Temperature-Dependent Lattice Vibration of Magnesium Hydride

The thermodynamic stability of magnesium hydride hinders its application as a hydrogen storage medium. Efforts to destabilize MgH2 demonstrate the undesired reduction of entropy concomitant with the decrement of enthalpy. Lattice vibration contributes to entropy as well as thermal properties, whereas its temperature dependence is rarely reported for MgH2. Here, vibrational spectra of MgH2 are detected by means of Raman scattering between room temperature and the critical temperature of significant dehydrogenation. The laser power is changed, and the localized sample temperature in the irradiated region is evaluated according to the bandshift of the incorporated carbon with MgH2. With increasing temperature, the frequency for the B-1g mode increases, while phonon softening is exhibited for both E-g and A(1g) modes. It is found that quasiharmonicity decides the thermal frequency shifts for Raman-active vibrations. Phonon-phonon interactions result in significant phonon broadenings for B-1g and E-g modes. By contrast, a decreasing line width of the Aig phonon as a function of temperature is observed, possibly arising from the electron-phonon coupling for the A(1g) mode in MgH2.