Double salt: Negative platinum X-rayed

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  • Published: Dec 1, 2016
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Double salt: Negative platinum X-rayed

Hydrogen storage

Cesium platinide hydride forms a translucent ruby red crystal and can exist only in an inert environment similar to conditions that exist in outer space. Credit: Ames Laboratory, US Department of Energy

The first intermetallic double salt containing the noble metal, platinum, has been synthesised by researchers at the US Department of Energy's Ames Laboratory in Ames, Iowa. The team used X-ray studies to verify their method. Materials scientists Anja-Verena Mudring and Volodymyr Smetana were the first to synthesise and accurately characterize the compound, caesium platinide hydride, or 4Cs2Pt⋅CsH. The compound exists as translucent ruby red crystals but persists only under an inert atmosphere, whimsically referred to as being similar to conditions that exist in outer space. This new member of a rare family of chemicals contains a metal as a negatively charged ion.

The team was investigating the potential of metal hydrides as storage materials for hydrogen gas, which might be used in a future low-carbon, hydrogen-based fuel economy as the energy source for fuel cells, for instance. "This hydride is a compound that as a researcher you have trouble envisioning that it can even exist, but once you do have it and can analyze it, it's nothing like what you expect," enthuses Mudring. "Instead of creating a gray, shiny alloy as typically observed for many hydrogen storage materials by reacting the metals caesium and platinum with hydrogen, these red crystals form. They are really quite beautiful."

Caesium melt

The team initially extracted the novel compound from a caesium melt but point out that it is highly unstable, as one might expect for a substance containing a negative metal ion. Indeed, the compound disintegrates when exposed to oxygen as the platinum ion reverts to its elemental state. This lack of stability meant it took the team a long time to determine its true chemical nature and obtain a proof of its composition using single crystal X-ray diffraction studies combined with solid-state nuclear magnetic resonance (NMR) spectroscopy and some deep theoretical investigations. Ultimately, they were able to characterise its unusual structure and properties, which they add are markedly different from typical intermetallic hydrides.

The structure as defined relies on the strong influence of relativistic effects on both caesium and platinum wherein the high speed of electrons is a large fraction of the speed of light and so Einstein's theory must be accommodated in the computational model to explain the nature of the structure. The fact that mercury is liquid at room temperature and gold a yellow colour rather than silvery are also explained by relativistic effects as are the properties of other rare metal complexes.

Relatively unique

This compound is unique, however, "It's the first example we have of a salt with so strongly negatively charged metal ions," adds Mudring. "Moreover, you mix an alloy with a salt and get another non-conducting salt. This allows for some deep insight into the nature of chemical bonding - and as Goethe wrote, ultimately what holds the world and its compounds together in its inmost form."

Related Links

Angew Chem Int Edn 2016, online: "Cesium Platinide Hydride 4Cs2Pt·CsH: An Intermetallic Double Salt Featuring Metal Anions"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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