Electron crystallography and dedicated electron-diffraction instrumentation.

Electron diffraction (known also as ED, 3D ED or microED) is gaining momentum in science and industry. The application of electron diffraction in performing nano-crystallography on crystals smaller than 1 µm is a disruptive technology that is opening up fascinating new perspectives for a wide variety of compounds required in the fields of chemical, pharmaceutical and advanced materials research. Electron diffraction enables the characterization of solid compounds complementary to neutron, powder X-ray and single-crystal X-ray diffraction, as it has the unique capability to measure nanometre-sized crystals. The recent introduction of dedicated instrumentation to perform ED experiments is a key aspect of the continued growth and success of this technology. In addition to the ultra-high-speed hybrid-pixel detectors enabling ED data collection in continuous rotation mode, a high-precision goniometer and horizontal layout have been determined as essential features of an electron diffractometer, both of which are embodied in the Eldico ED-1. Four examples of data collected on an Eldico ED-1 are showcased to demonstrate the potential and advantages of a dedicated electron diffractometer, covering selected applications and challenges of electron diffraction: (i) multiple reciprocal lattices, (ii) absolute structure of a chiral compound, and (iii) R-values achieved by kinematic refinement comparable to X-ray data.

Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF-4.

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

An "off-axis" Mn-Mn bond in Mn2(CO)10 at high pressure.

At variance with what was previously reported, Mn2(CO)10 does not transform its conformation from staggered to eclipsed in the high pressure crystal form. X-ray powder diffraction, DFT calculations and Raman spectroscopy show that the staggered conformation is retained. Instead, a rotation and a translation of the Mn(CO)5 pyramidal units produce an "off-axis" and rather shorter Mn-Mn bond.

Reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4).

We report the reversible pressure-induced amorphization of a zeolitic imidazolate framework (ZIF-4, [Zn(Im)(2)]). This occurs irrespective of pore occupancy and takes place via a novel high pressure phase (ZIF-4-I) when solvent molecules are present in the pores. A significant reduction in bulk modulus upon framework evacuation is also observed for both ZIF-4 and ZIF-4-I.

Thermal, spectroscopic, and ab initio structural characterization of carprofen polymorphs.

Commercial and recrystallized polycrystalline samples of carprofen, a nonsteroidal anti-inflammatory drug, were studied by thermal, spectroscopic, and structural techniques. Our investigations demonstrated that recrystallized sample, stable at room temperature (RT), is a single polymorphic form of carprofen (polymorph I) that undergoes an isostructural polymorphic transformation by heating (polymorph II). Polymorph II remains then metastable at ambient conditions. Commercial sample is instead a mixture of polymorphs I and II. The thermodynamic relationships between the two polymorphs were determined through the construction of an energy/temperature diagram. The ab initio structural determination performed on synchrotron X-Ray powder diffraction patterns recorded at RT on both polymorphs allowed us to elucidate, for the first time, their crystal structure. Both crystallize in the monoclinic space group type P2(1) /c, and the unit cell similarity index and the volumetric isostructurality index indicate that the temperature-induced polymorphic transformation I → II is isostructural. Polymorphs I and II are conformational polymorphs, sharing a very similar hydrogen bond network, but with different conformation of the propanoic skeleton, which produces two different packing. The small conformational change agrees with the low value of transition enthalpy obtained by differential scanning calorimetry measurements and the small internal energy computed with density functional methods.

Raman spectroscopic investigation of tetraethylammonium polybromides.

A large number of polyhalides, especially polyiodides, have been discovered and studied, but definitive studies on polybromides remain scarce. Br(3)(-) is the only monovalent polybromide with a known structure. Higher-order monovalent polybromide anions have been proposed but not structurally confirmed as discrete species. In this study tetraalkylammonium polybromides with molecular formulas R(4)NBr(2x+1) (R = ethyl; x = 1-4) were prepared by reacting tetraalkylammonium monobromide or tribromide salts with gas-phase bromine. Distinct and characteristic Raman spectra were obtained from the solid polybromides in the spectral range between 100 and 400 cm(-1). Experimental Raman spectra were compared to ab initio calculations to propose the structure of these polybromide anions. A general agreement between the experimental and theoretical results was observed. This study demonstrates that Raman spectroscopy is a sensitive technique for probing the structure of discrete monovalent polybromides.

Phase selection and energetics in chiral alkaline Earth tartrates and their racemic and meso analogues: synthetic, structural, computational, and calorimetric studies.

The hydrothermal reactions of calcium, strontium, and barium with l-, meso-, and d,l-tartaric acid were examined from room temperature to 220 degrees C. We report the synthesis of 13 new phases and crystal structures of 11 alkaline earth tartrates, including an unusual I(3)O(0) framework, [Ba(d,l-Tar)] (Tar = C(4)H(4)O(6)(2-)), with 3-D inorganic connectivity. Each alkaline earth exhibits different phase behavior in the reactions with the three forms of tartaric acid. Calcium forms unique l-, meso-, and d,l-tartrate phases which persist to 220 degrees C. Strontium forms three unique phases at lower temperatures, but above 180 degrees C reactions with l- and d,l-tartaric acid yield the meso phase. Likewise, Ba forms three unique low-temperature phases, but above 200 degrees C reactions with l- and meso-tartaric acid yield the d,l phase. Computational and calorimetric studies of the anhydrous calcium phases, [Ca(l-Tar)] and [Ca(meso-Tar)], strontium phases, [Sr(l-Tar)] and [Sr(meso-Tar)], and barium phases, [Ba(l-Tar)] and [Ba(d,l-Tar)], were performed to determine relative phase stabilities and elucidate the role of thermodynamic and kinetic factors in controlling phase behavior. The computational and calorimetric results were in excellent agreement. The [Ca(meso-Tar)] phase was found to be 9.1 kJ/mol more stable than the [Ca(l-Tar)] phase by computation (total electronic energies) and 2.9 +/- 1.6 kJ/mol more stable by calorimetry (enthalpies of solution). The [Sr(meso-Tar)] phase was found to be 13.4 and 8.1 +/- 1.4 kJ/mol more stable than [Sr(l-Tar)] by computation and calorimetry, respectively. Finally, the [Ba(l-Tar)] phase was found to be 6.4 and 7.0 +/- 1.0 kJ/mol more stable than the [Ba(d,l-Tar)] phase. Our results suggest that the calcium and strontium meso phases are the most thermodynamically stable phases in their systems over the temperature range studied. The phase transitions are controlled by relative thermodynamic stabilities but also by a kinetic factor, likely the barrier to isomerization/racemization of the tartaric acid, which is hypothesized to preclude phase transformations at lower temperatures. In the barium system we find the [Ba(l-Tar)] phase to be the most thermodynamically stable phase at low temperatures, while the [Ba(d,l-Tar)] phase becomes the thermodynamic product at high temperatures, due to a larger entropic contribution.

Calorimetric measurements of energetics of defect interactions in fluorite oxides.

Direct measurement by oxide melt solution calorimetry of energetics of mixing in rare earth and yttrium doped zirconia, hafnia, and ceria systems provides support for spectroscopic and computational studies of the location and clustering of vacancies in these systems. Strongly negative heats of mixing are seen when the vacancy is transferred from being nearest neighbor to Y or RE in the sesquioxide to being nearest neighbor to Zr or Hf in the cubic solid solution. In the absence of such redistribution, small positive enthalpies of mixing are seen in CeO2-YO1.5 and CeO2-REO.15 systems. Strongly positive enthalpies of mixing are seen in CeO2-ZrO2, which has a large difference in cation sizes and no vacancy formation. The system Ce0.8Y0.2O1.9-Zr0.8Y0.2O1.9 shows small positive heats of formation with less destabilization in the Ce-rich region, suggestive of "scavenging" of oxygen vacancies by Zr. The calorimetric data obtained in these studies offer direct comparison with the results of computations on defect clusters and their binding energies.