The　structural,　half-metallic　and　elastic　properties　of　the　half-Heusler　compounds　NiMnM　(M　=　Sb,　As　and　Si)　and　IrMnAs　were　investigated　using　first-principles　calculations　within　the　generalized　gradient　approximation　(GGA)　based　on　density　function　theory　(DFT).　The　most　stable　lattice　configurations　about　site　occupancy　are　(Ni)(4a)(Mn)(4c)(Sb)(4d),　(Ni)(4a)(Mn)(4c)(As)(4d),　(Ni)(4a)(Mn)(4c)(Si)(4d)　and　(Ir)(4a)(Mn)(4c)(As)(4d),　respectively,　and　the　exchange　of　elements　in　Wyckoff　position　4c　and　4d　results　in　an　identical　(symmetry-related)　phase.　The　half-Heusler　compounds　show　half-metallic　ferromagnetism　with　a　half-metallic　gap　of　0.168　eV,0.298　eV,　0.302　eV　and　0.109　eV,　respectively,　and　the　total　magnetic　moments　(M-tot)　are　4.00　mu　B,　4.00　mu　B,　3.00　mu　B　and　3.00　mu　B　per　formula　unit,　respectively,　which　agree　well　with　the　Slater-Pauling　rule　based　on　the　relationship　of　valence　electrons.　The　compound　(Ir)(4a)(Mn)(4c)(As)(4d)　with　half-metallic　ferromagnetic　character　was　reported　for　the　first　time.　The　individual　elastic　constants,　shear　modulus,　Young＇s　moduli,　ratio　BIG　and　Poisson＇s　ratio　were　also　calculated.　The　compounds　are　ductile　based　on　the　ratio　BIG.　The　Debye　temperatures　derived　from　the　average　sound　velocity　(nu(m))　are　327　K,　332　K,　434　K　and　255　K,　respectively.　The　predicted　Debye　temperature　for　NiMnSb　agrees　well　with　the　available　experimental　value,　and　the　Debye　temperatures　for　the　rest　three　compounds　were　reported　for　the　first　time.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.