Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

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Direct observation of the formation of polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ using neutron pair distribution function analysis

Relaxor property plays a key role in determining energy storage performance of dielectric capacitor. The usual method to improve relaxor property is to form substitutional solid solution. Here an unusually substitutional and interstitial hybrid perovskite solid solution 0.8(Ba0.85Ca0.15Zr0.1Ti0.9O3)‐0.2ZnO (BCZT:Zn) is designed, where one Zn2+ occupies B‐site and two Zn2+ share one A‐site of perovskite structure, and fabricated its epitaxial thin films. Compared to BCZT films, BCZT:Zn films have increased c/a ratio which leads to enhanced maximum polarization Pmax, wide band gap therefore high electrical breakdown strength EBDS, and enhanced relaxor degree thus suppressed remanent polarization Pr. As the results, dramatically improve energy storage property with the recoverable energy storage density Wrec of 75.9 J cm−3 and the energy storage efficiency η of 76.7% are obtained in BCZT:Zn films. This work not only provides a high performance relaxor ferroelectric material, but also impacts some new thinking to optimize die/ferro/piezoelectric properties by forming hybrid solid solutions.

High Energy Storage Performance in Ba0.85Ca0.15Zr0.1Ti0.9O3‐ZnO Hybrid Perovskite Solid Solution Thin Films

Synthetic macrocycles have worked as the principal tools for supramolecular chemistry, and also greatly extended the scope of organic charge transfer (CT) complexes as well as their practical value in solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycles-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycles-based crystalline CT assemblies, MCCAs for short), which are classified by their different types of donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts and host-guest D-A pairs. Particular attention will be focused on their diversiform functions and applications, as well as underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions.

Organic molecules with an active dipole moment have a natural propensity to align in an antiparallel fashion in the solid state, resulting in zero macroscopic polarization. This primary limitation makes the material unresponsive to switching with electric fields, mechanical forces, and to intense laser light. A single-component organic material that bestows macroscopic dipole-driven electro-mechanical and optical functions, e.g., piezoelectric, ferroelectric and nonlinear optical (NLO) activity, is unprecedented due to the design challenges imparted by crystal symmetry and dipole orientations. Herein we report a crystalline organic material that self-assembles with a polar order (P1), and is endowed with a high piezoelectric coefficient (d33–47 pm V−1), as well as ferroelectric and Debye-type relaxor properties. In addition, it shows second harmonic generation (SHG) activity, which is more than five times that of the benchmark potassium dihydrogen phosphate. Piezoelectric force microscopy (PFM) images validated electro-mechanical deformations. Piezoresponse force spectroscopy (PFS) studies confirmed a signature butterfly-like amplitude and a phase loop. To the best of our knowledge, this is the first report of a folded supramolecular π-system that manifests unidirectionally oriented dipoles and exhibits piezoelectricity, ferroelectricity, and has excellent ability to generate second harmonic light. These findings can herald new design possibilities based on folded architectures to explore opto-, electro- and mechano-responsive multifaceted functions.

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A folded π-system with supramolecularly oriented dipoles: single-component piezoelectric relaxor with NLO activity

Ferroelectricity in a hydrogen-bonded alternating donor-acceptor supramolecular copolymer.

This communication reveals co-assembly of an electron-deficient naphthalene-diimide (NDI)-appended polyurethane (P1) and electron-rich pyrene (Py), forming an organogel with prominent room-temperature ferroelectricity. In a non-polar medium, intra-chain hydrogen-bonding among the urethane groups of P1 produces a folded structure with an array of the NDIs in the periphery, which intercalate Py by charge-transfer (CT)-interaction. Such CT-complexation enables slow crystallization of the peripheral hydrocarbons, causing gelation with nanotubular morphology, in which the wall consists of the alternating NDI-Py stack. Such D-A assembly exhibits ferroelectricity (saturation polarization P s ~ 0.8 µC/cm 2 and coercive field E c ~ 8 kV/cm at 500V and 10Hz frequency) with Curie temperature ( T c ) of ~ 350 K, which can be related to the disassembly of the CT-complex. In the absence of Py, P1 forms spherical aggregates, showing dielectric behaviour.

A Hierarchical (Macro)molecular Assembly Assisted by Donor-Acceptor Charge-Transfer Interactions Exhibiting Room-Temperature Ferroelectricity.

Dielectric relaxation in ferroelectric perovskites can result from different inherent chemical and charge disorders within their crystal structures. Despite several theories on dielectric relaxations, the relationship between the concentration of oxygen vacancy (${V}_{\mathrm{O}}$) and dielectric relaxation has not been studied in perovskite oxide thin films. In this work, we report a systematic investigation on the influence of the ${V}_{\mathrm{O}}^{\ensuremath{'}}\mathrm{s}$ on the dielectric relaxation of ${\mathrm{Ba}}_{0.85}{\mathrm{Ca}}_{0.15}{\mathrm{Ti}}_{0.9}{\mathrm{Zr}}_{0.1}{\mathrm{O}}_{3}$ (BCZT) epitaxial thin films grown with ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ (LSMO) top and bottom electrodes on single-crystal ${\mathrm{SrTiO}}_{3}$ (100) substrates using pulsed laser deposition. The ambient oxygen pressures during film growth were systematically varied to control the concentration of ${V}_{\mathrm{O}}$ in the epitaxial BCZT thin films. Low ambient oxygen pressure was found to enhance the tetragonality of the BCZT films and a systematic decrease in the tetragonality was observed at high oxygen pressure conditions as evidenced from x-ray diffraction (XRD) studies. Temperature dependent XRD analyses indicated a low-temperature anomaly near $\ensuremath{\sim}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the BCZT/LSMO heterostructures. Low-temperature dielectric measurements revealed relaxor dielectric response with broad frequency dependence of the dielectric constant. It is proposed that the low-temperature dielectric relaxation is possibly caused by dipolar clusters that are formed by the aggregation of correlated ${V}_{\mathrm{O}}$-induced dipolar defects within the films deposited under different ambient oxygen pressures. High-resolution transmission electron microscopy suggested the presence of the nanometer-sized ${V}_{\mathrm{O}}$ clusters as represented by the structural defects of average diameters of $\ensuremath{\sim}5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ in the BCZT films grown at variable ambient oxygen pressure. Activation energies of the aforesaid dipolar clusters, obtained from the Vogel-Fulcher fitting of the dielectric dispersion, showed a gradual increase in magnitude, as well as decreasing freezing temperatures, with decreasing concentrations of ${V}_{\mathrm{O}}$ in the BCZT films. From the theoretical analysis of the experimental data, it is postulated that the presence of the ${V}_{\mathrm{O}}$-induced dipolar defects affects the low-temperature dielectric relaxation in the BCZT thin films. This work significantly extends the understanding of ${V}_{\mathrm{O}}$-mediated dielectric relaxor behavior in complex perovskite oxide films.

Shubhankar Barman

Papers

A Hierarchical (Macro)molecular Assembly Assisted by Donor-Acceptor Charge-Transfer Interactions Exhibiting Room-Temperature Ferroelectricity.

Role of oxygen vacancies on the low-temperature dielectric relaxor behavior in epitaxial Ba 0.85 Ca 0.15 Ti 0.9 Zr 0.1 O 3 thin films

Ferroelectricity in a hydrogen-bonded alternating donor-acceptor supramolecular copolymer.

Strain-Induced Enhanced Relaxor-Ferroelectricity and Large Electrocaloric Effect in Epitaxial Ba0.85ca0.15ti0.9zr0.1o3 Thin Film Heterostructures

Emerging ferroelectricity and piezoelectric energy harvesting properties in lead-free zinc titanate nanocrystals