Effects of deuterium content on the thermal stability and deuterium site occupancy of TiZrHfMoNb deuterides

Abstract A series of high entropy alloy (HEA) hydrides/deuterides were synthesized and characterized to explore the relationship between thermal stability and the structure of TiZrHfMoNb deuterides. The X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry results reveal that the lattice constant and cell volume of TiZrHfMoNb hydrides increase with the increasing hydrogen content. The desorption temperature decreases with the increasing hydrogen content, indicating that the increased lattice constant arising from the introduction of deuterium atoms will destabilize the HEA deuterides. A density functional theory calculations demonstrate that the binding energies between hydrogen and metals in BCC TiZrHfMoNb hydrides decline with the increasing hydrogen content, meaning that the thermal stability decreases with the increasing hydrogen content. The HEA deuterides are found to be more stable than their hydrides, implying the remarkable isotopic effect of TiZrHfMoNb HEA. Neutron powder diffraction refinements confirm that deuterium atoms prefer to occupy both of the tetrahedral and octahedral interstitials in BCC structure, while a prior occupation of tetrahedral interstitial sites in FCC structure. The deuterium/hydrogen concentrations and the lattice constants are proposed to be the major factor regulating the thermal stability of HEA deuteride, making them suitable for different application field of hydrogen storage.