What’s the most common substance in the human body?

Water, of course. That’s every kid's go-to science fact.

Well, what’s the most common substance in the world? Not water. Because yeah, if you look  at a map 70% of it is water.

But our planet isn’t flat. And remarkably, the most abundant stuff in the world is a mineral that’s exceptionally rare on the surface. In fact, it’s so rare there, scientists didn’t even have  a name for it until 2014.

And it’s part of a larger group of minerals, called perovskites, that have some pretty incredible properties… including possibly unlocking the next generation of green energy. [SciShow Intro] If you could dig down 660 kilometers beneath the Earth’s surface, you’d hit a layer of hot, dense rock called the Lower Mantle. And this is where the most  common mineral calls home. Of course, we’ve never succeeded in drilling down to anywhere near the upper mantle, let alone the lower mantle.

So we can’t just grab a  sample of that hot dense rock to figure out exactly what  minerals are down there. But scientists can and do conduct lab experiments where they put rocks under the extreme heat and pressure you’d find that deep. And it’s from these experiments that we’ve learned the most abundant mineral in the lower mantle has a structure with this chemical formula.

Each crystal of the stuff  is made of repeating units of one magnesium atom, one silicon atom, and three oxygen atoms, over and over. As for what that mineral is called, well, it turns out mineralogists are a bit weird when it comes to naming things. They have this rule where in order to give a mineral an official name, you must be able to study and describe a sample that comes from nature.

Making it in a lab isn’t good enough. And this mineral does not like to exist at the temperatures and pressures you find outside the lower mantle. The crystal structure can break down over time, turning it into a different mineral made up of the same ingredients.

So while scientists can occasionally stumble upon a mantle rock that’s made it to Earth’s surface, say by coming up though a volcano or from tectonic plates shifting, he mineral you can hold in your hand is often not the one you’re actually looking for. But after decades of searching, scientists finally struck  not-literal gold in 2014, when one team tried looking at rocks hat came not from the Earth, but from outer space. That’s right, they carefully studied the minerals in a meteorite.

Because when this space rock crashed into the planet’s surface, it momentarily experienced  around 24 gigapascals of pressure and got heated up to around 2300 Kelvin. In other words, conditions that match what you’d find  in Earth’s lower mantle. And upon an extremely close examination, the team found a tiny grain of  the mineral they were looking for.

With a naturally occurring sample in hand, the thing that makes up  38% of our planet’s volume, and the most common mineral in the world, could finally get a name: Bridgmanite. But here’s another weird  naming thing mineralogists do. Sometimes, they'll group a bunch of different minerals under one umbrella term not based on the elements they’re made of, but the ratio between the different elements they’re made of.

Since Bridgmanite crystals are made from repeating units of one magnesium, one silicon, and three oxygens, that makes it belong to a greater group of minerals called perovskites. Not because it’s made of magnesium, silicon, and oxygen, but because it fits into this generic formula: ABX3, where A and B are positively charged ions  and the Xs are negative. That’s what all perovskites have in common.

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The first perovskite mineral, which you can actually call plain ol’ perovskite, was discovered in Russia back in 1839. To distinguish it from all the other perovskites, you’ll also see it called calcium titanate… because its crystal unit is one calcium, one titanium, and three oxygen atoms. But nearly two centuries later, scientists have found a ton of  different element combinations that you can slot into this pattern.

In fact, of all the metallic elements on the periodic table, you can use over 90% of them  in a perovskite mineral. And I do mean “use”. Some perovskites are found in nature, and some are made in a lab.

And it’s those lab-grown perovskites which have some pretty tantalizing applications. Since 2009, scientists have been very interested in how halide metal perovskites … meaning those three X atoms are a halogen like fluorine or chlorine… could be used to revolutionize  our solar panel technology. Today, the light-collecting cells in most solar panels are made with silicon-based crystals.

They work pretty well, but they can be hard to manufacture. The silicon crystals themselves need to be practically flawless… with almost no impurities… for them to do their job efficiently. So they have to be heated up to like 900 degrees Celsius to make sure there aren’t too many defects.

Meanwhile, perovskite crystals can perform just as well as silicon ones with many more imperfections. And to process them, you only have to heat them up to like, 100 degrees, making perovskite solar panels not just cheaper to manufacture, but more environmentally-friendly because you don’t need as much energy. There’s also the fact that you can create different perovskites… each with its own preference for the kind of light it likes to gobble up… and combine them to maximize how much sunlight you’re converting into electricity.

Its all very exciting, I spend a lot of time reading about perovskites So why haven’t we all switched  to perovskite solar panels? Well, there are a few catches. Durability is the biggest one.

These synthetic perovskites have a tendency to decompose when they’re exposed to moisture, or oxygen…or light. And being able to withstand light is pretty important for, you know, a solar panel. So early versions of perovskite panels lasted for mere minutes.

And thanks to a decade plus of research, today’s can last for months. But that’s still not good  enough to compete with silicon, whose solar panels can last for decades. Another major issue is the fact that the best metal we’ve found to slot into that generic perovskite crystal structure is…lead.

Which is rather famous for being nasty to both people and the environment if you’re not very careful with it. But despite these hurdles, research into perovskite solar panels is moving very fast. In 2009, they had an efficiency of 3%... meaning they could only  convert 3% of the solar energy shining down on them into usable electricity.

Today, it’s over 25%, which rivals the best silicon solar panels. Researchers are also working to develop better seals that can keep moisture out, keep any lead in, and so forth. And in 2022, one team claimed that they’d developed a perovskite solar panel that can last for 30 years.

But one of the most promising ideas in researc h is to create tandem solar panels that combine the strengths of  both perovskite and silicon. In fact, this crystalline partnership seems to produce a more efficient panel than you’d get from either perovskite or silicon on its own. So while one perovskite gets to stand on a podium and hold up a little trophy for being the most common substance in the world, another might hold the key  to taking better care of it. [ OUTRO ]