Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations

Have you ever caught one of those diamonds are forever ads when you were younger? They had this way of showing diamonds sparkling like nothing else, making the phrase stick with you. I was just a kid, but deep down, I always had my doubts. Nothing really lasts forever, right? The universe doesn’t do perpetual motion; entropy only goes up, leading everything eventually into the abyss of a black hole, only to dissipate as Hawking radiation. Later on, when I started my studies in chemistry, I learned that diamonds are just a precious arrangement of carbon atoms. But here’s the kicker: they’re metastable. It’s a typical example of thermodynamics and kinetics at play. Thermodynamically, diamonds aren’t the most stable form of carbon—that honor goes to graphite, yes pencils. But thanks to kinetics, the rate at which they transform is incredibly slow, letting diamonds stick around for billions of years. So, how does this transformation happen? How does carbon decide to become a diamond and then, after eons, to switch back to graphite? Well, that’s what we’re here to explore today. Following the study of Liang and team, we’re about to dive into some molecular dynamics simulations that peel back the layers on the graphite to diamond phase transition. Ready to get a closer look at how nature pulls off one of its coolest tricks? Let’s go.

Exploring Diamond: A Closer Look at Carbon’s Masterpiece

Diamonds aren’t just a way to say I love you with a bit of a sparkle. They stand as nature’s nod to the elegance of high-pressure physics and chemistry, showcasing true marvels of material science.

Made entirely from carbon atoms arranged in what’s known as a cubic crystal structure, diamonds hit the peak of natural engineering.

The true allure of diamonds lies in their remarkable hardness—a feature that has made them indispensable in various industrial applications. We’re talking about cutting tools that breeze through the hardest materials and precision instruments for polishing that rely on diamonds. The secret behind this unbeatable toughness? It lies in the diamond’s structure, where each carbon atom is tetrahedrally bonded to four others, creating a robust three-dimensional lattice unparalleled in its resilience.

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Schematic animation of graphite converting into diamond | Credit

Diamonds are also top-notch thermal conductors, a feature that’s gold dust in electronics and manufacturing. This standout thermal conductivity, paired with their ability to interact with light thanks to a high refractive index and significant optical dispersion, is what makes diamonds so dazzling and sought after as gemstones.

And there’s more. Diamonds tell tales of Earth’s past. Born from the extreme squeeze and heat deep within Earth’s mantle, diamonds are relics, some as old as 1 billion to 3.5 billion years. They make their grand entrance from more than 150 kilometers below the surface, journeying through volcanic eruptions to finally be found and cherished by us.

Here’s where it gets really interesting—synthesizing diamonds is no longer just Mother Nature’s party trick. Yep, humans have cracked the diamond-making code, bringing a bit of the Earth’s deep magic right to the surface.

I know what you’re thinking… but the synthesis of diamond is not an easy feat.

Though attempts to synthetically create diamonds began as early as 1823, it wasn’t until the 1940s that the U.S., Sweden, and the Soviet Union really got the ball rolling. This led to the first reproducible synthesis of diamonds in 1954. Since then, we’ve seen the rise of high pressure high temperature and chemical vapor deposition methods, which are still at the forefront of synthetic diamond production today. And let’s not forget about detonation synthesis, which burst onto the scene in the late ’90s, creating diamonds from carbon-containing explosives.

The scientific endeavor to craft diamonds in labs essentially speeds up Mother Nature’s process, showcasing our knack for replicating her wonders. But, to get to the heart of the matter—how does this transformation occur? How does ordinary carbon, the same material found in your pencil’s graphite, transform into a sparkling diamond?

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Diamonds are brittle, but raising the temperature can soften and deform their crystal structure. Molecular dynamics simulation of a diamond fracture occurring at high temperatures | Credit

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Graphite To Diamond: A Harsh Phase Transition

Graphite—yep, the stuff in your everyday pencil—is carbon’s most stable form under normal conditions. It’s composed of stacked graphene layers. Each of these layers is a hexagonal lattice of carbon atoms, held together by van der Waals forces, making graphite an excellent lubricant and electrode material, among other applications. Despite its stability, when subjected to the precise blend of high pressure and temperature, graphite undergoes a remarkable transformation, emerging as diamond.

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Layers of graphene bound together via van der Waals interactions. Graphite model in QuantistryLab

This phase transition, mapped out through theoretical calculations and experimental evidence, shows a dynamic interplay of pressures and temperatures—from 1.7 GPa at 0 K to 12 GPa at 5000 K. But it’s not straightforward; a wide coexistence region around this equilibrium point complicates matters.

At room temperature and pressure, diamond exists in a metastable state, with its transition back to graphite happening at an extremely slow clip. Push the temperature above roughly 4500 K, and diamond’s stability wanes, making its way back to graphite. Achieving the reverse, however, asks for pressures well beyond the equilibrium line—think around 35 GPa at 2000 K, highlighting the extreme lengths we go to for diamond creation.

Here’s a quick detour for a fascinating fact I can’t help but share. What I find very intriguing is that under conditions similar to those within Neptune and Uranusplanets rich in carbondiamonds might melt into a metallic fluid (imagine the landscape!). This hypothesis could shed light on cosmic mysteries, including the misalignment of these planets’ geographic and magnetic poles.

So, the phase transition from graphite to diamond is a glimpse into the extremes governing carbon’s behavior, stretching from the Earth’s mantle to the vastness of space. Now, despite leaps in understanding and capability, crafting diamonds from graphite remains a sophisticated challenge. Yet, about twenty years ago, we hit a significant milestone with the successful synthesis of polycrystalline diamond from high-purity graphite, using ultra-high pressures and temperatures.

The resulting diamonds, with their intricate mix of internal structures, continue to puzzle and fascinate. Initially thought to form through diffusion and martensitic transformations, ongoing research hints at even deeper complexities.

So here we are, armed with knowledge yet brimming with questions: What precise conditions truly optimize the synthesis of polycrystalline diamond from high-purity graphite? In what ways do the mechanisms behind polycrystalline diamond’s internal structures affect its applications and characteristics? And what are the deep-down microscopic mechanisms that enable the transition from graphite to diamond?

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Water droplet on rigid graphite. Molecular Dynamics simulations | Credit

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Molecular Dynamics Simulations: From Graphite To Diamond

To answer those questions, a variety of methods have been applied at the experimental level. But as regular readers of From Atoms To Words know, I have a penchant for delving into simulations and the like. So, let’s talk molecular dynamics.

These simulations have really come into their own as a key player in unraveling unanswered puzzles in a variety of fields. On this blog, we have explored how they can be employed to study SARS-CoV-2 and DNA, or to understand complex processes like atomic layer deposition or catalysis.

Remember how they work, right? Well, let’s do a little refresher.

Molecular dynamics serves as a high-resolution computational microscope, zooming in on every single atom within a system to play out its dynamic interactions over time, grounded in a set of physical laws and parameters unique to each atom type. This detailed movie of the microscopic world requires a meticulously prepared script, the forcefield, which dictates how each atom—carbon, oxygen, or any other—behaves, allowing you to explore the complex dynamics of molecules from proteins to polymers with clarity and atomistic depth.

It turns out that molecular dynamics can also provide significant insights into the phase transition from graphite to diamond. Initially, accurately capturing the essence of the graphite phase posed a significant challenge. But thanks to the development of enhanced nearest neighbor potentials, we’ve seen a boost in the capabilities of these simulations, especially in studying the nucleation mechanism critical to the transition from graphite to diamond phase.

In today’s story, we’ll explore how Liang and team took this further. Using classical molecular dynamics simulations with the long-range carbon bond order potential (LCBOP II), they investigated the microscopic details of this transition, from graphite to diamond.

Craving some computational juice?

Keep reading. We’re about to dive deeper.

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Molecular dynamics simulation of three-point nanobending shows that hexagonal diamond is harder than cubic diamond | Credit

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The Wave-like Phase Transition From Graphite To Diamond

Let’s go into the nitty-gritty. What did Liang and team do? They leveraged molecular dynamics simulations, tapping into the LCBOP II potential, to zero in on how hexagonal graphite transforms into cubic diamond. They constructed a model comprising 192,000 atoms, carefully maintaining the initial interlayer distance (the distance between the graphene sheets in graphite) through a pre-equilibration phase lasting 10 picoseconds at a fiery 2000 Kelvin. Following this setup, they applied compression along the x-axis at a rapid pace of 100 meters per second, effectively unraveling the detailed mechanics of this high-pressure phase transition.

As we usually say on From Atoms To Words, a molecular dynamics simulation is only as good as the parameters it employs. Now, Liang’s work is made possible by the LCBOP II potential, which accounts not only for covalent bonds in the various hybridization forms of carbon but also for weaker nonbonded interactions, such as those between graphite layers. The LCBOP II potential has shown its value in several computational studies, from the estimation of grain boundary energy and elastic properties of graphene, to phonon dispersion, to the high-pressure high-temperature equation of state of graphite. In all these instances, the LCBOP II potential demonstrated good agreement with both quantum chemistry calculations and experimental results.

Now, after laying out the groundwork, I bet you are curious: what did Liang and team find out? Let’s dive into the key takeaways:

▸ Mechanism of Transformation

Liang and team have applied their computational set-up to study how compressed hexagonal graphite turns into cubic diamond. As you can see in the animation below, this change is propelled by what they’ve called a “wave-like buckling and slipping” mechanism occurring under extreme pressure, specifically targeting the graphite’s x[210] direction. At the outset, the graphite withstands the pressure, retaining its original structure. But, as pressure mounts to a tipping point, the layers start to warp, shifting their arrangement from ‘ABAB’ to ‘AAAA’, and finally to ‘ABC’ as the transition to cubic diamond begins. Liang’s results suggest a novel mechanism, standing apart from the conventional zigzag and armchair buckling conversion previously proposed. This insight offers a fresh perspective on graphite’s reaction to high pressures—and possibly that of other materials—opening new avenues for optimizing diamond synthesis.

▸ Stress-Strain Relationship

Digging into the nuts and bolts, Liang’s study pays close attention to the interplay between stress and strain in the phase transition from graphite to diamond. Initially, stress develops along the z[001] direction, attributed to the compression of the interlayer distance to just 0.240 nanometers. With ongoing pressure, stress intensifies in the x direction, surpassing that in the z direction, thereby triggering the transition into cubic diamond. This finding highlights how stress distribution in different directions influences the phase transition.

▸ Impact of Compressive Stresses and Interlayer Distances

An essential part of Liang’s work goes into understanding the effects of various compressive stresses and interlayer distances on the transition from graphite to diamond. It turns out that the initial interlayer distance is key in determining the grain size of the resulting cubic diamond. Specifically, when the interlayer distance hovers between 0.240 and 0.228 nanometers, you get a perfect cubic diamond. Stray from this sweet spot, and you might end up with polycrystalline cubic diamond or even hexagonal diamond, depending on the exact conditions.

▸ Hexagonal Graphite to Hexagonal Diamond Conversion

Liang’s research also delves into how hexagonal graphite can be directly transformed into hexagonal diamond, a route distinct from the cubic diamond process. This transition, occurring at an interlayer distance of 0.208 nanometers, bypasses the “wave-like buckling” seen in cubic diamond formation. Instead, it’s a straight shot to hexagonal diamond, demanding significantly less energy than its cubic counterpart. Besides the scientific interest, this finding underscores the potential for new methods in diamond synthesis.

Pretty cool, huh?

Liang and team have really hit something big with their finding of the “wave-like buckling and slipping” mechanism, shedding light on the intricate process that turns graphite to diamond. It’s a peek behind the curtain, showing us the critical roles of pressure and temperature, how stress distribution plays out, and the importance of the distances between graphene layers in graphite.

Graphite to Diamond: Mechanistic Insights from Molecular Dynamics Simulations | From Atoms To Words | Arturo Robertazzi
Liang’s molecular dynamics simulations of graphite to diamond. The animation depicts the “wave-like buckling and slipping” mechanism | Credit

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A Final Personal Touch

I honestly never grasped the allure of diamonds. They are just carbon, I thought. Why are we humans so drawn to them? And really, why spend a fortune on a shiny rock?

Digging into the topic today—from the notion of liquid diamonds on other planets to the subtleties of the phase transition from graphite to diamond—I must admit, my perspective has changed.

Diamonds are awesome. I might even go and buy one right now. But, hey, let’s get serious for a moment.

Liang and team have truly pushed the envelope in understanding how graphite turns into diamond, thanks to their robust classical molecular dynamics simulations. They’ve unveiled what they term a “wave-like buckling and slipping” mechanism, a crucial piece in decoding the shift from hexagonal graphite to that coveted cubic diamond structure.

This dive into the depths of high-pressure physics not only broadens our understanding of the transition phase from graphite to diamond but also illuminates new pathways for synthesizing diamonds in the lab, potentially revolutionizing traditional materials science and opening the door to groundbreaking industrial applications.

And I cannot help but marvel: isn’t this amazing? You devise some equations of motion, combine them with force fields, run your molecular dynamics simulations, and from the outcome, all within your computer, you gain a quantitative insight into natural processes that would be challenging or impossible to study experimentally.

And this holds true for a variety of phenomena, as we’ve explored on From Atoms To Words—from biological systems like DNA and proteins to materials. Isn’t this astounding?

So, let’s continue on the path of discovery. After all, when it comes to the pursuit of scientific understanding, the limit is now well beyond the sky.

If you enjoyed this dive into the phase transition of graphite to diamond, I’d love to hear your thoughts. Agree, disagree, or have a totally wild theory of your own? Let’s connect! Subscribe to my LinkedIn newsletter and let’s keep the conversation rolling.

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