Experimental and computational study on the C15 phase structure stability of YzNi2-yMny system

Abstract

Y0.95Ni2 intermetallic is a promising candidate for hydrogen storage applications, but currently suffers from hydrogen-induced amorphization (HIA) mainly caused by its low stability. The structure stability of AB2 Laves phase is mainly controlled by the geometric factor rA/rB. The present work is focused on the ternary Mn-Ni-Y system, as Mn addition helps achieving a close-to-ideal rA/rB (≤1.37) to avoid HIA or HID. Through a combination of X-ray diffraction, neutron diffraction, electron probe micro-analysis and first-principles calculations, new insight on the physicochemical properties and phase equilibrium of this ternary system was gained. Mn substitution is found to suppress the formation of a super-structure with ordered vacancies, in favor of a C15 Laves structure with a disordered distribution of Y vacancies. At low concentration Mn is accommodated only on the Ni sites, compensated by vacancies on the Y site (without long-range order). At high concentration, Mn is accommodated on both Ni and Y sites, with reduced Y vacancy concentration. The partitioning of Mn across the two sites allows to form a single-phase ternary intermetallic across a wide compositional range and suggests increased stability of the phase.