The influence of hydrogen contamination on the structural stability of CoSn under compression.

The binary CoSn compound has a unique ground state large-void crystal structure, whose stability under pressure has recently been examined. Whereas theoretical results predicted a series of phase transformations, the room temperature experiments did not observe any structural change. We suggest that the large void of a CoSn-type structure could contain natural impurities such as hydrogen, which can influence the thermodynamic stability of a CoSn system and explain the unusual disagreement between the theoretical and experimental results. Based on first-principles calculations we reveal that the contamination of CoSn by hydrogen only results in a subtle change of structural parameters and the equation of state of CoSn, but drastically increases the stability of the CoSn-type phase in comparison with the high-pressure phases predicted earlier. We argue that the hardly detectable natural impurities of light elements in porous compounds like CoSn are able to change the phase equilibria.