Martin Tolkiehn

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.

Iron phosphate materials have attracted a lot of attention due to their potential as cathode materials for lithium-ion rechargeable batteries. It has been shown that lithium insertion or extraction depends on the Fe mixed valence and reduction or oxidation of the Fe ions’ valences. In this paper, we report a new synthesis method for the Fe₃(PO₃OH)₄(H₂O)₄ mixed valence iron phosphate. In addition, we perform temperature-dependent measurements of structural and physical properties in order to obtain an understanding of electronic–structural interplay in this compound. Scanning electron microscope images show needle-like single crystals of 50 μm to 200 μm length which are stable up to approximately 200 °C, as revealed by thermogravimetric analysis. The crystal structure of Fe₃(PO₃OH)₄(H₂O)₄ single crystals has been determined in the temperature range of 90 K to 470 K. A monoclinic isostructural phase transition was found at ~213 K, with unit cell volume doubling in the low temperature phase. While the local environment of the Fe²⁺ ions does not change significantly across the structural phase transition, small antiphase rotations occur for the Fe³⁺ octahedra, implying some kind of electronic order. These results are corroborated by first principle calculations within density functional theory, which also point to ordering of the electronic degrees of freedom across the transition. The structural phase transition is confirmed by specific heat measurements. Moreover, hints of 3D antiferromagnetic ordering appear below ~11 K in the magnetic susceptibility measurements. Room temperature visible light absorption is consistent with the Fe²⁺/Fe³⁺ mixed valence.

The nuclear and magnetic structures of Mn₃Fe₂Si₃ are investigated in the temperature range from 20 to 300 K. The magnetic properties of Mn₃Fe₂Si₃ were measured on a single crystal. The compound undergoes a paramagnetic to antiferromagnetic transition at T _N2 ≃ 120 K and an antiferromagnetic to antiferromagnetic transition at T _N1 ≃ 69 K. A similar sequence of magnetic phase transitions is found for the parent compound Mn₅Si₃ upon temperature variation, but the field-driven transition observed in Mn₅Si₃ is not found in Mn₃Fe₂Si₃, resulting in a strongly reduced magnetocaloric effect. Structurally, the hexagonal symmetry found for both compounds under ambient conditions is preserved in Mn₃Fe₂Si₃ through both magnetic transitions, indicating that the crystal structure is only weakly affected by the magnetic phase transition, in contrast to Mn₅Si₃ where both transitions distort the nuclear structure. Both compounds feature a collinear high-temperature magnetic phase AF2 and transfer into a non-collinear phase AF1 at low temperature. While one of the distinct crystallographic sites remains disordered in the AF2 phase in the parent compound, the magnetic structure in the AF2 phase involves all magnetic atoms in Mn₃Fe₂Si₃. These observations imply that the distinct sites occupied by the magnetic atoms play an important role in the magnetocaloric behaviour of the family.

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.