Thermally and Photoinduced Spin-Crossover Behavior in Iron(II)-Silver(I) Cyanido-Bridged Coordination Polymers Bearing Acetylpyridine Ligands.

We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy)[Ag(CN)]} and {Fe(3acpy)[Ag(CN)]} (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties.

Effect of a flat band on a multiband two-dimensional Lieb lattice with intra- and interband interactions.

In this study, we explore the effect of a single flat band in the electronic properties of a ferromagnetic two-dimensional Lieb lattice using the multiband Hubbard model with polarized carriers, spin-up and spin-down. We employ the self-consistent dynamical mean field theory and a Green functions cumulant expansion around the atomic limit to obtain the correlated densities of states while varying the intra- and interband interactions. Our findings demonstrate a renormalization of the correlated density of states in both the spin-up and spin-down carriers as we varied the intra- and interband interactions.

Emergence of Rashba-/Dresselhaus effects in Ruddlesden-Popper halide perovskites with octahedral rotations.

Ruddlesden-Popper halide perovskites are highly versatile quasi-two-dimensional energy materials with a wide range of tunable optoelectronic properties. Here we use the all-inorganic Csn+1PbX3n+1Ruddlesden-Popper perovskites with X = I, Br, and Cl to systematically model the effect of octahedral tilting distortions on the energy landscape, band gaps, macroscopic polarization, and the emergence of Rashba-/Dresselhaus splitting in these materials. We construct all unique = 1 and = 2 structures following from octahedral tilts and use first-principles density functional theory to calculate total energies, polarizations and band structures, backed up by band gap calculations using theapproach.

From an antiferromagnetic insulator to a strongly correlated metal in square-lattice MCl(pyrazine) coordination solids.

Electronic synergy between metal ions and organic linkers is a key to engineering molecule-based materials with a high electrical conductivity and, ultimately, metallicity. To enhance conductivity in metal-organic solids, chemists aim to bring the electrochemical potentials of the constituent metal ions and bridging organic ligands closer in a quest to obtain metal-d and ligand-π admixed frontier bands. Herein, we demonstrate the critical role of the metal ion in tuning the electronic ground state of such materials.

Antiferroelectric negative capacitance from a structural phase transition in zirconia.

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO and ZrO) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field.

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo-BiVO Thin Films.

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi-modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo-BiVO . By using scanning transmission X-ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X-ray absorption spectra corresponding to grain centers and grain boundaries.

An automatically curated first-principles database of ferroelectrics.

Ferroelectric materials have technological applications in information storage and electronic devices. The ferroelectric polar phase can be controlled with external fields, chemical substitution and size-effects in bulk and ultrathin film form, providing a platform for future technologies and for exploratory research. In this work, we integrate spin-polarized density functional theory (DFT) calculations, crystal structure databases, symmetry tools, workflow software, and a custom analysis toolkit to build a library of known, previously-proposed, and newly-proposed ferroelectric materials.

Formation of the layered conductive magnet CrCl(pyrazine) through redox-active coordination chemistry.

The unique properties of graphene, transition-metal dichalcogenides and other two-dimensional (2D) materials have boosted interest in layered coordination solids. In particular, 2D materials that behave as both conductors and magnets could find applications in quantum magnetoelectronics and spintronics. Here, we report the synthesis of CrCl(pyrazine), an air-stable layered solid, by reaction of CrCl with pyrazine (pyz).

Electron delocalization and charge mobility as a function of reduction in a metal-organic framework.

Conductive metal-organic frameworks are an emerging class of three-dimensional architectures with degrees of modularity, synthetic flexibility and structural predictability that are unprecedented in other porous materials. However, engendering long-range charge delocalization and establishing synthetic strategies that are broadly applicable to the diverse range of structures encountered for this class of materials remain challenging. Here, we report the synthesis of K Fe(BDP) (0 ≤ x ≤ 2; BDP = 1,4-benzenedipyrazolate), which exhibits full charge delocalization within the parent framework and charge mobilities comparable to technologically relevant polymers and ceramics.

Reducing Coercive-Field Scaling in Ferroelectric Thin Films via Orientation Control.

The desire for low-power/voltage operation of devices is driving renewed interest in understanding scaling effects in ferroelectric thin films. As the dimensions of ferroelectrics are reduced, the properties can vary dramatically, including the robust scaling relationship between coercive field ( E) and thickness ( d), also referred to as the Janovec-Kay-Dunn (JKD) law, wherein E ∝ d. Here, we report that whereas (001)-oriented heterostructures follow JKD scaling across the thicknesses range of 20-330 nm, (111)-oriented heterostructures of the canonical tetragonal ferroelectric PbZrTiO exhibit a deviation from JKD scaling wherein a smaller scaling exponent for the evolution of E is observed in films of thickness ≲ 165 nm.

Critical Role of Methylammonium Librational Motion in Methylammonium Lead Iodide (CHNHPbI) Perovskite Photochemistry.

Raman and photoluminescence (PL) spectroscopy are used to investigate dynamic structure-function relationships in methylammonium lead iodide (MAPbI) perovskite. The intensity of the 150 cm methylammonium (MA) librational Raman mode is found to be correlated with PL intensities in microstructures of MAPbI. Because of the strong hydrogen bond between hydrogens in MA and iodine in the PbI perovskite octahedra, the Raman activity of MA is very sensitive to structural distortions of the inorganic framework.

Lead-free antiferroelectric: xCaZrO3-(1 -x)NaNbO3 system (0 ≤x≤ 0.10).

This study demonstrates that antiferroelectricity can be stabilized in NaNbO(3) (NN) based ceramics by lowering the tolerance factor. Through consideration of the crystal chemistry via the Goldschmidt tolerance factor and polarizability, we show that simultaneous substitution of Zr(4+) and Ca(2+) ions in the Nb and Na sites, respectively, lowers the polarizability and tolerance factor of the (Na(1-x)Ca(x))(Nb(1-x)Zrx)O(3) (CZNN100x) solid solution, while maintaining charge neutrality. Structural investigations using both X-ray diffraction and transmission electron microscopy (TEM) indicated an enhancement of antiferroelectric (AFE) superlattice peaks with CaZrO(3) substitution.