Gregor Kieslich

Bond strength dependent superionic phase transformation in the solid solution series Cu2ZnGeSe4−xSx

Recently, copper selenides have shown to be promising thermoelectric materials due to their possible superionic character resulting from mobile copper cations. Inspired by this recent development in the class of quaternary copper selenides we have focused on the structure-to-property relationships in the solid solution series Cu2ZnGeSe4−xSx. The material exhibits an insulator-to-metal transition at higher temperatures, with a transition temperature dependent on the sulfur content. However, the lattice parameters show linear thermal expansion at elevated temperatures only and therefore no indication of a structural phase transformation. 63Cu nuclear magnetic resonance shows clear indications of Cu located on at least two distinct sites, which eventually merge into one (apparent) site above the phase transformation. In this manuscript the temperature dependent lattice parameters and electronic properties of the solid solution Cu2ZnGeSe4−xSx are reported in combination with 63Cu NMR, and an attempt will be made to relate the nature of the electronic phase transformation to a superionic phase transformation and a changing covalent character of the lattice upon anion substitution in this class of materials.

The same and not the same: molecular perovskites and their solid-state analogues

There has been enormous interest in ABX3 perovskite-type materials where at least one ion, usually A or X, is replaced with a molecular building unit. In this Minireview we highlight the unique properties that arise as a result of these molecular building units, drawing on topical examples of the recent literature. We focus in particular on phenomena related to low energy lattice modes that tie the molecular building-units to the fascinating properties observed in molecular perovskites.

Defective Metal‐Organic Frameworks

The targeted incorporation of defects into crystalline matter allows for the manipulation of many properties and has led to relevant discoveries for optimized and even novel technological applications of materials. It is therefore exciting to see that defects are now recognized to be similarly useful in tailoring properties of metal-organic frameworks (MOFs). For instance, heterogeneous catalysis crucially depends on the number of active catalytic sites as well as on diffusion limitations. By the incorporation of missing linker and missing node defects into MOFs, both parameters can be accessed, improving the catalytic properties. Furthermore, the creation of defects allows for adding properties such as electronic conductivity, which are inherently absent in the parent MOFs. Herein, progress of the rapidly evolving field of the past two years is overviewed, putting a focus on properties that are altered by the incorporation and even tailoring of defects in MOFs. A brief account is also given on the emerging quantitative understanding of defects and heterogeneity in MOFs based on scale-bridging computational modeling and simulations.

How Strong Is the Hydrogen Bond in Hybrid Perovskites

Hybrid organic–inorganic perovskites represent a special class of metal–organic framework where a molecular cation is encased in an anionic cage. The molecule–cage interaction influences phase stability, phase transformations, and the molecular dynamics. We examine the hydrogen bonding in four AmBX3 formate perovskites: [Am]Zn(HCOO)3, with Am+ = hydrazinium (NH2NH3+), guanidinium (C(NH2)3+), dimethylammonium (CH3)2NH2+, and azetidinium (CH2)3NH2+. We develop a scheme to quantify the strength of hydrogen bonding in these systems from first-principles, which separates the electrostatic interactions between the amine (Am+) and the BX3– cage. The hydrogen-bonding strengths of formate perovskites range from 0.36 to 1.40 eV/cation (8–32 kcalmol–1). Complementary solid-state nuclear magnetic resonance spectroscopy confirms that strong hydrogen bonding hinders cation mobility. Application of the procedure to hybrid lead halide perovskites (X = Cl, Br, I, Am+ = CH3NH3+, CH(NH2)2+) shows that these compounds have significantly weaker hydrogen-bonding energies of 0.09 to 0.27 eV/cation (2–6 kcalmol–1), correlating with lower order–disorder transition temperatures.

Solution synthesis of nanoparticular binary transition metal antimonides.

The preparation of nanoengineered materials with controlled nanostructures, for example, with an anisotropic phase segregated structure or a regular periodicity rather than with a broad range of interparticle distances, has remained a synthetic challenge for intermetallics. Artificially structured materials, including multilayers, amorphous alloys, quasicrystals, metastable crystalline alloys, or granular metals, are mostly prepared using physical gas phase procedures. We report a novel, powerful solution-mediated approach for the formation of nanoparticular binary antimonides based on presynthesized antimony nanoparticles. The transition metal antimonides M-Sb (M = Co, Ni, Cu(2), Zn) were obtained with sizes ranging from 20 and 60 nm. Through careful control of the reaction conditions, single-phase nanoparticular antimonides were synthesized. The nanophases were investigated by powder X-ray diffraction and (high resolution) electron microscopy. The approach is based on activated metal nanoparticles as precursors for the synthesis of the intermetallic compounds. X-ray powder diffraction studies of reaction intermediates allowed monitoring of the reaction kinetics. The small particle size of the reactants ensures short diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis.

Using water adsorption measurements to access the chemistry of defects in the metal–organic framework UiO-66

Tailoring defects in metal–organic frameworks is important for enhancing sorption and reaction properties. Defects in UiO-66 have been characterized for the first time by using water adsorption measurements. We found that the defect-induced hydrophilicity, quantitatively expressed by the Henry constant and the saturation water uptake, correlates well with the catalytic performance in the cyanosilylation of benzaldehyde.

Thermodynamic and Kinetic Effects in the Crystallization of Metal–Organic Frameworks

The evolution of metal-organic frameworks (MOFs) has been one of the most exciting aspects of materials chemistry over the last 20 years. In this Account, we discuss the development during this period in our understanding of the factors that control the crystallization of MOFs from solution. Both classical porous MOFs and dense MOF phases are considered. This is an opportune time at which to examine this complex area because the experimental tools now available to interrogate crystallization processes have matured significantly in the last 5 years, particularly with the use of in situ synchrotron X-ray diffraction. There have also been impressive developments in the use of density functional theory (DFT) to treat not only the energies of very complex structures but also their entropies. This is particularly important in MOF frameworks because of their much greater flexibility compared with inorganic structures such as zeolites. The first section of the Account describes how early empirical observations on the crystallization of dense MOFs pointed to a strong degree of thermodynamic control, with both enthalpic and entropic factors playing important roles. For example, reactions at higher temperatures tend to lead to denser structures with higher degrees of framework connectivity and lower levels of solvation, and polymorphs tend to form according to their thermodynamic stabilities. In the case of metal tartrates, these trends have been validated by calorimetric studies. It has been clear for more than a decade, however, that certain phases crystallize under kinetic control, especially when a change in conformation of the ligand or coordination around a metal center might be necessary to form the thermodynamically preferred product. We describe how this can lead to time-dependent crystallization processes that evolve according to the Ostwald rule of stages and can be observed by in situ methods. We then consider the crystallization of porous MOFs, which presents additional challenges because of solvation effects. In spite of these problems, much has been learned about the energetics of the underlying frameworks, where the relationship between porosity and stability initially seemed to mirror the behavior of zeolites, with more porous structures being less stable. Recently, however, this simple relationship has had to be reconsidered with the emergence of some very flexible structures wherein the open structures are more stable than their denser analogues at finite temperatures because of their large vibrational entropies. In the final section we describe how the concepts developed in the MOF work have been extended into the closely related area of hybrid organic-inorganic perovskites. We describe recent studies on polymorphism in hybrid perovskites, which is amenable to total free energy calculations using a combination of DFT and lattice dynamics methods.

Thermoelectric properties of spark-plasma sintered nanoparticular FeSb2 prepared via a solution chemistry approach

Nanoparticular FeSb2 was prepared in solution from cyclopentadienyl iron(ii) dicarbonyl dimer [Fe(Cp(CO)2)]2 and antimony nanoparticles. Spark plasma sintering was used as consolidation method to maintain the particle size. The thermoelectric performance of FeSb2 is limited by its high thermal conductivity. In this work, the thermal conductivity was suppressed by nearly 80% compared to the bulk value by introducing grain boundary scattering of phonons on the nanoscale. The thermoelectric properties of the consolidated FeSb2 emphasize the possibility of altering thermal transport of promising thermoelectric compounds by phonon scattering by engineering the interfaces at the nanoscale.

Tuning the Mechanical Response of Metal–Organic Frameworks by Defect Engineering

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal-organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs.

Tolerance factors of hybrid organic–inorganic perovskites: recent improvements and current state of research

In 2014 we applied Goldschmidts concept of ionic tolerance factors to the large family of hybrid organic–inorganic perovskites. Initially seen as a guiding concept for the discovery of new hybrid organic–inorganic perovskites, the tolerance factor concept has also proven to be a valuable tool for understanding and manipulating the phase stability and properties of existing phases. Since our initial report, there have been many research examples in which tolerance factors were used to understand the existence and stability of certain hybrid organic–inorganic perovskites, while the concept itself has been continuously improved. Here we give an update on the current state of the concept, reviewing the different improvements that have been made over the past few years and drawing on topical examples in which tolerance factors have played a major role.