Crystal polymorphism

Some things can change shape. They stay the same inside. But they look different. They can be hard or soft.

This happens when tiny parts move. The parts stay the same. But they line up in new ways. This can change how the thing looks.

Heat can cause this change. High pressure can do it too. Some things have two shapes. Others have three or more.

Scientists study these shapes. They use special tools like light. They also use X-rays to see.

Knowing these shapes is helpful. It helps us make good food. It also helps us make medicine.

Some things can change their shape. They stay the same on the inside. This is called polymorphism. It means a substance can make different crystal structures.

In this way, the parts stay the same. They just line up in new ways. Heat or pressure can cause these changes. For example, iron has different forms. It changes based on how hot it is. Carbon is another example. Diamond and lonsdaleite are polymorphs. They have the same parts but different shapes.

Scientists use many tools to study these shapes. They use X-ray crystallography. This uses X-rays to see how parts are arranged. They also use light through a microscope. This helps them see the changes as they happen. Some people use computers to predict new shapes. This is called crystal structure prediction. Knowing these shapes is very useful. It helps people make better medicine, food, and dyes.

Have you ever thought about how things are built? Most substances have a specific way their tiny parts line up to make a solid. This is called a crystal structure. Sometimes, the exact same substance can arrange itself in more than one way. This amazing phenomenon is called polymorphism. It means a single compound can form different types of crystals. These different versions are called polymorphs. It is important to know that the chemistry does not change. The substance stays the same, but its physical shape is different. This can change how a material looks or works.

How does this change actually happen? It usually works because of changes in temperature or pressure. Scientists call these shifts polymorphic transitions. When the heat or pressure reaches a certain point, called an inversion point, the crystal structure shifts. The molecules move into a new pattern while staying the same substance.

This is a reversible process, meaning it can go back and forth. For example, if you change the temperature, the crystals might switch from one shape to another. This is different from melting, where the solid turns into a liquid. In polymorphism, the substance stays a solid throughout the change.

People have been studying these crystal shapes for a long time. In the early 1800s, Eilhard Mitscherlich and Jöns Jacob Berzelius studied phosphates. They measured angles in crystals to show that identical salts had different forms. Later, in the 1830s, Moritz Ludwig Frankenheim used a microscope to study these changes. Around 1877, Otto Lehmann invented a hot stage for microscopes. This tool allowed scientists to watch crystals change while they heated them up. These early discoveries helped us understand how much heat causes a shift. Over many years, these tools became much more advanced.

Today, scientists use very powerful tools to see these tiny structures. One main method is X-ray crystallography. This uses X-rays to find the unique pattern of each polymorph. They also use vibrational spectroscopy, which looks at how molecules move. This includes tools like Raman spectroscopy and infrared spectroscopy. These methods can show how molecules are bonded together. Even computers are used now for crystal structure prediction. This helps scientists model and find new crystal shapes before they even see them in a lab.

Polymorphism is all around us in science and industry. It is very important for making medicines, foods, and dyes. For instance, the organic compound benzamide has three different crystal forms. Another example is carbon, which is a nonmetal. Carbon can form diamond and lonsdaleite, which are polymorphs of each other. However, graphite is a different type of carbon and is not a polymorph of diamond. Metals like iron also show this. Iron has three forms that change based on how hot the metal is. Understanding these shapes helps us make better products every day.

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.