K, Xe atoms occupy all 32 f positions (8 atoms per pore) while at 150 K the occupancy descends to 25% (2 atoms per pore). No Xe occupation of the small pores is observed by neither experimental measurements nor theoretical studies.

Diffuse x-ray scattering intensities from a single crystal of 0.96(Bi1/2Na1/2TiO3)–0.04(BaTiO3) have been collected at room temperature with and without application of an electric field along the [100] direction. Distinct features in the diffuse scattering intensities indicate correlations on a nanometer length scale. It is shown that locally correlated planar-like structures and octahedral tilt-domains within the room temperature rhombohedral R3c phase are both electrically active and are irreversibly affected by application of an electric field of 4.3 kV/mm. The field dependence of these nanoscale structures is correlated with the relaxor behavior of the material by macroscopic permittivity measurements.

Highly conductive PrBaCo2O5.5+δ thin films were grown on (001) SrTiO3 by pulsed laser deposition. Microstructural studies from synchrotron x-ray diffraction and transmission electron microscopy reveal that the as-grown PrBaCo2O5.5+δ films are a-axis oriented with excellent single crystallinity. Two domain structures with c axes in the plane directions parallel to the substrate surface were found with an atomic sharp interface. Transport property and isothermal magnetoresistance measurements for both as-grown and post-annealing were done to understand the physical properties of the films. An abnormal positive magnetoresistance effect was observed in the postannealed films, which suggests that the spins in the system are polarized easier.

The second harmonic (SH) generation from the highly epitaxial Al-doped ZnO film on sapphire was measured, using the femtosecond Ti:Sapphire laser at the near-resonant SH wavelength, in reflection geometry to avoid the sapphire's contribution in the conventional Maker fringes technique. By investigating SH intensities as a function of the azimuthal angle along the film's normal, we found that the sapphire substrate had a negligible contribution to the reflective SH signal and the film had a pure and well-aligned c-domain. We also developed a new method to calculate the component's ratios of the nonlinear susceptibility tensor by analyzing the polarization diagrams of SH intensities under the incidence with two different angles. The ratios indicate that Kleinman's symmetry is broken due to the absorption at SH wavelength and the dominant component of the nonlinear susceptibility tensor is d(33). Calibration using the Z-cut quartz shows a possible overestimate of the nonlinear response by Maker fringes technique.

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The oxygen exchange kinetics of thin films of the oxygen-deficient double perovskite PrBaCo2O5+δ (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The ECR and IEDP measurements reveal that the PBCO films have high electronic conductivity and rapid surface exchange kinetics, although the ECR data indicate the presence of two distinct kinetic pathways. The rapid surface kinetics compared with those of other perovskites suggest the application of PBCO as a cathode material in intermediate-temperature solid oxide fuel cells.

Structural studies done on HgSe nanoclusters fabricated in the pores of two types of zeolite frameworks, LTL (tubular pore) and Nd-Y (nearly spherical pore), are presented. Nanoclusters were synthesized in these zeolite frameworks via chemical vapor deposition. Anomalous X-ray scattering (AXS) studies were performed on both HgSe/zeolite-Y and HgSe/zeolite-L systems. As a result, we present two distinct structural models for NdY/HgSe and LTL/HgSe systems. A remarkable feature present in our X-ray diffraction (XRD) patterns, diffuse scattering, will also be discussed using our initial results of the atomic pair distribution function (PDF) studies. X-ray structural work is complemented using the results of optical and Raman studies.

The thesis aims to find possible approaches for improved electromechanical properties in KNN-based piezoelectric single crystals. Both submerged-seed and top-seeded solution growth techniques were employed to produce single crystals. Conclusions from the crystal growth aspect are: (i) For individual elements, segregation coefficients highly rely on the initial concentration in the liquid solution. (ii) A competition between elements occupied on the same lattice site was found. (iii) The very low Li segregation coefficient in the KNN matrix is responsible for the occurrence of a secondary phase with the tetragonal tungsten bronze structure. (iv) Observed optically-cloudy regions in as-grown crystals decrease the electrical response and can be reduced by thermal treatment with slow cooling. In the second part, we used three approaches to enhance the piezoelectric and ferroelectric behavior of KNN-based single crystals. Ta or Sb substitutions indicates that enhanced electromechanical response is achieved when the orthorhombic-tetragonal phase transition is near room temperature. Thermal treatment in pure O2 atmosphere resulted in a twofold increase of the piezoelectric coefficient and ferroelectric parameters of a (K,Na,Li)(Ta,Nb,Sb)O3 single crystal. The highest room-temperature piezoelectric coefficient in annealed KNN-based single crystals of 732 pC/N was obtained. The third approach, doping with Mn ions in (K,Na,Li)(Ta,Nb)O3 single crystals, is also presented.; Cette thèse vise à trouver des approches possibles pour l’amélioration des propriétés électromécaniques de monocristaux piézoélectriques à base de KNN. La TSSG et la SSSG sont entreprises afin de faire croître des monocristaux La conclusion de l'aspect de croissance cristalline est: (1) Pour chaque élément pris individuellement, leurs coefficients de ségrégation reposent fortement sur leurs concentrations initiales dans la solution liquide. (2) La compétition entre éléments occupant le même site du réseau est démontrée. (3) Le très faible coefficient de ségrégation de Li dans la matrice KNN est responsable de l'apparition d'une phase secondaire présentant la structure bronze de tungstène quadratique. (4) Les régions optiquement laiteuses observées dans les monocristaux diminuent la réponse électrique et peuvent être réduites par traitement thermique et refroidissement lent. Dans la deuxième partie, nous avons utilisé trois approches pour améliorer le comportement piézo/ferroélectrique des monocristaux à base de KNN. La Ta ou Sb substitution indique qu'une réponse électromécanique améliorée est obtenue lorsque la transition orthorhombique-quadratique est à proximité de la température ambiante. Le traitement thermique sous atmosphère d'O2 pur a conduit au doublement de la valeur du coefficient piézoélectrique et des paramètres ferroélectriques d'un monocristal de (K,Na,Li) (Ta,Nb,Sb)O3. Son coefficient piézoélectrique à la température ambiante, qui constitue un record mondial à l’heure actuelle vis-à-vis de ce qui est reporté dans la littérature internationale, vaut 732 pC/N. La troisième approche consiste au dopage des monocristaux de (K,Na,Li)(Ta,Nb)O3 avec Mn.