Carsten Paulmann

The discovery of an in-situ method to grow flexible single crystals of methanol solvated cocrystals of caffeine and 4-chloro-3-nitrobenzoic acid by desolvation-resolvation of the solvent-grown material is reported. The in-situ obtained crystals are isomorphous to the starting Form with orthorhombic space group symmetry Fdd2, show similar habit and demonstrate macroscopic flexibility but possess greater unit cell volume. Crystal structures of the in-situ Form obtained employing synchrotron radiation suggest that the solvent channels are enlarged by an additional ~½ methanol molecule which leads to elongation of the unit cell axis parallel to the crystal growth direction. The new in-situ Form conserves the hydrogen bond geometries of the solvent grown material, while caffeine–coformer dimers along π···π stacks parallel to methanol channels are re-oriented by increased intermolecular distances and tilt of their mean planes. The most striking impact of the greater density of solvent molecules is in the thermal expansion of the in-situ crystals which is ~ 73 % lower than that of the solvent grown Form. The present results demonstrate the strength of in-situ techniques which not only depicts an unprecedented “breathing property” of these elastic crystals similar to zeolites, MOFs and COFs but also the potential of tuning their thermal response.

Relations between structural properties and their variations due to phase transitions under variable thermodynamic conditions is of central interest in material science. More than a decade ago, the discovery of elastic bending in single crystals of caffeine cocrystal solvates opened up a new area of research in mechanical properties of molecular crystals. Here we report structural transition at T > 320 K that is accompanied by negative area expansion of the plane normal to the bending axis and non-linear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles from 360 K ≤ T ≤ 380 K to T = 100 K, three structurally irreversible states are discovered that are characterized by hysteresis in the area of the planes and intermolecular distances. All the phase transformations are associated with significant crystal degradation as suggested by increasing mosaicity but not decomposition. These results suggest that the loss of elastic flexibility above room temperature is governed by increased crystal defects rather than previously proposed loss of solvent molecules from the structure. During these various thermal processes, the acid-base dimers rotate as well as move. The ratio (η) between variation in rotations and distances reveals a striking similarity in its order in the high T phase as well as those of irreversible structural states to that calculated from reported deformations during mechanical bending at T = 100 K.

The pressure dependence of structural behavior in the orthorhombic (Pccn, PI) and monoclinic (P2₁/c, PII) polymorphs of the compound [Fe(PM-BiA)₂(NCS)₂], where PM–BiA = (N–(2′–pyridylmethylene)–4-amino–bi–pheynyl), is studied with synchrotron single-crystal X-ray diffraction and vibrational spectroscopy. Both polymorphs are stable up to ∼1.5 GPa, with a spin state transition occurring only in polymorph PII under hydrostatic conditions as documented by single-crystal synchrotron diffraction. The diffraction data also provide evidence of the formation of superstructures for both PI, with a doubled c axis, and PII, with a doubled b axis, on applying pressures above 2 GPa. The LS and HS states seem to coexist at high-pressures for both polymorphs studied with synchrotron infrared spectroscopy at quasi-hydrostatic conditions. Such results indicate that the occurrence of spin-crossover transformations in [Fe(PM-BiA)₂(NCS)₂] might strongly depend on the stress in the sample.

Structural and spatial aspects of cell degradation are studied using a combination of diffraction-and imaging-based tools applying laboratory X-rays, neutron scattering and synchrotron radiation with electrochemical and thermal characterization. Experimental characterization is carried out on cylindrical cells of 21700-type, where four regimes of cell degradation are identified, which are supplemented by an increased cell resistance and surface temperature during cell operation. The amount of intercalated lithium in the fully charged anodes in the fresh and aged states is determined by ex situ X-ray diffraction radiography and in situ X-ray diffraction computed tomography. The qualitatively similar character of the results revealed a loss of active lithium along with the development of a complex heterogeneous distribution over the electrode stripe.

This study explains the ionic conductivity in the mineral sugilite (idealized formula: Fe₂Na₂K[Li₃Si₁₂O₃₀]) by resolving the dynamic disorder of both Li and Na cations using synchrotron X-ray single-crystal diffraction from 298 K to 1023 K. Non-zero anharmonic atomic displacement parameters at Na and Li sites at 1023 K adumbrated long-range charge transport routes for Li and Na cations commonly parallel to the (a–b) plane. Temperature-enhanced diffuse residuals in Fourier maps could unambiguously localize two interstitial sites suitable for Li, as well as three for Na. Each two-dimensional (2D) network of Li and Na interstitials was formed parallel to each other, providing Li and Na hopping pathways. The higher concentration of Na cations hopping in short distances of 2.0962(4)–2.3015(5) Å could be the main reason for the higher bulk conductivity values evaluated by impedance spectra of sugilite in comparison to those of its structural relatives with low Na contents, e.g., the mineral sogdianite ((Zr,Al,Fe)₂Na_0.36K[Li₃Si₁₂O₃₀]). Bond valence sum landscape maps supported the critical role of dynamic disorder of Na⁺ over densely packed 2D interstitial networks for combined ionic conductivity along with mobile Li⁺ in sugilite-type compounds.

EuAl₄ possesses the BaAl₄ crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge density wave (CDW) transition at T _CDW = 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the incommensurately modulated crystal structure of EuAl₄ in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q = (0,0,0.1781 (3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00σ)s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the Al atoms as the location of the CDW. The temperature dependence of the specific heat reveals an anomaly at T _CDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl₄ leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

The crystal structure of CrAs was investigated using synchrotron X-ray single-crystal diffraction for separate dependences on temperature (30–400 K) and on pressure (0–9.46 GPa). The isosymmetrical magnetostructural phase transition at T _N = 267 K can induce a change in the microstructure by twinning due to a crossing of the orthohexagonal setting of the unit-cell parameter ratio c/b. Within the crystal structure, one particular Cr–Cr distance exhibits anomalous behavior in that it is nearly unaffected by temperature and pressure in the paramagnetic phase, which is stable above 267 K and at high pressures. The distinction of this shortest Cr–Cr distance might be of importance for the superconducting properties of CrAs.

New possibilities are presented for the characterization ofA^IIIB^Vmixed superlattice compounds by the complementary use of synchrotron diffraction topography and rocking curves. In particular, using a synchrotron white beam and the section diffraction pattern of a 5 µm slit taken at a 10 cm film-to-crystal distance, it was possible to reproduce a set of stripes corresponding to interference fringes. These are analogous to the interference maxima revealed in high-resolution rocking curves, but are created by the changes in orientation of the planes inclined to the surface which are induced by unrelaxed strain. The section diffraction topographic method enabled examination of the sample homogeneity along the narrow intersecting beam. This was important in the case of the present sample containing a twin lamella in the InP substrate wafer. Both the section and projection Bragg case topographic methods enabled the crystallographic identification of the twin lamella. Another characteristic feature indicated in the section topography was the bending of the stripes corresponding to the superlattice peaks close to the boundaries of the twin lamella. The most probable interpretation of this phenomenon is an increase in the thickness of the deposited layers close to the lamella, together with possible changes in the chemical composition, leading to a decrease in the mean lattice parameter in the superlattice.