Polymorphism (materials science)

In materials science, polymorphism describes the existence of a solid material in more than one form or crystal structure. Polymorphism is a form of isomerism. Any crystalline material can exhibit the phenomenon. Allotropy refers to polymorphism for chemical elements. Polymorphism is of practical relevance to pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives. According to IUPAC, a polymorphic transition is "A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point) to another phase of the same chemical composition with a different crystal structure." According to McCrone, polymorphs are "different in crystal structure but identical in the liquid or vapor states." Materials with two polymorphs are called dimorphic, with three polymorphs, trimorphic, etc. In some cases, polymorphism was "discovered" on a computer by crystal structure prediction first, before chemists actually synthesize the crystal in the

Direct Visualization of Atomic-Scale Heterogeneous Structure Dynamics in MnO2 Nanowires

Xin Peng, Yuxi Zhang, Kangning Zhao

Manganese oxides are attracting great interest owing to their rich polymorphism and multiple valent states, which give rise to a wide range of applications in catalysis, capacitors, ion batteries, and so forth. Most of their functionalities are connected to transitions among the various polymorphisms and Mn valences. However, their atomic-scale dynamics is still a great challenge. Herein, we discovered a strong heterogeneity in the crystalline structure and defects, as well as in the Mn valence state. The transitions are studied by in situ transmission electron microscopy (TEM), and they involve a complex ordering of [MnO6] octahedra as the basic building tunnels. MnO2 nanowires synthesized using solution-based hydrothermal methods usually exhibit a large number of multiple polymorphism impurities with different tunnel sizes. Upon heating, MnO2 nanowires undergo a series of stoichiometric polymorphism changes, followed by oxygen release toward an oxygen-deficient spinel and rock-salt phase. The impurity polymorphism exhibits an abnormally high stability with interesting small-large-small tunnel size transition, which is attributed to a preferential stabilizer (K+) concentration, as well as a strong competition of kinetics and thermodynamics. Our results unveil the complicated intergrowth of polymorphism impurities in MnO2, which provide insights into the heterogeneous kinetics, thermodynamics, and transport properties of the tunnel-based building blocks.

AMER CHEMICAL SOC2021

Two new polymorphic structures of human full-length alpha-synuclein fibrils solved by cryo-electron microscopy

Ricardo Cesar Guerrero Ferreira, Pratibha Kumari, Henning Paul-Julius Stahlberg

Intracellular inclusions rich in alpha-synuclein are a hallmark of several neuropathological diseases including Parkinson’s disease (PD). Previously, we reported the structure of alpha-synuclein fibrils (residues 1–121), composed of two protofibrils that are connected via a densely-packed interface formed by residues 50–57 (Guerrero-Ferreira, eLife 218;7:e36402). We here report two new polymorphic atomic structures of alpha-synuclein fibrils termed polymorphs 2a and 2b, at 3.0 Å and 3.4 Å resolution, respectively. These polymorphs show a radically different structure compared to previously reported polymorphs. The new structures have a 10 nm fibril diameter and are composed of two protofilaments which interact via intermolecular salt-bridges between amino acids K45, E57 (polymorph 2a) or E46 (polymorph 2b). The non-amyloid component (NAC) region of alpha-synuclein is fully buried by previously non-described interactions with the N-terminus. A hydrophobic cleft, the location of familial PD mutation sites, and the nature of the protofilament interface now invite to formulate hypotheses about fibril formation, growth and stability.

2019

Controlling PbI2 Stoichiometry during Synthesis to Improve the Performance of Perovskite Photovoltaics

Vikas Kumar, Mohammad Khaja Nazeeruddin

Over the past decade, remarkable progress has advanced the field of perovskite solar cells to the forefront of thin film solar technologies. The stoichiometry of the perovskite material is of paramount importance as it determines the optoelectronic properties of the absorber and hence the device performance. However, little published work has focused on the synthesis of fully stoichiometric precursor materials of high purity and at high yield. Here, we report a low-cost, energy-efficient, and solvent-free synthesis of the lead iodide precursor by planetary ball milling. With our synthetic approach, we produce low-oxygen, single or multiple polytypic phase PbI2 with tunable stoichiometry. We determine the stoichiometry and the polytypes present in our synthesized materials and further compare them to commercially available materials, using X-ray diffraction, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. Both the stoichiometric PbI2 we synthesized and a substoichiometric commercially available PbI2 (where the iodide content is below the optimum Pb:I atomic ratio of 1:2) were used to grow methylammonium lead iodide microcrystals (which corrects the iodide content). Perovskite solar cells were then produced using stoichiometric and substoichiometric PbI2 mixed with an equimolar amount of methylammonium iodide and compared to devices produced from re-dissolved microcrystals. The photoactive perovskite layer deposition was processed in air, enabled by the use of a single low-toxicity solvent (dimethyl sulfoxide) combined with vacuum-assisted solvent evaporation. We find that the device performance is strongly dependent upon the stoichiometry of the lead iodide precursor, reaching champion efficiencies over 17%, with no obvious correlation with its polytypic phases. This work highlights the critical role of PbI2 stoichiometry in hybrid perovskites as well as demonstrating synthesis methods and perovskite layer fabrication protocols suitable for low-cost solar energy harvesting.

2021

Controlling PbI2 Stoichiometry during Synthesis to Improve the Performance of Perovskite Photovoltaics

Vikas Kumar, Mohammad Khaja Nazeeruddin

2021