New materials: Synthetic pathway for promising nitride compounds discovered

Ruddlesden-Popper compounds are a class of materials with a special layered structure that makes them interesting for numerous applications — as superconductors or catalysts, for example, or for use in photovoltaics. There have been many halides and oxides of this structural type before now, but no nitrides. Although scientists expected Ruddlesden-Popper nitrides to have outstanding material properties, they were unable to actually manufacture them.

Now researchers led by Dr. Simon Kloß from the Department of Chemistry at LMU have developed a special synthetic pathway which has enabled them to manufacture nitride materials that crystallize in the Ruddlesden-Popper structural type, as they report in the journal Nature Chemistry.

Stability of nitrogen posed a challenge

The stability of the triple bond in the nitrogen molecule (N₂) and the low electron affinity of the element made it very challenging for the chemists to manufacture the nitrogen-rich Ruddlesden-Popper nitrides. They achieved a breakthrough by carrying out the syntheses under extreme conditions. Employing large-volume presses, they compressed their samples at pressures of 8 gigapascals, which is equivalent to 80,000 bars. Then they used an active nitrogen source such as sodium azide to prepare the rare-earth transition-metal nitride compounds.

“We think we can systematically investigate Ruddlesden-Popper nitrides compounds with our new synthesis strategy,” says Kloß. The scientists demonstrated this by investigating three new compounds of this materials class — a cerium-tantalum nitride (Ce₂TaN₄) and praseodymium- and neodymium-rhenium nitrides (Ln₂ReN₄ (Ln = Pr, Nd)). “These three initial materials already exhibit a rich variety of structural, electronic, and magnetic properties,” says Kloß.

The praseodymium and the neodymium compounds displayed exciting magnetic characteristics. For example, the neodymium compound is a remarkable hard ferromagnet with irreversible magnetic behavior. Meanwhile, the tantalum compound is a semiconductor with properties that make it exciting for applications in the energy conversion domain or as a ferroelectric material. “The same synthetic method will probably lead to other Ruddlesden-Popper nitride compounds and their derivatives,” explains Kloß. “Consequently, a large new class of nitrides is waiting to be researched.”