Using nitrogen as a raw material (“feedstock”) for industrial usage is accomplished by a reaction known as “nitrogen fixation.” In this reaction, molecular nitrogen (or “dinitrogen”; N₂) is split into two atoms of nitrogen that can then be connected to other elements like hydrogen or carbon, which allow nitrogen to be stored as ammonia or converted directly in higher value compounds.

But ammonia is not easy to make on an industrial level; the main process, called “Haber-Bosch,” uses an iron-based catalyst at temperatures around 450oC and pressures of 300 bar — almost 300 times the pressure at sea level. In order to make the process more cost-effective, chemists have focused on the development of new systems that can transform nitrogen into useful compounds using mild low-energy conditions.

In 2017, the lab of Marinella Mazzanti at EPFL was able to convert molecular nitrogen into ammonia in ambient conditions by synthesizing a compound containing two uranium(III) ions and three potassium centers held together by a nitride group.

Now, the group, in collaboration with other EPFL groups, has shown that by replacing the nitride bridge in the uranium system with an oxo bridge they can still bind dinitrogen. In addition, the bound dinitrogen can be easily cleaved in ambient conditions by carbon monoxide to make cyanamide, a compound that is widely used in agriculture, pharmaceuticals, and various organic compounds.

The reactivity of the oxo-bridged dinitrogen complex was remarkably different compared to the previous nitride complex and the few other nitrogen complexes known in the field. Computational studies then allowed the scientists to relate these differences in reactivity to the bonding in the uranium-oxo/-nitride bridge.

“These findings provide important insight into the relation between structure and reactivity that should extend to nitride and oxide materials,” says Marinella Mazzanti. “Moreover, the implementation of these compounds in catalytic systems could ultimately lead to a lower cost access to fertilizers.”

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Researchers at MIT and at Brookhaven National Laboratory have demonstrated that they can control the magnetic properties of a thin-film material simply by applying a small voltage. Changes in magnetic orientation made in this way remain in their new state without the need for any ongoing power, unlike today’s standard memory chips, the team has found.

The new finding is being reported today in the journal Nature Materials, in a paper by Geoffrey Beach, a professor of materials science and engineering and co-director of the MIT Materials Research Laboratory; graduate student Aik Jun Tan; and eight others at MIT and Brookhaven.

Spin doctors

As silicon microchips draw closer to fundamental physical limits that could cap their ability to continue increasing their capabilities while decreasing their power consumption, researchers have been exploring a variety of new technologies that might get around these limits. One of the promising alternatives is an approach called spintronics, which makes use of a property of electrons called spin, instead of their electrical charge.

Because spintronic devices can retain their magnetic properties without the need for constant power, which silicon memory chips require, they need far less power to operate. They also generate far less heat — another major limiting factor for today’s devices.

But spintronic technology suffers from its own limitations. One of the biggest missing ingredients has been a way to easily and rapidly control the magnetic properties of a material electrically, by applying a voltage. Many research groups around the world have been pursuing that challenge.

Previous attempts have relied on electron accumulation at the interface between a metallic magnet and an insulator, using a device structure similar to a capacitor. The electrical charge can change the magnetic properties of the material, but only by a very small amount, making it impractical for use in real devices. There have also been attempts at using ions instead of electrons to change magnetic properties. For instance, oxygen ions have been used to oxidize a thin layer of magnetic material, causing a extremely large changes in magnetic properties. However, the insertion and removal of oxygen ions causes the material to swell and shrink, causing mechanical damage that limits the process to just a few repetitions — rendering it essentially useless for computational devices.

The new finding demonstrates a way around that, by using hydrogen ions instead of the much larger oxygen ions used in previous attempts. Since the hydrogen ions can zip in and out very easily, the new system is much faster and provides other significant advantages, the researchers say.

Because the hydrogen ions are so much smaller, they can enter and exit from the crystalline structure of the spintronic device, changing its magnetic orientation each time, without damaging the material. In fact, the team has now demonstrated that the process produces no degradation of the material after more than 2,000 cycles. And, unlike oxygen ions, hydrogen can easily pass through metal layers, which allows the team to control properties of layers deep in a device that couldn’t be controlled in any other way.

“When you pump hydrogen toward the magnet, the magnetization rotates,” Tan says. “You can actually toggle the direction of the magnetization by 90 degrees by applying a voltage — and it’s fully reversible.” Since the orientation of the poles of the magnet is what is used to store information, this means it is possible to easily write and erase data “bits” in spintronic devices using this effect.

Beach, whose lab discovered the original process for controlling magnetism through oxygen ions several years ago, says that initial finding unleashed widespread research on a new area dubbed “magnetic ionics,” and now this newest finding has “turned on its end this whole field.”

Essentially, Beach explains, he and his team are “trying to make a magnetic analog of a transistor,” which can be turned on and off repeatedly without degrading its physical properties.

Just add water

The discovery came about, in part, through serendipity. While experimenting with layered magnetic materials in search of ways of changing their magnetic behavior, Tan found that the results of his experiments varied greatly from day to day for reasons that were not apparent. Eventually, by examining all the conditions during the different tests, he realized that the key difference was the humidity in the air: The experiment worked better on humid days compared to dry ones. The reason, he eventually realized, was that water molecules from the air were being split up into oxygen and hydrogen on the charged surface of the material, and while the oxygen escaped to the air, the hydrogen became ionized and was penetrating into the magnetic device — and changing its magnetism.

The device the team has produced consists of a sandwich of several thin layers, including a layer of cobalt where the magnetic changes take place, sandwiched between layers of a metal such as palladium or platinum, and with an overlay of gadolinium oxide, and then a gold layer to connect to the driving electrical voltage.

The magnetism gets switched with just a brief application of voltage and then stays put. Reversing it requires no power at all, just short-circuiting the device to connect its two sides electrically, whereas a conventional memory chip requires constant power to maintain its state. “Since you’re just applying a pulse, the power consumption can go way down,” Beach says.

The new devices, with their low power consumption and high switching speed, could eventually be especially useful for devices such mobile computing, Beach says, but the work is still at an early stage and will require further development.

“I can see lab-based prototypes within a few years or less,” he says. Making a full working memory cell is “quite complex” and might take longer, he says.

The team used a technique called micro-electron diffraction (MicroED), which had been used in the past to learn the 3-D structures of larger molecules, specifically proteins. In this new study, the researchers show that the technique can be applied to small molecules, and that the process requires much less preparation time than expected. Unlike related techniques — some of which involve growing crystals the size of salt grains — this method, as the new study demonstrates, can work with run-of-the-mill starting samples, sometimes even powders scraped from the side of a beaker.

“We took the lowest-brow samples you can get and obtained the highest-quality structures in barely any time,” says Caltech professor of chemistry Brian Stoltz, who is a co-author on the new study, published in the journal ACS Central Science. “When I first saw the results, my jaw hit the floor.” Initially released on the pre-print server Chemrxiv in mid-October, the article has been viewed more than 35,000 times.

The reason the method works so well on small-molecule samples is that while the samples may appear to be simple powders, they actually contain tiny crystals, each roughly a billion times smaller than a speck of dust. Researchers knew about these hidden microcrystals before, but did not realize they could readily reveal the crystals’ molecular structures using MicroED. “I don’t think people realized how common these microcrystals are in the powdery samples,” says Stoltz. “This is like science fiction. I didn’t think this would happen in my lifetime — that you could see structures from powders.”

The results have implications for chemists wishing to determine the structures of small molecules, which are defined as those weighing less than about 900 daltons. (A dalton is about the weight of a hydrogen atom.) These tiny compounds include certain chemicals found in nature, some biological substances like hormones, and a number of therapeutic drugs. Possible applications of the MicroED structure-finding methodology include drug discovery, crime lab analysis, medical testing, and more. For instance, Stoltz says, the method might be of use in testing for the latest performance-enhancing drugs in athletes, where only trace amounts of a chemical may be present.

“The slowest step in making new molecules is determining the structure of the product. That may no longer be the case, as this technique promises to revolutionize organic chemistry,” says Robert Grubbs, Caltech’s Victor and Elizabeth Atkins Professor of Chemistry and a winner of the 2005 Nobel Prize in Chemistry, who was not involved in the research. “The last big break in structure determination before this was nuclear magnetic resonance spectroscopy, which was introduced by Jack Roberts at Caltech in the late ’60s.”

Like other synthetic chemists, Stoltz and his team spend their time trying to figure out how to assemble chemicals in the lab from basic starting materials. Their lab focuses on such natural small molecules as the fungus-derived beta-lactam family of compounds, which are related to penicillins. To build these chemicals, they need to determine the structures of the molecules in their reactions — both the intermediate molecules and the final products — to see if they are on the right track.

One technique for doing so is X-ray crystallography, in which a chemical sample is hit with X-rays that diffract off its atoms; the pattern of those diffracting X-rays reveals the 3-D structure of the targeted chemical. Often, this method is used to solve the structures of really big molecules, such as complex membrane proteins, but it can also be applied to small molecules. The challenge is that to perform this method a chemist must create good-sized chunks of crystal from a sample, which isn’t always easy. “I spent months once trying to get the right crystals for one of my samples,” says Stoltz.

Another reliable method is NMR (nuclear magnetic resonance), which doesn’t require crystals but does require a relatively large amount of a sample, which can be hard to amass. Also, NMR provides only indirect structural information.

Before now, MicroED — which is similar to X-ray crystallography but uses electrons instead of X-rays — was mainly used on crystallized proteins and not on small molecules. Co-author Tamir Gonen, an electron crystallography expert at UCLA who began developing the MicroED technique for proteins while at the Howard Hughes Medical Institute in Virginia, said that he only started thinking about using the method on small molecules after moving to UCLA and teaming up with Caltech.

“Tamir had been using this technique on proteins, and just happened to mention that they can sometimes get it to work using only powdery samples of proteins,” says Hosea Nelson (PhD ’13), an assistant professor of chemistry and biochemistry at UCLA. “My mind was blown by this, that you didn’t have to grow crystals, and that’s around the time that the team started to realize that we could apply this method to a whole new class of molecules with wide-reaching implications for all types of chemistry.”

The team tested several samples of varying qualities, without ever attempting to crystallize them, and were able to determine their structures thanks to the samples’ ample microcrystals. They succeeded in getting structures for ground-up samples of the brand-name drugs Tylenol and Advil, and they were able to identify distinct structures from a powdered mixture of four chemicals.

The UCLA/Caltech team says it hopes this method will become routine in chemistry labs in the future.

“In our labs, we have students and postdocs making totally new and unique molecular entities every day,” says Stoltz. “Now we have the power to rapidly figure out what they are. This is going to change synthetic chemistry.”