Crystal polymorphism

In the field of crystallography, polymorphism is a phenomenon where a single chemical compound or element can crystallize into more than one distinct crystal structure. This means that while the chemical identity of the substance remains exactly the same, the physical arrangement of its molecules or atoms in a solid state changes. These different versions of the same substance are known as polymorphs. If a material possesses two polymorphic phases, it is called dimorphic, while three phases make it trimorphic. Understanding polymorphism is vital because these structural differences can significantly alter a material's physical properties. This science is highly relevant to industries such as pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives.

A polymorphic transition is the process by which one solid crystalline phase shifts into another. According to the International Union of Pure and Applied Chemistry (IUPAC), this is a reversible transition occurring at a specific temperature and pressure known as the inversion point. During this transition, the substance stays in a solid state, which distinguishes it from melting or vaporization. The chemical composition does not change during polymorphism. This is different from tautomerism, which involves a chemical change called dynamic isomerization. Instead, polymorphism involves a change in how the molecules are organized. For example, in organic compounds, this often results from conformational polymorphism, where the shape of the molecules themselves changes.

Scientists have developed many ways to detect and identify these different structures. In the 20th century, X-ray crystallography became a primary method for study. Researchers use single crystal X-ray diffraction or powder X-ray diffraction to measure the crystal unit cell. Because every polymorph has a unique crystal structure, each will produce a different X-ray diffraction pattern. Another major method is vibrational spectroscopy, which includes infrared (IR), Raman, and terahertz spectroscopy. These techniques are sensitive to changes in hydrogen bonding patterns and intermolecular interactions. By observing these vibrational modes, scientists can uncover the three-dimensional differences between polymorphs. Today, computational chemistry is also used for crystal structure prediction to model and predict new polymorphs before they are even found in a lab.

The history of studying polymorphism began in the early 1800s. Eilhard Mitscherlich and Jöns Jacob Berzelius conducted early studies on phosphates and arsenates. They measured interfacial angles of crystals to prove that chemically identical salts could exist in different forms. Mitscherlich originally referred to this discovery as isomorphism. Later, in the 1830s, Moritz Ludwig Frankenheim used the microscope to demonstrate how to induce crystal phase changes. Around 1877, Otto Lehmann invented the hot stage for use with a polarized light microscope. This allowed scientists to observe polymorphic transitions and determine melting points by heating the samples. These advancements moved the field from simple observation to precise measurement of heat flow and structural shifts.

Organic compounds provide many fascinating examples of this phenomenon. The compound benzamide has three known polymorphs. The least stable form, orthorhombic form II, is created through flash cooling. The monoclinic form III was observed by Friedrich Wöhler and Justus von Liebig in 1832. The most stable version is monoclinic form I. While their hydrogen bonding is similar, their pi-pi interactions differ strongly. In 2006, a new polymorph of maleic acid was discovered 124 years after the first was studied. This new form was created by dissolving a caffeine and maleic acid co-crystal in chloroform. Other organic compounds show extreme variety; acridine has eight polymorphs, while aripiprazole has nine. The compound known as ROY holds the record for the largest number of well-characterized polymorphs.

Inorganic matter, including elements and metal oxides, also exhibits polymorphism. When an element shows different forms, the term allotropy is used. It is important to note that while some allotropes are polymorphs, not all allotropes are polymorphs. For example, carbon has several allotropes like graphite, diamond, and lonsdaleite. Graphite is not a polymorph of diamond because it is chemically distinct. However, diamond and lonsdaleite are polymorphs because they are chemically identical and only differ in structure. Metals like iron also show this; alpha-iron exists at room temperature, but above 910 degrees, gamma-iron appears. Tin also transitions from white tetragonal beta-tin to gray cubic diamond alpha-tin when cooled below 13.2 degrees.

Binary metal oxides are also highly significant due to their economic value. Silicon dioxide (SiO2) is a famous example, forming many polymorphs such as alpha-quartz, tridymite, cristobalite, coesite, and stishovite. Each of these forms exists under different conditions of temperature and pressure. For instance, alpha-quartz exists under ambient conditions, while stishovite requires much higher pressure. Understanding these complex relationships between chemistry, temperature, and structure allows scientists to predict how materials will behave in different environments, from the human body to industrial factories.