Structural information on ball milled magnesium hydride from vibrational spectroscopy and ab-initio calculations

Ball milled magnesium hydride with an average size of about 40 nm and bulk magnesium hydride have been studied with vibrational spectroscopy together with density functional computer calculations. Using this combination of techniques structural information can now be obtained on a nanometer scale, which is especially important for nanosized samples. Such samples exhibit very broad diffraction lines, from which limited information about the structure can be extracted. It was found that ball milling distorts the vibrational spectra due to distribution in stresses over the sample. Cycling of the hydrogen content of ball milled samples results in the spectrum of unmilled samples, while the particle size remains small and hydrogen storage characteristics continue to be better for ball milled samples. We conclude that improved performance for hydrogen storage applications of ball milled magnesium hydride has to be attributed to the reduction of the particle size, while defect densities inside the particles play less of a role.