The use of the highest purity starting materials in fundamental research seems to be an obvious choice and is a priori assumed in experimental science, including rare earth metallurgy. Yet an ambiguous “99.9%” purity reported by commercial vendors for the rare earths, in almost all cases refers to the purity with respect to only the other rare earth elements, and generally does not include other metals and more importantly the presence of the interstitial non-metallic elements oxygen, nitrogen, hydrogen, and carbon. In a recently published paper, Ames Laboratory scientists found that interstitial impurities present in small concentrations in gadolinium germanide can significantly change its fundamental properties such as the magnetic ordering temperature and the structural orthorhombicity as the compound magnetically orders. They discovered that Gd5Ge3 compound prepared using high purity Materials Preparation Center gadolinium exhibits a strong structural distortion from the hexagonal Mn5Si3-type structure into a new orthorhombic crystal structure at Neél antiferromagnetic ordering temperature, TN. The same intermetallic compound prepared using a commercial grade of gadolinium shows much weaker structural distortion at TN, which also occurs 19 K below that of the pure Gd5Ge3. In fact, using a low resolution diffraction instrument to study the crystal structure of a typical Gd5Ge3 sample prepared with impure Gd, one may not detect such a minor distortion. This may explain why the orthorhombic structure was not reported in earlier studies. It is also possible that samples with high interstitial levels may prevent the orthorhombic phase from forming. First principles electronic structure calculations confirm that the antiferromagnetic orthorhombic structure is the ground state of Gd5Ge3. They also show that interstitial oxygen, which was the main impurity in the commercial alloy, decreases the TN of Gd5Ge3, while interstitial nitrogen and carbon, detected in much smaller amounts, should potentially increase the magnetic ordering temperature.
Low-temperature crystal structure and magnetic properties of Gd5Ge3