Solid nitrogen

Solid nitrogen is a special form of the element nitrogen that can be found as solid pieces. It was first discovered in 1884. Scientists study it mostly for research, but it also has an important role in space. In the outer parts of our Solar System, solid nitrogen exists in very cold and low-pressure conditions. It is part of planets and other objects there.

In very high temperatures and pressures, solid nitrogen can become a powerful explosive. It holds more energy than any non-nuclear material. This makes it interesting for scientists who study explosions and materials, even though we don’t see it in everyday life.

Generation

In 1884, a scientist named Karol Olszewski discovered solid nitrogen. He used liquid nitrogen to freeze nitrogen and made a very cold temperature of 48 K, which was a record then.

Today, scientists 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. The New Horizons space probe first saw it there in July 2015. It is also present on the moon Triton, which the Voyager 2 space probe observed in August 1989.

Even in very cold places, solid nitrogen can change 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 on Pluto’s surface.

On Triton, solid nitrogen looks like 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. When pressure goes up, the temperature at which nitrogen melts also goes up. This keeps happening up to a pressure of 50 GPa. Scientists think that at this pressure, liquid nitrogen changes its structure. After this pressure, the melting temperature goes down.

Sublimation

Nitrogen can also change from a solid to a gas without becoming a liquid first. This happens at very low pressures, at the triple point. This point is at 63.14±0.06 K and 0.1255±0.0005 bar. At these low pressures, solid nitrogen has 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 with liquid hydrogen. Scientists found that at very cold temperatures, liquid hydrogen can dissolve a few nitrogen molecules. Nitrogen and oxygen can mix when they are liquids but separate when they become solids. This means extra nitrogen or oxygen freezes first, and a special mix of liquid air freezes at even colder temperatures.

Crystal structure

Dinitrogen crystals

Nitrogen forms pairs of atoms called molecules. At low temperatures, these molecules stick together, 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.

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.

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. When it is very cold, it becomes stronger and stiffer. Scientists study these properties to learn how nitrogen behaves on icy worlds far from the Sun.

How 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 when stretched or squeezed. The speed of sound in solid nitrogen also changes with temperature.

Related substances

Under pressure, nitrogen can mix with other molecules to form special structures. It can combine with methane at high pressure and with helium to make a unique compound. Nitrogen also mixes with carbon monoxide, which is found on 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 cool slush. This helps keep tiny living cells safe, like in sperm cryopreservation.

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

Reactions

When solid nitrogen is hit by fast-moving particles, it makes 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.