A High‐Pressure Polymorph of Phosphorus Nitride Imide

Phosphorus nitride imide, PN(NH), is of great scientific importance because it is isosteric with silica (SiO₂). Accordingly, a varied structural diversity could be expected. However, only one polymorph of PN(NH) has been reported thus far. Herein, we report on the synthesis and structural investigation of the first high‐pressure polymorph of phosphorus nitride imide, β‐PN(NH); the compound has been synthesized using the multianvil technique. By adding catalytic amounts of NH₄Cl as a mineralizer, it became possible to grow single crystals of β‐PN(NH), which allowed the first complete structural elucidation of a highly condensed phosphorus nitride from single‐crystal X‐ray diffraction data. The structure was confirmed by FTIR and ³¹P and ¹H solid‐state NMR spectroscopy. We are confident that high‐pressure/high‐temperature reactions could lead to new polymorphs of PN(NH) containing five‐fold‐ or even six‐fold‐coordinated phosphorus atoms and thus rivalling or even surpassing the structural variety of SiO₂.     

A High‐Pressure Polymorph of Phosphorus Oxonitride with the Coesite Structure

The chemical and physical properties of phosphorus oxonitride (PON) closely resemble those of silica, to which it is isosteric. A new high‐pressure phase of PON is reported herein. This polymorph, synthesized by using the multianvil technique, crystallizes in the coesite structure. This represents the first occurrence of this very dense network structure outside of SiO₂. Phase‐pure coesite PON (coe‐PON) can be synthesized in bulk at pressures above 15 GPa. This compound was thoroughly characterized by means of powder X‐ray diffraction, DFT calculations, and FTIR and MAS NMR spectroscopy, as well as temperature‐dependent diffraction. These results represent a major step towards the exploration of the phase diagram of PON at very high pressures and the possibly synthesis of a stishovite‐type PON containing hexacoordinate phosphorus.     

Polymorph of Dibenzo‐24‐Crown‐8 and its Mercury Complex

Dibenzo‐24‐crown‐8 is studied herein as a flexible ligand able to adopt different conformations, as well as for the complexation of mercury. The recrystallization of dibenzo‐24‐crown‐8 (DB24C8) from dry THF gives a new polymorphic structure of this ligand. This new structure is described and compared to the literature compound. Additionally, coordination of this ligand to mercury iodide HgI₂ is studied.     

Monoclinic Low-Temperature Polymorph of Cesium Nitrate

A new CsNO₃ polymorph with space group P2₁/c, Z = 4, a = 4.5699(9) Å, b = 11.1871(10) Å, c = 9.1484(18) Å, β = 131.24(3)° has been prepared by crystallization from a mixture of water and DMSO. Flat triangles NO₃ are located in the (010) and (020) planes between the layers formed by coordination polyhedra of Cs atoms in the (040) plane. In contrast to the previously known low-temperature polymorph, the new modification is characterized by the crystal equivalence of all Cs and NO₃ groups.     

Polymorph transition kinetics by DTA

The polymorph transition kinetics of the changes vaterite to calcite and aragonite to calcite have been followed using a DTA technique. The method eliminates the grinding process present in other analytical techniques and so has certain advantages. For both transitions the kinetic equation describing the process is found to be: $$\frac{{\partial \alpha }}{{\partial t}} = \frac{4}{3}k[ - \ln (1 - \alpha )]^{3/4} (1 - \alpha ).$$ Values of activation energy were found to be 452±19 KJ mole^?1 and 208±8 KJ mole^?1 for aragonite and vaterite transitions respectively using a procedure which enabled the calculation of kinetic parameters from a rising temperature experiment.     

Polymorph selectivity under nanoscopic confinement

Polymorph selectivity has been achieved during crystallization of anthranilic acid (AA) and 5-methyl-2-[(2-nitrophyenyl)amino]-3-thiophenecarbonitrile (ROY), both considered benchmarks of polymorphic behavior, within nanoporous glass beads and polymer monoliths. Whereas polymorph III of AA crystallizes from the melt on nonporous glass beads or within larger pores, the metastable polymorph II crystallizes in pores with diameters <23 nm, with the selectivity toward this form increasing with decreasing pore size. Of the six ROY polymorphs characterized by single-crystal X-ray diffraction, the yellow form (Y) crystallizes during evaporation of pyridine solutions imbibed by the 30-nm cylindrical pores of porous polycyclohexylethylene (p-PCHE) monoliths. Although both R and ON grow from the melt on the external surfaces of PCHE, only the red form (R) crystallizes in the pores. Amorphous ROY also forms in p-PCHE pores during evaporation from pyridine solutions, subsequently crystallizing to the R nanocrystals upon heating. Although heterogeneous nucleation on the pore walls may play a role, these observations suggest that nucleation and polymorph selectivity is governed by critical size constraints imposed by the ultrasmall pores. The ability to achieve polymorph selectivity in both glass and polymer matrices suggests wide-ranging compatibility with various organic crystalline solids, promising a new approach to controlling polymorphism and searching for unknown polymorphs.     

Polymorph of 2,9-dichloroquinacridone and its electronic properties

The title compound (2,9-DClQA) is a hydrogen-bonded, bluish-red pigment. However, red and black single crystals have been isolated from the vapor phase in the higher- and lower-temperature regions around 380 and 150 degrees C, respectively. Because of this, correlation has been studied in the present investigation between the crystal structure and the shade in the solid state. The red phase is found to correspond to the commercial product as characterized by two-dimensional NH...O hydrogen bonds between the NH group of one molecule and the O atom of the neighboring one. On the other hand, the molecule of the black phase is heavily deformed and no intermolecular hydrogen bonds are recognized. The molecular distortion induced by crystallization in the black phase is found to displace the absorption band toward longer wavelengths to bring about the black color. In addition, the black phase is observed to be transformed into the red one around 200 degrees C.     

Polymorph formation from solvate desolvation

Spironolactone (C₂₄H₃₂O₄S) usually crystallizes into an orthorhombic phase, named Form II. Another orthorhombic phase, named Form I, is also known but seems difficult to obtain. Studies of the kinetics of desolvation of the ethanol solvate at room temperature showed that these two forms can be obtained through different mechanisms of desolvation.     

Pentacoordinate Phosphorus in a High‐Pressure Polymorph of Phosphorus Nitride Imide P4N6(NH)

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high‐pressure polymorph of highly condensed phosphorus nitride imide P₄N₆(NH) representing a new framework topology. This is the first example of phosphorus in trigonal‐bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi‐anvil technique. γ‐P₄N₆(NH) has been thoroughly characterized using X‐ray diffraction, solid‐state NMR and FTIR spectroscopy. The synthesis of γ‐P₄N₆(NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high‐pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.     

UF4 and the High-Pressure Polymorph HP-UF4

A laboratory-scale synthesis of UF₄ is presented that utilizes the reduction of UF₆ with sulfur in anhydrous hydrogen fluoride. An excess of sulfur can be removed by vacuum sublimation, yielding pure UF₄, as shown by powder X-ray diffraction, micro X-ray fluorescence analysis, infrared and Raman spectroscopy, as well as magnetic measurements. Furthermore, a single-crystalline, high-pressure modification of UF₄ was obtained in a multi-anvil press at elevated temperatures. The high-pressure polymorph HP-UF₄ was characterized by means of single-crystal and powder X-ray diffraction, as well as by magnetic measurements, and presents a novel crystal structure type. Quantum-chemical calculations show the HP-modification to be 10 kJ mol⁻¹ per formula unit higher in energy compared to UF₄.     

The Synthesis,Full Characterisation and Utilisation of Template‐Free Silica Sodalite,a Novel Polymorph of Silica

Empty glass : Subjecting ethylene glycol silica sodalite to heat (680 °C) under a nitrogen atmosphere (i) successfully removes the templating agent to give cubic silica sodalite, which, upon consequent heating under an oxygen atmosphere (ii), transforms into a rhombohedral form of the empty sodalite, in effect a novel polymorph of silica.

     

Polymorph selection during the crystallization of Yukawa systems

Using molecular-dynamics simulations, we study the crystallization of supercooled liquids of charge-stabilized colloidal suspensions, modeled by the Yukawa (screened-Coulomb) potential. By modifying the value of the screening parameter lambda, we are able to invert the stability of the body-centered cubic (bcc) and face-centered cubic (fcc) polymorphs and study the crystal nucleation and growth in the domain of stability of each polymorph. We show that the crystallization mechanism strongly depends on the value of lambda. When bcc is the stable polymorph (lambda=3), the crystallization mechanism is straightforward. Both kinetics and thermodynamics favor the formation of the bcc particles and polymorph selection takes place early during the nucleation step. When fcc is the stable polymorph (lambda=10), the molecular mechanism is much more complex. First, kinetics favor the formation of bcc particles during the nucleation step. The growth of the post-critical nucleus proceeds through the successive cross-nucleation of the stable fcc polymorph on the metastable hcp polymorph as well as of the hcp polymorph on the fcc polymorph. As a result, polymorph selection occurs much later, i.e., during the growth step, than for lambda=3. We then extend our findings established in the case of homogeneous crystal nucleation to a situation of practical interest, i.e., when a seed of the stable polymorph is used. We demonstrate that the growth from the (111) face of a perfect fcc crystal into the melt proceeds through the same mechanisms.