Phase Stability, Structural Transition, and Hydrogen Absorption−Desorption Features of the Polymorphic La₄MgNi₁₉ Compound

Abstract

The phase stability, structural transition, and hydrogen storage characteristics of the polymorphic La4MgNi19 compound are systematically investigated in the present work. The La4MgNi19 compound has two variants in the temperature range of 840−960 °C with a preference for a hexagonal structure (2H) at a higher temperature and a rhombohedral structure (3R) at a lower temperature. Because the structure and composition are almost identical, the polymorphic transition between the 2H and 3R phases may proceed through short-range displacements of their subunit layers. Detailed analyses on XRD patterns for the hydrided or dehydrided samples indicate that the equilibrium pressures of hydrogen absorption and desorption for the 2H phase are lower than the pressures for the 3R phase, but the pressure gap between the two phases is too small to be identified from the measured pressure−composition (P−C) isotherms. The average hydriding and dehydriding enthalpies are determined to be −32.1 and 31.5 kJ/mol H2, respectively. As hydrogen atoms enter into the lattices, the strains in both the 2H and the 3R phases increase first in the region of the dilute H-dissolved solid solutions and then dramatically decrease once the dilute solid solutions transform into the corresponding hydrides.

Crystal structure and cyclic properties of hydrogen absorption–desorption in Pr2MgNi9

“…4. The enthalpy of hydride formation was established as À37 kJ/ mol of H 2 , which is close to those for the LaNi 5 eH 2 system [10] and the La 4 MgNi 19 eH 2 system [9]. The cyclic properties of LaNi 5 were reported by Goodell [15].…”

“…The electrochemical capacity of the alloy is higher than that of conventional MmeNi alloys with the CaCu 5 -type structure. Zhang et al investigated the crystal structures and PeC isotherms of polymorphic La 4 MgNi 19 with superlattice structures [9]. Sm 5 Co 19 -type (2H) and Ce 5 Co 19 -type (3R) structures coexisted in the X-ray diffraction (XRD) profile of La 4 MgNi 19 quenched from 1173 K. Crystal structure analysis using the Rietveld method indicated that the mass fractions of the 2H and 3R phases were 43% and 57%, respectively.…”

Changes of crystal structure and hydrogen storage performances for multiphase La0.7Mg0.3Ni3 alloy upon gas–solid cycling

“…5d. The result resembles the tendency [21,22]. However, there is no evident plateau between 0.025 and 0.200 MPa.…”

“…Thus, Location C is determined to be A 5 B 19 -type phase. Two allotropes are reported in A 5 B 19 -type structures, i.e., Pr 5 Co 19 -type (2H) and Ce 5 Co 19 -type (3R) structures [16,21,22]. According to the literatures of Nakamura et al [21] and Zhang et al [22], the specific peak of Ce 5 Co 19 -type structure at 31°-32°does not appear, instead that the peak of Pr 5 Co 19 -type structure at 32°-33°is clearly as shown in Fig.…”

Study on solid solubility of Mg in Pr 3−x Mg x Ni 9 and electrochemical properties of PuNi 3 -type single-phase RE–Mg–Ni (RE = La, Pr, Nd) hydrogen storage alloys

“…PuNi 3 -type structure is constructed by stacking of alternating AB 5 and A 2 B 4 units (Lavestype) in 1:1 ratio [7]. Other similar to A 2 B 4 /AB 5 mixed structure alloys, such as A 2 B 7 -type (Ce 2 Ni 7 or Gd 2 Ni 7 ) and A 5 B 19 -type (Pr 5 Co 19 and Ce 5 Co 19 ) alloys are also proposed as metal hydride alloys [8][9][10][11].…”