The elements of the periodic table are the building blocks of everything around us. All matter is composed of at least one element or a combination of two or more elements. The elements make up the structures of everything we see and touch, and they carry out many processes essential to life.
Studies of elements can be used to predict chemical reactions and processes, and accurate data are always required. The element carbon has two common forms, graphite and diamond. Graphite is used in many devices, from odour-eating insoles, to lithium-ion batteries, and diamond has uses in wear-resistant coatings and acoustic devices.
A recent study led by Mary Anne White, a Dalhousie chemist and fellow of the Royal Society of Canada, answered a fundamental question concerning a basic property of carbon. Which is lower in energy and therefore, more stable – graphite or diamond? The study was led by Dalhousie’s Faculty of Science and involved five research groups in three countries.
“Graphite and diamond are two different forms of carbon, and it has long been thought that graphite is more stable, except at high pressure in which case diamond is more stable,” says Dr. White. “A recent investigation had suggested that, with no applied pressure, graphite would become higher in energy — less stable — than diamond as the temperature approaches absolute zero.”
If graphite is more stable than diamond, it won’t turn into diamond, but that diamond could turn into graphite. This relative stability plays an important role in understanding topics as diverse as chemical reactions, to the geologic history of metamorphic rocks on Earth and other rocky planets.
Gaps in understanding
Dr. White’s experiments determined that our previous understanding of the relative stability of graphite and diamond had significant gaps.
Their main finding, just published in a leading journal from the German Chemical Society, Angewandte Chemie, determined that without applied pressure, graphite is more stable than diamond for all temperatures below 400 Kelvin (which is 127 oC), down to absolute zero. They determined accurate values of the difference in energy between graphite and diamond throughout this temperature range, with excellent agreement between experiment and new theoretical calculations.
This work definitively answered a question concerning carbon that many thought was already understood.
“This is a crucial discovery as accurate knowledge of the details of the stability of diamond relative to graphite underpins accurate prediction of many chemical reactions and properties. As a result of this study, numbers in high school chemistry textbooks will have to be modified. Furthermore, we showed theoretically that the reasons for graphite’s greater stability are its bonding arrangements and its more flexible lattice (the way in which atoms are arranged), compared with diamond’s stiffer lattice,” she says.
The work used diamonds that had been studied at the National Research Council in Ottawa in the 1950s, and a gem-quality diamond borrowed from a Halifax jeweller. Some of the work required to determine the heat of combustion involved burning diamonds. That part of the investigation used diamond powder, with each little crystal in the powder about the width of a hair. In the combustion experiments, the diamond sample is placed in a sealed container that is filled with oxygen. It is then lit with an electrical spark and the container heats up as the diamond sample burns. The heat given off is the heat of combustion.
“The difference in heats of combustion of diamond and graphite is the difference in energy of graphite and diamond, at the temperature of the combustion,” says Dr. White.
An international effort
Very few labs can do this type of experiment with high accuracy, especially since the results require precise knowledge of the very small difference between two very large numbers. This experiment had been carried out before, but, for diamonds, not since 1968 (and the technique is now more accurate), and only with a few replicates.
“The combustion experiments were critical to an accurate final result, so we engaged two leaders in this field, Maria Ribeiro da Silva at the Universidade do Porto in Portugal and Sergey Verevkin at the University of Rostock in Germany, to join the project. It was very useful to have two labs doing these experiments, as their techniques were slightly different. In the end, their results were consistent which was excellent news. This truly shows the power of international collaboration.”
The heat-capacity measurements allow calculation of the difference in energy of graphite and diamond at other temperatures. In Dr. White’s lab, they have considerable experience and top-of-the-line equipment to carry out high-accuracy low-temperature heat-capacity measurements, which was a major part of the study. Dr. White collaborated with Dalhousie research associate Samer Kahwaji on this aspect.
Fortunately for Dr. White, she had other outstanding theoretical chemists at Dal to work with. Josef Zwanziger and Erin Johnson are both leaders in calculations such as the ones required here, and they use different theoretical approaches. Joseph Weatherby, a master’s student with Dr. Johnson, also joined the team. A powerful team with top research groups in each of the required areas, giving accurate results, both experimentally and theoretically was the major reason for the success of the study, says Dr. White.
"With this research, we're providing an underpinning for a better understanding of many materials and we're adding to the fundamental knowledge of nature," she says. "All matter is made of elements and there are only about 90-some naturally occurring elements, with about 30 in high abundance. When we understand how they form different structures (such as diamond and graphite, both made purely of carbon), we can advance our understanding of why different bonding arrangements can occur. Such fundamental information allows us to better predict new structures with properties tailored to specific applications.”
For more in-depth details, view the publication.
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