Phosphorus Family: a Tale of Isomorphous Crystallization

PHOSPHORUS IN LIFE AND AIRTIST

In fact, the blue flame inside the ladybird is the light emitted by the "phosphorus" inside the corpse, which causes the spontaneous combustion of the human body (of course, the literature uses exaggeration).

Those of you who enjoy reading supernatural and thriller novels are probably familiar with will-o'-the-wisps. In reality, I actually encountered them when I was a child on rural roads during dusk. They would even follow me as I walked, and it scared me quite a bit (I was timid, belonging to the category of both being easily frightened and an adventurous spirit). As a dedicated fan of horror novels, I can assure you that these experiences added to my fear.However, knowledge has given me courage, and now I know that this phenomenon is also caused by the spontaneous combustion of "phosphorus."

This combustible nature of "phosphorus" can certainly lead to safety incidents. In recent years, there have been several cases of white phosphorus self-igniting, causing fires. Although these incidents didn't result in significant casualties, the subsequent release of "acidic fog" into the environment is not favorable. It has made people anxious, and we should certainly be cautious of its existence.

How is it that matches can ignite with a simple strike? This is also due to the presence of red phosphorus in the "striking material" on the sides of matchboxes. So, it seems that phosphorus has indeed permeated our lives and artistic works. Even if you might not have seen its "true face" in person, you've likely memorized the periodic table of elements in middle school chemistry more than once. Today, I'm here to unveil the true nature of phosphorus and see what this rather "explosive" element is really like.

Classification of Phosphorus

In the phosphorus family, there are several members: white phosphorus, red phosphorus, and black phosphorus. They are all allotropes of phosphorus, meaning they have different molecular structures, resulting in different crystal configurations, and ultimately exhibiting different physical and chemical properties. Next, let's introduce these members one by one.

Isomerism refers to the phenomenon where a single chemical element can form different substances with distinct physical and chemical properties due to variations in arrangement. For example, oxygen and ozone, or diamond and graphite, are isomers of the same chemical element.

Polymorphism, on the other hand, is the occurrence where substances with the same chemical composition can crystallize into two or more distinct crystal structures under different physical and chemical conditions.

White phosphorus

White phosphorus, whether in a solution or gaseous state, exists in the form of P4 molecules, forming a molecular crystal structure. Pure white phosphorus is a colorless, transparent crystal, but it undergoes a weak oxidation reaction upon exposure to light, leading to the formation of complex phosphorus oxides, which turn it yellow. Therefore, it is also known as yellow phosphorus.

The discovery of the element phosphorus can be traced back to the medieval period in Europe during the practice of alchemy. During that time, many alchemists were fervently searching for the mythical "philosopher's stone" that could transmute base metals into gold. One such figure was a merchant named Hennig Brand, who collected 50 barrels of human urine and subjected it to a heating process along with other substances, hoping to create gold. Instead of gold, he obtained a white waxy substance that emitted a blue-green glow in the dark. He named this new substance "phosphorus," derived from the Latin word "Phosphorum," meaning "light-bringer" or "bearer of light," due to its ability to produce cold light.

White phosphorus is the most reactive and volatile member of the phosphorus family. This is because its molecular structure consists of four phosphorus atoms arranged in a tetrahedron, theoretically forming bond angles of 60°. However, in reality, about 98% of the P-P bonds in its molecule are formed by the overlap of 3p orbitals of adjacent phosphorus atoms, resulting in covalent bonds. The typical bond angle for p-orbital overlap is 90°, so to approach 60°, the P-P bonds experience significant strain and become highly unstable. As a result, white phosphorus exhibits very high chemical reactivity, igniting at just 40°C and self-igniting in air, producing dense smoke. Due to this property, it cannot be easily analyzed using X-ray diffraction at room temperature.

(Polymorphism of white phosphorus)

White phosphorus exists in three different crystal forms, known as α-phase, β-phase, and γ-phase, depending on temperature and pressure conditions. At room temperature, it typically exists in the α-phase, which has a structure similar to the α-phase of metallic manganese. The α-phase belongs to the cubic crystal system, and each unit cell contains 58 P4 tetrahedra. In this phase, each P4 molecule undergoes dynamic rotation around its center, leading to its needle-like appearance under a microscope.

(The image shows the crystal structure of metal manganese in its α-phase. The P4 molecules replace each metal manganese atom to form the α-phase structure of white phosphorus.)

Bridgman discovered that the α-phase of white phosphorus can transform into the β-phase under specific conditions. This transformation is reversible, and under appropriate conditions, it can revert to the normal α-phase of white phosphorus. In the β-phase, the arrangement of P4 molecules is more ordered, but it has reduced symmetry compared to the α-phase. Similar to the α-phase, its structure is closely related to the structure of metallic elements, resembling the γ-phase structure of metal plutonium. This phase belongs to the orthorhombic crystal system.

(The image shows the crystal structure of β-phase white phosphorus, where the P4 molecules replace each metal plutonium atom to form the β-phase structure of white phosphorus.)

Spiess and colleagues discovered the existence of another crystal structure of white phosphorus - the γ-phase. It forms when the α-phase is quenched to -165 °C and held at that temperature for several hours. This is also a reversible phase transition, and when warmed back to -115 °C, the γ-phase transforms into the β-phase. However, cooling again does not yield the γ-phase. The unit cell belongs to the monoclinic crystal system, with the centers of P4 molecules forming a twisted hexagonal lattice. Each hexagon adopts a chair-like conformation, and these hexagonal lattices stack in the c-direction, resulting in a distorted body-centered cubic structure, as shown in the image below:

(The picture shows the crystal structure of γ-type white phosphorus)

White phosphorus, the most reactive and volatile member of the phosphorus family, has several applications, including the production of smoke bombs and incendiary devices. It is highly toxic and can be absorbed through the skin. In case of contact, immediate rinsing with a large amount of water is crucial, followed by treatment with copper sulfate or silver nitrate solutions. If accidentally ingested, seek medical attention promptly. In industry, it is used to produce high-purity phosphoric acid, red phosphorus, rodenticides, and organophosphate compounds.

Red phosphorus

Red phosphorus, on the other hand, is less reactive and stable at room temperature. It requires higher temperatures, around 240°C, to ignite. Unlike white phosphorus, it is not toxic. Red phosphorus was discovered in 1845 and is considered an allotrope of phosphorus. Its structure is more complex than that of white phosphorus, and there is no specific molecular formula for it. Instead, it is often represented by the chemical symbol "P."

(The Polymorphism of Red Phosphorus)

Red phosphorus exhibits polymorphism, meaning it can exist in different forms. Researchers have identified at least five different polymorphs of red phosphorus labeled as types I to V. Type I is the amorphous or non-crystalline form of red phosphorus, and the other four types (II, III, IV, and V) can be obtained by annealing or heating type I red phosphorus to specific temperatures. Each polymorph has its own unique crystal structure.

(I-type, amorphous red phosphorus, chain trapezoidal structure, proposed by Haser in 1990)

Type II red phosphorus exhibits a helical chain structure composed of P8-P4 or P10-P2 rings. This structure is similar to the recently discovered polycrystalline phosphorus nanorods.

Type IV red phosphorus, also known as violet phosphorus, is characterized by a tubular structure composed of P2-P8-P2-P9 rings. It was first discovered by Hittorf in 1865.

(V-type phosphorus, also known as fibrous phosphorus, represents one of the unique allotropes of phosphorus. It is characterized by a layered or laminar structure formed by repeating cycles of P2-P8-P2-P9.)

Note: The III-phase crystal structure of phosphorus has not been definitively confirmed or characterized.

The early matchbox side rub material was originally made of white phosphorus, but it was easy to spontaneous combustion and unsafe, and the discovery of red phosphorus realized the perfect replacement and has been used until now. It can also be used to produce flame retardants, organophosphorus pesticides and so on. In addition, red phosphorus also has good application prospects in the field of optoelectronics: The amorphous red phosphorus film synthesized by high-energy ultrasonic technology can be used as the anode of lithium-ion battery, with a theoretical specific capacity of 2137 mA·h·g-1. After adding reduced graphene oxide material, even after 200 cycles, The specific capacity of the battery can still reach 706 mA·h·g-1 (while the theoretical specific capacity of the general graphite negative electrode is only 372 mA·h·g-1). Crystalline red phosphorus (such as fiber phosphorus, nano phosphorus, purple phosphorus, etc.) can also be used as electrode materials, or as semiconductor materials with good performance. The conversion of amorphous red phosphorus to crystalline red phosphorus was achieved by metal-bismuth catalysis, elements-assisted chemical vapor transfer reaction and mild solvothermal synthesis.

When white phosphorus is heated or irradiated by ultraviolet light or heated to 273 ° C under isolated air, it will be converted to red phosphorus, and red phosphorus will be sublimed and condensed into white phosphorus under isolated air heated to 416 ° C.

Black phosphorus

Black phosphorus is the most stable big brother in the home, the best temper and the most calm, it is a black crystal with a metallic luster, can conduct electricity, the ignition point is 490 ° C, and it is usually difficult to react chemically. In 1914, the American physicist Bridgman first synthesized black phosphorus with white phosphorus under high pressure and high temperature conditions. Like red phosphorus, it has an extremely complex spatial structure, so it can only use P as its chemical formula.

(Crystal structure of black phosphorus)

Black phosphorus crystals are formed by double folds and belong to the orthorhombic system, where each layer consists of a parallel zigzag chain of interconnected phosphorus atoms. It has a graphite-like sheet structure (wavy layer structure), and each phosphorus atom in the structure has 5 valence electrons in the 3p orbital, which are saturated by forming covalent bonds, resulting in sp3 hybridization. Where the two covalent bonds of adjacent phosphorus atoms are in the same plane, and the third bond connects the above/below phosphorus atoms, thus connecting the top/bottom parts of the double layer together, as shown below:

(The image above illustrates the crystal structure of black phosphorus)

Due to its similarity in structure to graphite, black phosphorus shares some physical properties and applications. It can conduct electricity and is used in the production of electronic materials such as field-effect transistors and sensors. Black phosphorus is also employed in various applications, including water splitting, photovoltaic solar cells, and photodetectors. Like red phosphorus, it can be directly utilized as an anode material in lithium-ion batteries, offering a theoretical specific capacity of 2596 mA·h·g-1, which helps enhance the energy density of batteries. However, the long-standing challenge with black phosphorus is its air stability; it degrades when exposed to the environment, forming phosphorus oxides on the surface, eventually converting to phosphoric acid. To address this issue, researchers are increasingly exploring the combination of black phosphorus with various carbon allotropes to create more stable nanocomposite materials.

(An image depicting the application of black phosphorus in lithium-ion batteries)

Currently, there are several methods available to achieve the transformation of white phosphorus or red phosphorus into black phosphorus. These methods often involve controlling factors such as temperature, pressure, and heating/cooling rates to achieve the desired crystal formation. Additionally, mechanical methods and non-metallic catalysis can also be employed to produce black phosphorus at lower temperatures and pressures, eliminating the need for high-temperature, high-pressure conditions.

(Summary of methods for synthesizing black phosphorus starting from white phosphorus/red phosphorus)

SUMMARY

By now, you should have a basic understanding of the "appearance and temperament" of each member of the Phosphorus family. Using science to conquer the fear of the unknown, I believe that when you find yourself walking alone on a rural road at night and encounter the eerie phosphorescent glow of phosphorus, you won't let your imagination run wild and instead, you'll keep a steady pace.

While there are indeed some "troublemakers" with bad tempers in the family, every member of the Phosphorus family can be useful under the right conditions, bringing us more convenience in our technological lives in their unique ways.