What Is the Melting Point of Diamond?

Diamonds are renowned for their exceptional hardness and thermal conductivity, but their behavior at extreme temperatures is less commonly discussed. Unlike most materials that transition predictably from solid to liquid, diamond’s melting point depends heavily on pressure, making it a complex subject of study.

Understanding how diamond responds to high temperatures provides insights into its stability, potential applications, and the fundamental properties of carbon-based materials.

Thermal Threshold at Different Pressures

The melting point of diamond is not fixed but varies with pressure. At ambient pressure, diamond does not transition directly to a liquid; instead, it sublimates at approximately 3,600 to 3,800°C, as carbon atoms gain enough energy to break free from the lattice and disperse as gas. However, under extreme pressures, diamond can melt before becoming a gas.

Experiments using laser-heated diamond anvil cells show that diamond melts at pressures exceeding 10 GPa, with temperatures rising to 4,000–4,500°C. At around 100 GPa, conditions similar to Earth’s deep mantle, diamond may remain solid even at 5,000°C. High-pressure phase diagrams confirm that diamond is the most stable form of carbon under extreme conditions, requiring both high temperature and pressure to liquefy.

This relationship between pressure and melting behavior has implications for planetary science, particularly in understanding the interiors of carbon-rich exoplanets and gas giant cores. In environments with extreme pressures, diamond may exist in a partially molten state, affecting planetary thermal and mechanical properties. Studies suggest that liquid carbon at high pressures exhibits metallic characteristics, influencing conductivity and magnetic fields in celestial bodies.

Laboratory Techniques for High-Temperature Measurement

Determining diamond’s melting point requires sophisticated experimental setups capable of generating extreme conditions while maintaining precise measurements. Traditional furnaces are inadequate, so researchers use techniques such as laser-heated diamond anvil cells (LHDACs), high-power resistive heating, and shock compression. Each method presents challenges, particularly in maintaining controlled environments and preventing contamination.

The laser-heated diamond anvil cell is a key tool for high-temperature studies. A small diamond sample is placed between two opposing diamond anvils, generating pressures exceeding 100 GPa. A high-powered laser then heats the sample, raising its temperature to several thousand degrees Celsius. Spectroradiometry, which analyzes thermal radiation, is used to monitor temperature changes, though calibration is crucial to avoid errors. A chemically inert environment, often using argon or neon, prevents unwanted reactions that could alter results.

Dynamic compression techniques, such as shock wave experiments with high-velocity impactors or laser pulses, provide another approach. These methods generate transient high-pressure, high-temperature conditions lasting only microseconds, allowing researchers to capture phase transitions in real time. Optical pyrometry, which measures emitted light intensity, is commonly used for temperature determination. However, the short timescales make it difficult to distinguish equilibrium melting from metastable phase transitions.

Structural Changes in the Carbon Lattice

At extreme temperatures, diamond’s atomic structure begins to change. The rigid, three-dimensional network of sp³-hybridized carbon atoms, responsible for its hardness, starts to lose integrity as thermal energy disrupts covalent bonds. This process does not occur uniformly; localized bond weakening leads to gradual disorder and a breakdown of the crystalline framework.

At high pressures, diamond can exhibit signs of graphitization, where carbon atoms rearrange into sp²-hybridized configurations, forming layers characteristic of graphite. Unlike diamond’s dense tetrahedral structure, graphite consists of planar sheets held together by weak van der Waals forces, making it much softer. Raman spectroscopy detects this transition by identifying characteristic graphite peaks, showing that even before complete melting, diamond can partially degrade.

The nature of these structural changes depends on temperature and pressure. At lower pressures, diamond tends to revert to graphite before melting, whereas under extreme pressures, it retains its tetrahedral bonding longer. Computational simulations using density functional theory reveal that as thermal energy increases, bond lengths fluctuate, and defects in the lattice emerge, acting as precursors to larger-scale restructuring.

Observations of Diamond in a Liquid Phase

Observing liquid diamond is challenging due to the extreme conditions required to induce melting. Unlike many materials that transition smoothly from solid to liquid, diamond’s behavior is complex. At temperatures above 4,000°C and pressures exceeding 10 GPa, its rigid lattice collapses, and carbon atoms shift into a densely packed, disordered state.

Advanced experimental setups, including laser-heated diamond anvil cells and shock compression techniques, have provided glimpses into this phase. Spectroscopic data indicate that liquid carbon at extreme pressures exhibits metallic properties, meaning it conducts electricity similarly to molten metals. This suggests that in astrophysical environments, such as carbon-rich exoplanets, liquid diamond could influence magnetic field generation. High-speed imaging has captured rapid transitions between solid and liquid states, showing that diamond can melt and recrystallize almost instantly as pressures fluctuate.

Comparison With Other Carbon Allotropes

Diamond’s behavior at extreme temperatures contrasts with other carbon allotropes, which have distinct phase transitions. Graphite, the most thermodynamically stable form of carbon at standard conditions, melts at approximately 3,600°C under normal atmospheric pressure. Unlike diamond, which sublimates at ambient pressure, graphite transitions into a liquid without requiring extreme conditions.

Fullerenes, such as C₆₀, decompose rather than melt, breaking apart into smaller carbon fragments at high temperatures. Carbon nanotubes, composed of sp²-hybridized carbon, exhibit high thermal stability, with degradation temperatures exceeding 3,500°C. Their cylindrical structure influences their thermal resilience, making them less prone to direct melting. Amorphous carbon, including soot and glassy carbon, lacks a well-defined lattice and undergoes gradual structural reorganization instead of a sharp phase transition.