Temperature-dependent Single-crystal X-ray Diffraction Research Articles

Ferroelastic phase transition and dielectric anomalies in 2,4,6-trimethylanilinium perchlorate

The crystal structure of the crystalline 1 : 1 complex of 2,4,6-trimethylaniline with perchloric acid, (C9H14N)+·ClO4− (1), was determined at 25 °C and 110 °C, respectively. The crystal structural analysis of both temperatures show that 1 undergoes a reversible phase transition from triclinic P (No.2) to monoclinic P21/m (No.11) with a distinct change of the cell parameters. It is a ferroelastic phase transition with an Aizu notation of 2/mF. DSC (differential scanning calorimetry) measurement confirms that this compound undergoes a reversible first-order solid-state structural phase transition at about 48 °C with a thermal hysteresis of 3 °C. Usually, phase transition is accompanied by an anomaly of the dielectric constant near phase transition temperature (Tc). The DSC measurement, temperature-dependent single-crystal X-ray diffraction and dielectric studies reveal that 1 displays structural phase transitions and distinctly step-like dielectric anomalies at 48 °C probably supporting that this phase transition is of the first-order.

Polymorph Selective Growth of Sodium Tetrakis(2-pyridinecarboxylato) Lanthanides and Their Structure Sensitive Properties

Four new single crystals of sodium tetrakis(2-pyridinecarboxylato)Ln(III), where Ln is europium and terbium, with general formulas of Na[Ln(C6H4NO2)4]·2H2O were prepared from a reaction of solvent-mediated polymorph selection. All the crystal structures were determined by using temperature-dependent single crystal X-ray diffraction at 100(2) and 298(2) K. No phase transition was observed within this temperature range. Two isostructural compounds, IM (Eu3+ core) and IIIM (Tb3+ core), crystallize in the monoclinic space group C2h6 - C2/c (No. 15) and the other two compounds, IIH (Eu3+ core) and IVH (Tb3+ core), in the enantiomorphic hexagonal space group D63 - P6522 (No. 179). Both IM and IIH (or both IIIM and IVH) are homeotypic. Each structure consists of an extensive network of organo-lanthanide complex and has extremely long-ranged interlayer distances, about 10 Å. The parallel interlayer in IM (or IIIM) can be reconstructed into IIH (or IVH) via 6-fold angularly step-staggered packing. Such a phenomenon of crystal packing strongly depends not only on the nature of the applied solvent system but also on long-range ordering of the organo-lanthanides. Transmission electron microscopy analysis of IIH and IVH reveals the absolute configuration of parallel helical assemblies. Their optical and magnetic data show the salient features of topospecific chirality over those racemic counterparts.

An Application of the “Coloring Problem”: Structure−Composition−Bonding Relationships in the Magnetocaloric Materials LaFe13-xSix

The LaFe(13)-(x)Si(x) (1.0 < or = x < or = 5.0) series is studied experimentally and theoretically to gain possible understanding for the relationships among geometrical structure, chemical composition, magnetic behavior, and physical properties as related to the magnetocaloric effect in these compounds. As the Si concentration increases, LaFe(13)-(x)Si(x) exhibits a structural transformation from the cubic NaZn(13) structure type to a tetragonal derivative due primarily to preferential ordering of Fe and Si atoms. At room temperature, LaFe(13)-(x)Si(x) crystallize in the cubic structure for the range 1 < or = x < or = 2.6 and in the tetragonal for 3.2 < or = x < or = 5. In the range 2.6 < or = x < or = 3.2, it shows a two-phase mixture. Temperature-dependent single-crystal X-ray diffraction experiments near the corresponding Curie temperatures were performed on the room-temperature cubic phases to examine the origin of the large isothermal magnetic entropy changes. A thorough statistical and structural analysis of the data indicates that the noncentrosymmetric F43c space group provides a more adequate atomic arrangement than the centrosymmetric Fm3c space group. This change in space group leads to divergence for specific sets of Fe-Fe distances below the Curie temperature that arises from tilting of Fe-centered [Fe(12)-(x)Si(x)] icosahedra. The noncentrosymmetric space group also agrees with the predominance of icosahedral clusters lacking local inversion symmetry. From extended Hückel and tight-binding linear muffin-tin orbital (TB-LMTO) electronic structure calculations on various model structures, the F43c model is more energetically favorable than the Fm3c model. Extended Hückel calculations on various icosahedral [Fe(12)-(n)Si(n)] (n = 1-5) clusters and TB-LMTO calculations on "LaFe(13)," LaFe(11)Si(2), and LaFe(9)Si(4) have also been carried out to study the effects of a main group element (Si) on stabilizing the cubic NaZn(13)-type structure, influencing the transformation between cubic and tetragonal symmetries, and to study relationships among their chemical bonding and magnetic properties.

Enol-enamine tautomerism in crystals of 1,3-bis(pyridin-2-yl) propan-2-one: a combined crystallographic and quantum-chemical investigation of the effect of packing on tautomerization processes.

The enolpyridine, OH-ketoenamime, NH equilibrium in crystals of 1,3-bis(pyridin-2-yl)propan-2-one was studied using temperature-dependent single-crystal X-ray diffraction. The relative population of the different tautomers was found to be sensitive to the temperature in the range of 100-300 K, illustrating the small thermodynamic difference between these two tautomers. This energy resemblance is partially attributed to the molecular packing in the crystal, where the molecules are arranged in the form of dimers. Ab initio electronic energy calculations (HF/6-31G** and MP2/6-31G**) reveal the effect of dimerization in the crystal on the electronic energy levels. Several tautomeric states were identified in the dimer of 1,3-bis(pyridin-2-yl)propan-2-one. A model is proposed in which four of these dimer states are populated in the crystal at ambient temperatures. The crystallographic data were treated according to this four-state dimer model, suggesting that the free energy of the OH-NH dimers is higher than that of the OH-OH dimers by 120 +/- 10 cal mol(-1) and that the NH-NH dimers are yet higher in free energy by 50 +/- 10 cal mol(-1).