High-Pressure Synthesis of Quantum-Stabilised Manganese Superhydride MnH₇

Through high-temperature, high-pressure diamond anvil cell studies combined with in situ synchrotron powder x-ray diffraction, we investigate the reactivity between manganese and hydrogen up to 182 GPa. Laser heating manganese in an $H_2$ medium at 118 GPa yields body-centred tetragonal ${\mathrm{MnH}}_7$ , which has the highest hydrogen content ever observed in the binary hydrides of group IV-XII transition metals. In addition, we also observe two other new hydrides as minor phases, ${\mathrm{MnH}}_{\sim 2}$ with a tetragonal crystal structure and cubic ${\mathrm{MnH}}_{3.67}$ , formed at the pressures of 105 and 118 GPa, respectively. Upon decompression, ${\mathrm{MnH}}_{3.67}$ and ${\mathrm{MnH}}_7$ remain stable down to $\sim$ 30 GPa before decomposing into the monohydride. Our ab initio quantum chemistry computations employing density-functional theory reveal that the key stabilisation factor of ${\mathrm{MnH}}_{3.67}$ and ${\mathrm{MnH}}_7$ is zero-point energy, however, no isotope effect is observed on the crystal structures of the deuterated samples.