Robert E. Dinnebier

Real-time and in situ monitoring of mechanochemical milling reactions

Chemical and structural transformations have long been carried out by milling. Such mechanochemical steps are now ubiquitous in a number of industries (such as the pharmaceutical, chemical and metallurgical industries), and are emerging as excellent environmentally friendly alternatives to solution-based syntheses. However, mechanochemical transformations are typically difficult to monitor in real time, which leaves a large gap in the mechanistic understanding required for their development. We now report the real-time study of mechanochemical transformations in a ball mill by means of in situ diffraction of high-energy synchrotron X-rays. Focusing on the mechanosynthesis of metal-organic frameworks, we have directly monitored reaction profiles, the formation of intermediates, and interconversions of framework topologies. Our results reveal that mechanochemistry is highly dynamic, with reaction rates comparable to or greater than those in solution. The technique also enabled us to probe directly how catalytic additives recently introduced in the mechanosynthesis of metal-organic frameworks, such as organic liquids or ionic species, change the reactivity pathways and kinetics.

Electronic Phase Separation in the Slightly Underdoped Iron Pnictide Superconductor Ba1-xKxFe2As2

Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba1-xKxFe2As2 by means of x-ray powder diffraction, neutron scattering, muon-spin rotation (microSR), and magnetic force microscopy (MFM). Static antiferromagnetic order sets in below T{m} approximately 70 K as inferred from the neutron scattering and zero-field-microSR data. Transverse-field microSR below Tc shows a coexistence of magnetically ordered and nonmagnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting- or normal-state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.

Refinement of modulated structures against X-ray powder diffraction data with JANA2000

JANA is a computer program for the refinement and analysis of periodic and aperiodic (incommensurately modulated structures and composite crystals) crystal structures. Here a new module is introduced that allows Rietveld refinements against powder diffraction data. It is shown that JANA2000 provides a state-of-the-art description of the peak profiles. A re-analysis of the low-temperature structure of (CO)xC60 showed that the application of icosahedral symmetry restrictions to the C60 molecule leads to a better description of the electron density and to a corrected position of the CO molecule as compared with a rigid-body refinement. The incommensurately modulated structure of NbTe4 has been successfully refined against X-ray powder diffraction data. The structural parameters are equal to, but less accurate, than the parameters obtained from a single-crystal study.

In situ X-ray diffraction monitoring of a mechanochemical reaction reveals a unique topology metal-organic framework.

Chemical and physical transformations by milling are attracting enormous interest for their ability to access new materials and clean reactivity, and are central to a number of core industries, from mineral processing to pharmaceutical manufacturing. While continuous mechanical stress during milling is thought to create an environment supporting nonconventional reactivity and exotic intermediates, such speculations have remained without proof. Here we use in situ, real-time powder X-ray diffraction monitoring to discover and capture a metastable, novel-topology intermediate of a mechanochemical transformation. Monitoring the mechanochemical synthesis of an archetypal metal-organic framework ZIF-8 by in situ powder X-ray diffraction reveals unexpected amorphization, and on further milling recrystallization into a non-porous material via a metastable intermediate based on a previously unreported topology, herein named katsenite (kat). The discovery of this phase and topology provides direct evidence that milling transformations can involve short-lived, structurally unusual phases not yet accessed by conventional chemistry.

Single Crystals Popping Under UV Light: A Photosalient Effect Triggered by a [2+2] Cycloaddition Reaction†

The extremely rare examples of dynamic single crystals where excitation by light or heat induces macroscopic motility present not only a visually appealing demonstration of the utility of molecular materials for conversion of energy to work, but they also provide a unique opportunity to explore the mechanistic link between collective molecular processes and their consequences at a macroscopic level. Here, we report the first example of a photosalient effect (photoinduced leaping) observed with crystals of three coordination complexes which is induced by a [2+2] photocycloaddition reaction. Unlike a plethora of other dimerization reactions, when exposed to even weak UV light, single crystals of these materials burst violently, whereby they are propelled to travel several millimeters. The results point to a multistep mechanism where the strain energy that has been accumulated during the dimerization triggers a rapid structure transformation which ultimately results in crystal disintegration.

Real-time in situ powder X-ray diffraction monitoring of mechanochemical synthesis of pharmaceutical cocrystals.

Looking in: The penetrating power of high-energy X-rays provides a means to monitor in situ and in real time the course of ball-milling reactions of organic pharmaceutical solids by detecting crystalline phases and assessing the evolution of their particle sizes. Upon switching from neat grinding to liquid-assisted grinding, cocrystal formation is enabled or tremendously accelerated, while the reaction mechanism alters its course.

Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (α-ε) that are related by four phase transitions including one thermosalient transition (α↔γ). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 × 10(-6)  K(-1)) with volumetric expansion coefficients (≈250 × 10(-6)  K(-1)) that are among the highest values reported in molecular solids thus far. The habit plane advances at ~10(4) times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.

Hyperhoneycomb Iridate β − Li 2 IrO 3 as a Platform for Kitaev Magnetism

A complex iridium oxide β-Li(2)IrO(3) crystallizes in a hyperhoneycomb structure, a three-dimensional analogue of honeycomb lattice, and is found to be a spin-orbital Mott insulator with J(eff)=1/2 moment. Ir ions are connected to the three neighboring Ir ions via Ir-O(2)-Ir bonding planes, which very likely gives rise to bond-dependent ferromagnetic interactions between the J(eff)=1/2 moments, an essential ingredient of Kitaev model with a spin liquid ground state. Dominant ferromagnetic interaction between J(eff)=1/2 moments is indeed confirmed by the temperature dependence of magnetic susceptibility χ(T) which shows a positive Curie-Weiss temperature θ(CW)∼+40  K. A magnetic ordering with a very small entropy change, likely associated with a noncollinear arrangement of J(eff)=1/2 moments, is observed at T(c)=38  K. With the application of magnetic field to the ordered state, a large moment of more than 0.35  μ(B)/Ir is induced above 3 T, a substantially polarized J(eff)=1/2 state. We argue that the close proximity to ferromagnetism and the presence of large fluctuations evidence that the ground state of hyperhoneycomb β-Li(2)IrO(3) is located in close proximity of a Kitaev spin liquid.

Structures of three dehydration products of bischofite from in situ synchrotron powder diffraction data (MgCl2.nH2O; n = 1, 2, 4).

High-quality in situ synchrotron powder diffraction data have been used to investigate the decomposition products of bischofite in the temperature range 298 </= T </= 873 K. At least eight phases could be identified: MgCl(2).nH(2)O (n = 1, 2, 4 and 6), MgOHCl.nH(2)O (0 </= n </= 1.0), MgCl(2) and MgO. The crystal structures of three magnesium chloride hydrates MgCl(2).nH(2)O (n = 1, 2, 4) were determined ab initio, replacing published Rietveld refinements from low-quality powder diffraction data based on similarity criteria. MgCl(2).4H(2)O was found to be disordered and has been correctly determined for the first time. The crystal structures of bischofite and MgCl(2).4H(2)O consist of discrete Mg(H(2)O)(6) and MgCl(2)(H(2)O)(4) octahedra, respectively. The crystal structure of MgCl(2).2H(2)O is formed by single chains of edge-sharing MgCl(2)(H(2)O)(4) octahedra, while in the case of MgCl(2).H(2)O double chains of edge-sharing MgCl(H(2)O)(5) octahedra are found. The phases in the system MgCl(2)-H(2)O are intermediates in the technologically important process of MgO and subsequently Mg production. The same phases were recently found to be of key importance in the understanding of cracks in certain magnesia concrete floors.

In situ and real-time monitoring of mechanochemical milling reactions using synchrotron X-ray diffraction

We describe the only currently available protocol for in situ, real-time monitoring of mechanochemical reactions and intermediates by X-ray powder diffraction. Although mechanochemical reactions (inducing transformations by mechanical forces such as grinding and milling) are normally performed in commercially available milling assemblies, such equipment does not permit direct reaction monitoring. We now describe the design and in-house modification of milling equipment that allows the reaction jars of the operating mill to be placed in the path of a high-energy (∼90 keV) synchrotron X-ray beam while the reaction is taking place. Resulting data are analyzed using conventional software, such as TOPAS. Reaction intermediates and products are identified using the Cambridge Structural Database or Inorganic Crystal Structure Database. Reactions are analyzed by fitting the time-resolved diffractograms using structureless Pawley refinement for crystalline phases that are not fully structurally characterized (such as porous frameworks with disordered guests), or the Rietveld method for solids with fully determined crystal structures (metal oxides, coordination polymers).