Solid nitrogen

Solid nitrogen is a special form of the element nitrogen that can be found in solid pieces. It was first discovered in 1884. While it is mostly studied by scientists for research, it also plays an important role in space. In the outer parts of our Solar System, solid nitrogen can be found in very cold and low-pressure conditions, making up parts of planets and other objects there.

In very high temperatures and pressures, solid nitrogen can become a very powerful explosive. It holds more energy than any other material that is not a nuclear bomb. This makes it interesting for scientists who study explosions and materials, even though it is not something we see in everyday life.

Generation

In 1884, a scientist named Karol Olszewski discovered solid nitrogen. He did this by first turning hydrogen into a liquid using liquid nitrogen, and then freezing the nitrogen with that liquid hydrogen. This process also created a very cold temperature of 48 K, which was a world record at the time.

Today, scientists usually make solid nitrogen in labs by letting liquid nitrogen evaporate in a vacuum. The solid nitrogen they create has many tiny holes.

Occurrence in nature

Solid nitrogen is found on the surface of Pluto, where it mixes with solid carbon monoxide and methane. It was first seen there in July 2015 by the New Horizons space probe. It is also present on the moon Triton, observed by the Voyager 2 space probe in August 1989.

Even in very cold conditions, solid nitrogen can change directly into a gas to form an atmosphere, or turn back into frost. It flows like glaciers when there is not much pressure on it. However, it is denser than water ice, so blocks of water ice can float on top of it. The New Horizons probe saw this happening on Pluto’s surface.

On Triton, solid nitrogen appears as frost crystals and a clear layer of ice. Jets of nitrogen gas have been seen erupting from near the south pole. One idea is that sunlight passes through the clear ice layer, warming the layers below. The nitrogen then turns into gas and bursts through holes in the upper layer, carrying dust and creating dark lines.

Transitions to fluid allotropes

Melting

At normal air pressure, solid nitrogen turns into liquid nitrogen at a temperature of 63.23 K. As pressure increases, the temperature at which nitrogen melts also rises. This continues up to a pressure of 50 GPa, where scientists think liquid nitrogen changes its structure. Beyond this pressure, the melting temperature actually goes down.

Sublimation

Nitrogen can also change directly from a solid to a gas without becoming liquid first. This happens at very low pressures, specifically at the triple point, which is at 63.14±0.06 K and 0.1255±0.0005 bar. At these low pressures, solid nitrogen exists in two different forms: α-nitrogen (below 35 K) and β-nitrogen (between 35 K and 63 K).

Solubility in common cryogens

Solid nitrogen can mix a little bit with liquid hydrogen. Scientists figured out that at very cold temperatures, liquid hydrogen can dissolve a small number of nitrogen molecules. Nitrogen and oxygen can mix when they are liquids but separate when they become solids. This means that extra nitrogen or oxygen will freeze first, and a special mix of liquid air will freeze at even colder temperatures.

Crystal structure

Dinitrogen crystals

Nitrogen forms pairs of atoms called molecules. At low temperatures, these molecules stick together because of weak forces, forming solid nitrogen.

Solid nitrogen can change shape depending on temperature and pressure. Below a certain temperature, it forms a cube-like shape. As it gets warmer, it changes into a hexagonal shape, where the molecules tilt at an angle.

There are many other special shapes nitrogen can take under very high pressure. Some of these shapes are very stiff and hold a lot of energy, which makes them interesting for scientific studies.

Oligomer crystals

Scientists have made nitrogen form chains of atoms under very high pressure and heat. These chains can stay stable even when the pressure is lowered.

Amorphous and network allotropes

Under extreme pressure, nitrogen can form shapes without a clear pattern. These shapes can act like semiconductors or even metals. One special form, called cubic-gauche nitrogen, is very stiff and holds a lot of energy, making it interesting for use in powerful fuels.

Others

There are many other special shapes that nitrogen can form under different conditions.

Bulk properties

Solid nitrogen has different strengths and behaviors depending on its temperature. At very cold temperatures, it becomes stronger and stiffer. Scientists study these properties to understand how nitrogen behaves in places like icy worlds far from the Sun.

The way solid nitrogen lets heat move through it also changes with temperature. It looks clear when very cold and white when even colder. Its density is close to that of water, and it can change shape in different ways depending on how much it is stretched or squeezed. The speed at which sound travels through solid nitrogen also varies with temperature.

Related substances

Under pressure, nitrogen can combine with other molecules to form special structures. It can mix with methane above a certain pressure and with helium to create a unique compound. Nitrogen also forms mixtures with carbon monoxide, which is found on places like Pluto and Triton.

Nitrogen can include atoms from noble gases like neon, xenon, and argon in its solid forms. With dideuterium, it can form another special mixture. Nitrogen can also mix with oxygen while keeping its shape.

Use

Solid nitrogen can be mixed with liquid nitrogen to make a slush that cools things faster. This is helpful for keeping tiny living cells safe, like in sperm cryopreservation.

Scientists also use solid nitrogen to hold and study very reactive chemicals, such as free radicals or single atoms. This helps them learn about special combinations of metals called dinitrogen complexes.

Reactions

When solid nitrogen is hit by fast-moving particles, it creates several reactive forms. These include single nitrogen atoms, positively charged nitrogen, a type of nitrogen made of two atoms, groups of three nitrogen atoms, and a special form called azide.