TITLE:
A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal X2YZ Compounds
AUTHORS:
Shuchun Wu, S. Shahab Naghavi, Gerhard H. Fecher, Claudia Felser
KEYWORDS:
Elastic Constants, Elastic Stability, Tetragonal Heusler Compounds, Cubic Instability, Phase Transition
JOURNAL NAME:
Journal of Modern Physics,
Vol.9 No.4,
March
30,
2018
ABSTRACT: In the present work, the elastic constants and derived properties of tetragonal Heusler compounds were calculated using the high accuracy of the full-potential linearized augmented plane wave (FPLAPW) method. To find the criteria required for an accurate calculation, the consequences of increasing the numbers of k-points and plane waves on the convergence of the calculated elastic constants were explored. Once accurate elastic constants were calculated, elastic anisotropies, sound velocities, Debye temperatures, malleability, and other measurable physical properties were determined for the studied systems. The elastic properties suggested metallic bonding with intermediate malleability, between brittle and ductile, for the studied Heusler compounds. To address the effect of off-stoichiometry on the mechanical properties, the virtual crystal approximation (VCA) was used to calculate the elastic constants. The results indicated that an extreme correlation exists between the anisotropy ratio and the stoichiometry of the Heusler compounds, especially in the case of Ni2MnGa. Metastable cubic Ni2MnGa exhibits a very high anisotropy (≈28) and hypothetical cubic Rh2FeSn violates the Born-Huang stability criteria in the L21 structure. The bulk moduli of the investigated tetragonal compounds do not vary much (≈130 ...190 GPa). The averaged values of the other elastic moduli are also rather similar, however, rather large differences are found for the elastic anisotropies of the compounds. These are reflected in very different spatial distributions of Young’s moduli when comparing the different compounds. The slowness surfaces of the compounds also differ considerably even though the average sound velocities are in the same order of magnitude (3.2 ... 3.6 km/s). The results demonstrate the importance of the elastic properties not only for purely tetragonal Heusler compounds but also for phase change materials that exhibit magnetic shape memory or magnetocaloric effects.