We report relaxation dynamics of glycerol-water mixtures as probed by megahertz-to-terahertz dielectric spectroscopy in a frequency range from 50 MHz to 0.5 THz at room temperature. The dielectric relaxation spectra reveal several polarization processes at the molecular level with different time constants and dielectric strengths, providing an understanding of the hydrogen-bonding network in glycerol-water mixtures. We have determined the structure of hydration shells around glycerol molecules and the dynamics of bound water as a function of glycerol concentration in solutions using the Debye relaxation model. The experimental results show the existence of a critical glycerol concentration of ~7.5 mol %, which is related to the number of water molecules in the hydration layer around a glycerol molecule. At higher glycerol concentrations, water molecules dispersed in a glycerol network become abundant and eventually dominate and four distinct relaxation processes emerge in the mixtures. The relaxation dynamics and hydration structure in glycerol-water mixtures are further probed with molecular dynamics simulations, which confirm the physical picture revealed by the dielectric spectroscopy.

We report the dynamics of soft phonon modes and their role towards the various structural transformations in Aurivillius materials by employing terahertz frequency-domain spectroscopy, atomic pair distribution function analysis, and first-principles calculations. We have chosen Bi4Ti3O12 as a model system and identified soft phonon modes associated with the paraelectric tetragonal to the ferroelectric monoclinic transition. Three soft phonon modes have been discovered which exhibit a strong temperature dependence. We have determined that the anharmonicity in Bi−O bonds plays a significant role in phonon softening and that Bi cations play an important role in the emergence of ferroelectricity.The knowledge of soft phonon mode properties is crucial for understanding the origin of lattice instabilities and structural phase transitions in bismuth layered ferroelectrics (Aurivilliustype structures represented as [Bi2O2] , where m = 3, A = Bi, and B=Ti, for Bi4Ti3O12). Typically, ferroelectric-paraelectric phase transitions in these materials occur with the heavily damped phonons in the terahertz (THz) frequencies [1,2]. Additionally, there could be subtle structural distortions below Curie temperature (Tc), which are often difficult to correlate with phonon dynamics. Since structural changes drive many material properties, a fundamental understanding of dynamics of these phonon modes is critical for designing high performance ferroelectric materials and devices [3]. The number of phonon modes are defined by the nature of changes in the symmetry during the transitions. The phase transitions involving more than one soft phonon modes [4] and corresponding order parameters, may induce structural transformations at temperatures below Tc. However, the condensation of more than one phonon modes at a single transition is quite unusual [5].Here, we employed Bi4Ti3O12 (BiT) as a model system to understand phonon modes related to phase transitions in Aurivillius materials. Interest in layered structures, such as Aurivillius compounds, Dion-Jacobson phases, and Ruddlesden-Popper phases, has been increasing rapidly due to their relevance for 2D materials based electronics along with high Curie temperature (Tc for BiT > 600 o C) [6]. The ferroelectric members in these families have potential for high temperature sensors and fatigue-free ferroelectric memory devices etc. [7]. Furthermore, these layered materials exhibit anisotropic and very low thermal conductivity due to effective phonon scattering [8]. The structure of BiT consists of perovskite-like block (Bi2Ti3O10) 2interleaved with fluorite like (Bi2O2) 2+ layers perpendicular to pseudo-tetragonal c-axis [9] which results in relatively higher polarization [10]. In terms of phase transformation characteristics, BiT Supplemental material

non-centrosymmetric structures which can possess a spontaneous electric polarization which can be controlled using applied electric fields (Figure 1). Ferroelectrics themselves are inherently hierarchical materials-wherein picometer ionic displacements give rise to polarization which can collectively extend over millimeters or self-organize into complex mesoscopic structures or collectively reorient under applied stimuli (e.g., electric fields, temperature, or stress). Understanding these complex behaviors necessitates a multilevel approach, wherein atomistic, microscopic, mesoscopic, and macroscopic properties are studied in concert. Parallel advances in synthesis, characterization, and simulation have enabled such multimodal studies and provided a methodology through which a multitude of ferroelectric functionalities can now be achieved and studied.The promise of utilizing these functionalities in a number of applications kick-started the modern era of ferroelectric research in the mid-20th century. [1] Looking further back, in the 1920s, the ability to switch the polarization of sodium potassium tartrate tetrahydrate (commonly known as Rochelle salt) was first observed, along with dielectric and piezoelectric anomalies near the ferroelectric transition-now called the Curie point. In the 1940s, as part of the accelerated research push associated with World War II, ferroelectric BaTiO 3 was discovered by accident. When modifying TiO 2 with BaO to enhance its dielectric properties, a record-high dielectric permittivity was discovered and subsequent studies demonstrated a hysteretic switchable polarization. This discovery ushered in a new understanding of ferroelectricity as more than a rare phenomenon associated with salts that contained hydrogen bonding, but rather as a phenomenon that could exist in simple oxides like perovskites. Over the subsequent decades, the number of ferroelectric compositions exploded, particularly within the perovskite oxides, introducing new chemistries including LiNbO 3 and the PbZr x Ti 1−x O 3 system. Meanwhile, theoretical descriptions of ferroelectricity were advanced through lattice-dynamical models invoking a soft-mode optical phonon. Studies of the piezoelectric, thermodynamic, and optical properties in ferroelectric ceramics allowed for their deployment in a number of applications including piezoelectric sensors and pyroelectric infrared detectors. By the 1960s, increased interest in using ferroelectric polarization for nonvolatile memory was driving research into Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length sc...

B site have shown high activities and represent a cost-effective alternative to expensive noble metal-based catalysts; however, gas exchange at these perovskite surfaces remains the limiting factor for high efficiency in electrochemical devices. [1][2][3] The development of perovskites with high activity has historically relied on brute-force compositional screening involving the synthesis and testing of numerous bulk-ceramic samples to establish an intuition for chemical trends that can guide researchers to highly active compositions. More recently, however, there has been a significant push to develop electronic descriptors for predicting electrochemical activity. [4][5][6][7] This work has led to the identification of several electronic descriptors that are closely correlated with high activity for ORR and OER catalysts, including the relative position of the oxygen 2p band center, [6] the BO bond covalency, [8] and, in particular, the occupancy of the e g orbitals. [5,9] For example, e g orbital occupation has been shown to be strongly correlated with the B-site-dependent chemical activity trends, and an "ideal" e g occupancy of ≈1.2 electrons has been used as the justification for high activities in LaNiO 3 and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) catalysts. [5,9,10] As such, the development of electronic descriptors has already proved fruitful in identifying compositions with high activities and, in turn, developing synthetic control of materials properties to tune the electronic structure toward these ideal values could unlock a new way to optimize materials.In parallel to the development of these electronic descriptors, researchers have leveraged thin-film-based studies to understand how manipulation of the lattice structure of these perovskites can affect the electrochemical activity. Several studies have focused on the role of epitaxial strain in perovskite oxides for controlling electrochemical activity. [11][12][13][14][15][16][17] Reports have noted enhanced electrochemical activities in cobalt-and ironbased perovskites under biaxial-tensile strain driven by straininduced changes to surface chemistry [12] or suggested electronic contributions as the driving force for enhanced electrochemical activity under tensile strain. [13,14,18] Meanwhile, strain-induced changes to e g occupancy have been proposed as the mechanism of enhanced ORR/OER activities in LaNiO 3 films under biaxial compressive strain. [16] This discrepancy suggests that straininduced changes to electronic structure may be dependent on the identity of the B-site cation and calls for more direct studies Epitaxial strain has been shown to produce dramatic changes to the orbital structure in transition metal perovskite oxides and, in turn, the rate of oxygen electrocatalysis therein. Here, epitaxial strain is used to investigate the relationship between surface electronic structure and oxygen electrocatalysis in prototypical fuel cell cathode systems. Combining high-temperature electricalconductivity-relaxation studies and synchrotron-...

Epitaxial PbHf1–x Ti x O3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1–x Ti x O3 system. End members, PbHfO3 and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3 and PbHf0.25Ti0.75O3 solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/cm2) structures, respectively. For intermediate PbHf1–x Ti x O3 compositions (e.g., x = 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R3c, monoclinic (Cm), and P4mm symmetries and, in all cases, were ferroelectric with remanent (5–22 μC/cm2) and saturation (18.5–36 μC/cm2) polarization and coercive field (24–34.5 kV/cm) values increasing with x. While the dielectric constant was the largest for PbHf0.6Ti0.4O3, the MPB is thought to be near x = 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement–voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr0.52Ti0.48O3 films in the same geometry. Thereby, the PbHf1–x Ti x O3 system is a viable alternative to the PbZr1–x Ti x O3 system offering comparable performance.

Dynamic fluctuations in hydrogen-bond network of water occur from femto- to nano-second timescale and provides insights into structural/dynamical aspects of water at ion-water interfaces. Employing terahertz spectroscopy assisted with molecular dynamics simulations, we study aqueous chloride solutions of five monovalent cations, namely, Li, Na, K, Rb and Cs. We show that ions modify the behavior of surrounding water molecules and form interfacial layers of water around them with physical properties distinct from that of bulk water. Small cations with high charge densities influence the kinetics of water well beyond the first solvation shell. At terahertz frequencies, we observe an emergence of fast relaxation processes of water with their magnitude following the ionic order Cs>Rb>K>Na>Li, revealing an enhanced population density of weakly coordinated water at ion-water interface. The results shed light on the structure breaking tendency of monovalent cations and provide insights into the properties of ionic solutions at the molecular level.

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