Polymorphs with Remarkably Distinct Physical and/or Chemical Properties

Polymorphism in crystals is known since 1822 and the credit goes to Mitscherlich who realized the existence of different crystal structures of the same compound while working with some arsenate and phosphate salts. Later on, this phenomenon was observed also in organic crystals. With the advent of different technologies, especially the easy availability of single crystal XRD instruments, polymorphism in crystals has become a common phenomenon. Almost 37 % of compounds (single component) are polymorphic to date. As the energies of the different polymorphic forms are very close to each other, small changes in crystallization conditions might lead to different polymorphic structures. As a result, sometimes it is difficult to control polymorphism. For this reason, it is considered to be a nuisance to crystal engineering. It has been realized that the property of a material depends not only on the molecular structure but also on its crystal structure. Therefore, it is not only of interest to academia but also has widespread applications in the materials science as well as pharmaceutical industries. In this review, we have discussed polymorphism which causes significant changes in materials properties in different fields of solid-state science, such as electrical, magnetic, SHG, thermal expansion, mechanical, luminescence, color, and pharmaceutical. Therefore, this review will interest researchers from supramolecular chemistry, materials science as well as medicinal chemistry.

Solid-State Fluorescence of A Quasi-Isostructural Polymorphic Biphenyl Based Michael Addition Product

Polymorphic materials have gained significant attention owing to their fascinating physicochemical properties. Herein, a biphenyl based Michael addition product (Compound A) with an active methylene group (dimedone) was synthesized. Compound...

Phase Transformation of a K2GeF6 Polymorph for Phosphors Driven by a Simple Precipitation-Dissolution Equilibrium and Ion Exchange

Tuning crystal phase transformations is very important for obtaining polymorphs for phosphors with the ideal optical properties and stability. Mn⁴⁺-doped K₂GeF₆ (KGF) is a typical polymorphic phosphor, but the thermodynamic and kinetic mechanism of its phase transformation is still unclear. Herein, the phase transformation of polymorphs varying from P6₃mc KGF and trigonal KGF to P6₃mc Si⁴⁺-doped KGF is realized by introducing the synergistic action of an HF solution and Si⁴⁺ ions. The full structural refinements of KGF polymorphs at room temperature and the electronic band structure calculations were performed. The results show that the Si⁴⁺-doped hexagonal KGF polymorph with good photoluminescence properties is the most stable phase according to the calculated total energy landscape and relative formation energy. The morphologic changes were monitored in situ to clearly understand the rapid phase transformation mechanism, which proves that the phase transformation is driven by a simple precipitation-dissolution equilibrium and ionic exchange.