Hydrogen Storage in Martensite Ti-Zr-Ni Alloy: A Density Functional Theory Study
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The hydrogen storage potential of TiNi-based shape memory alloys is an attractive but experimentally challenging issue. We employ the FP (L)APW+lo method, based on the density functional theory (DFT), in order to address the electronic structure of the low-temperature, martensitic phase of Ti0.67Zr0.33Ni and its hydrides. Further, the thermodynamics of hydride formation in the martensitic Ti-Zr-Ni alloys is studied, and some unanswered questions regarding the hydriding of martensite TiNi are resolved. The calculated formation energy of the orthorhombic beta- and gamma-hydrides of martensitic Ti0.67Zr0.33Ni is, respectively, -14.2 kJ/mol(H) and -29.6 kJ/mol(H), showing that isostructural (Ti,Zr)Ni hydrides with a larger amount of titanium have improved potential for hydrogen storage applications. Furthermore, based on the calculation results, it is unlikely that the hydriding of TiNi martensite would lead to the formation of orthorhombic beta-hydride in analogy to the pseudobinary compo...und. The formation of Ti0.67Zr0.33Ni hydrides leads to significant changes in electronic structure, causing shifting of some metal states to lower energies due to their interaction with hydrogen s states, and the increase of the number of states at Fermi energy. By modeling intermediate structures, the process of hydride formation in martensitic Ti0.67Zr0.33Ni is resolved to reveal the effects of crystal structure change, volume increase, and hydrogen-metal interactions on the band structure, charge transfer, and thermodynamics. A dominant, stabilizing effect in the process of hydride formation was found to come from the chemical interaction of hydrogen and metal atoms.