Polymorph Competition and Metastable Half-Metallicity in NiMnAs: A First-Principles Study of Hexagonal Ground-State Stability and Coherent-Strain Screening

We investigate polymorph competition, stability, and half-metallicity in NiMnAs using spin-polarized GGA-PBE calculations. Cubic, hexagonal, tetragonal, and orthorhombic structures with different atomic orderings are examined. Static total-energy comparisons identify ferromagnetic hexagonal $\alpha -P\overline{6}2m$  as the lowest-energy examined phase, whereas cubic C1b NiMnAs is a metastable half-metal. Elastic constants and phonon spectra support mechanical and local harmonic dynamical stability of the cubic, orthorhombic, and hexagonal phases. Their electronic structures and vibrational thermodynamic properties are analyzed. Cubic NiMnAs exhibits 100% spin polarization, an integer magnetic moment of 4.00 µ_B per formula unit, and a minority-spin gap of 0.73 eV, while the hexagonal and orthorhombic phases are metallic. An analytical coherent-strain model based on cubic elastic constants estimates the elastic cost of biaxial deformation. Simple cube-on-cube matching identifies GaAs and Ge as low-mismatch candidates for future epitaxial investigations. Harmonic phonon calculations characterize the vibrational thermodynamic response. The strain model excludes interface chemistry and strain-induced electronic changes, while the thermal analysis omits thermal expansion and explicit anharmonicity. Consequently, these results provide a screening framework without establishing epitaxial stabilization, preservation of half-metallicity under strain, or finite-temperature phase ordering. NiMnAs exhibits polymorph-dependent magnetism, with a metastable cubic half-metallic phase offering motivation for future synthesis and spintronic studies.