The pressure effect on the tunneling mode and vibrational spectra of hydrogen in $\alpha -{\text{MnH}}_{0.07}$ has been studied by inelastic neutron scattering. Applying hydrostatic pressure of up to 30 kbar is shown to shift both the hydrogen optical modes and the tunneling peak to higher energies. First-principles calculations show that the potential for hydrogen in $\alpha$-Mn becomes overall steeper with increasing pressure. At the same time, the barrier height and its extent in the direction of tunneling decrease and the calculations predict significant changes of the dynamics of hydrogen in $\alpha$-Mn at 100 kbar, when the estimated tunneling splitting of the hydrogen ground state exceeds the barrier height.

• Received 29 July 2016

DOI:https://doi.org/10.1103/PhysRevB.94.134301