By Robert Hazen, Ph.D., George Mason University
Phase transformation is the property of some materials to alter their atomic structure when subjecting them to heat, high pressure, or some other external influence. Phase transformations play key roles in the dynamics of the Earth and in life. Our universe is enriched by the great principle that elements and compounds combine in chemical reactions to form new materials.
Change in Vibration
If chemicals, once formed, never changed, the world would be a very boring place. Change is a hallmark of the material world. Wood burns, water boils, glue hardens, eggs cook, dead organisms decay. These obvious physical transformations reflect the change in the arrangement of atoms and their chemical bonds. All of these changes are the result of the thermal vibrations of atoms.
At lower temperatures, atoms and molecules don’t vibrate as much. See, the atomic scale structure is so critical to properties. But, as you go to higher temperatures, atoms and molecules vibrate more and more, until they become so agitated that they become a liquid. And then those liquid molecules and atoms fly off and become a gas at even higher temperatures. And at the highest temperatures of all, a plasma forms because the gas molecules hit each other and strip off electrons.
Many common substances occur in more than one arrangement of atoms. For example, there are 18 different isomers of octane. That’s a carbon-based molecule that has 8 carbon atoms and then hydrogens all around, so it’s C8H18. So there are 18 different molecular structures for that compound alone.
Learn more about the four states of matter.
Remarkable Change in Water
Let’s focus now on an important substance called water. Solid water is remarkable for its many phase transformations. As temperature and pressure are varied, H2O, water undergo at least 10 different kinds of transformations to different crystal structures. Each of these different materials—all of them H2O, all of them solid in crystalline form—has a different arrangement of those H2O molecules. So they’re different crystal structures with different properties.
The phase diagram of water, which has pressure plotted against temperature, shows that in various pressure-temperature regimes, you have these different solid forms. It’s a very useful device for scientists because then you can predict at what particular temperature and pressure you want to study water, go to it, and see that crystal form.
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The Case of Gillespite
Well, not all phase transformations result in such drastic changes in the crystal structure. A beautiful red mineral called gillespite is very interesting. This very rare mineral, a barium copper silicate, is found only in a few locations, for example, in California. But it forms magnificent, square, red crystals.
The thing that’s so neat about this mineral is when you take it up to high pressure, to about 20,000 atmospheres, that beautiful red crystal suddenly, instantaneously, changes to a deep blue color. Now, we know that all physical properties are the result of the atoms and how they’re bonded together. So there has to be a change in atomic structure.
Studying the crystal structure at low pressure and at high pressure resulted in finding a very interesting change. At low pressure, there are iron atoms in this mineral that are surrounded by a perfect plane of four oxygens, a square of oxygens with iron in the center. There’s an electronic transition—an electron jump—in that iron atom that gave the mineral its red color.
But as you go to higher pressure, that square plane buckles, and so two of the atoms go a little bit higher, and two of the atoms go a little bit lower than the iron. You no longer have a plane of atoms, and the electronic structure of the iron atom changes too. That subtle change in the iron causes the color to change from red to blue. It’s an absolutely magnificent phase transition.
Learn more about the difference between phase transformations and chemical reactions.
Applying High Pressure to Material
The most dramatic phase transitions in many compounds occur at extremely high pressures where scientists expect that all compounds—water, gas, diamond, whatever compound you can think of—will transform to a metal. Now, the reason is this: As you go to high pressure, you’re squeezing the atoms closer and closer together. Eventually, the electrons start to overlap from adjacent atoms, and that’s an unhappy situation for those electrons. They want to get out of there. And so what the electrons do is delocalize, and they become a metal.
Recently scientists have actually turned oxygen into metal at a million atmospheres. And they’ve done it with sulfur as well. But one of the great holy grails of high-pressure physics is to turn hydrogen, element number 1, into that most primitive of all metals. This is a challenge that may require five million atmospheres of pressure, and a lot of people are still trying to do that.
One of the great things about high pressure is that it holds the promise of producing so many new materials. As you squeeze virtually any compound, it will change, it will transform again and again and again. So simply by applying pressure to all the known compounds, we could conceivably triple or even quadruple the number of known materials that are available to us. That’s the lure of high pressure.
Common Questions about Phase Transformations in Crystal Structures
When applying high pressure and heat, water can change to 10 different crystal structures which are different yet still water.
Because the most dramatic transitions, we can potentially see in materials are when applying high pressure. This can lead to new and different materials being born, so it’s exciting to test such pressures on various materials.
When applying high pressure or other significant changes to a material, the atoms and molecules of a structure will start to vibrate. Based on this vibration, we may see phase transformations like a solid turning into a liquid.