MnFeAs crystallizes in two phases under different synthetic conditions:(i)the tetragonal Cu2Sb-type structure(isostructural to superconductors LiFeAs and NaFeAs)at normal conditions; or(ii)the hexagonal Fe2P-type structure at high pressure and temperature.In both phases,Mn atom is square pyramidally coordinated by As atoms,while Fe atoms occupy half of the tetrahedron holes formed by As atoms.The tetragonal phase is antiferromagnetic(AFM)(TN~463 K)with a doubled crystallographic c-axis,whereas the hexagonal phase is ferromagnetic(FM)with TC near 190 K.Effective exchange parameters obtained from SPRKKR calculations indicate that both direct and indirect exchange couplings play essential roles in understanding the magnetic orderings observed in both tetragonal and hexagonal phases.The nature of nearest-neighbor direct exchange couplings,i.e.,either FM or AFM,were predicted by analyzing the nature of metal-metal orbital interactions,i.e.,bonding,non-bonding,or antibonding,near the Fermi level.The indirect exchange couplings were rationalized by both RKKY and superexchange mechanism.In addition,the structure change under pressure was studied by examining total energy variation as volume.