In a companion paper, the properties of the other polymorphs of Mn and their relative stability will be discussed. However, it is suggested that the magnetism leads to a splitting of the crystallographically inequivalent sites into a larger number of magnetic subgroups than deduced from the magnetic neutron diffraction data, but in accordance with NMR experiments. The calculated crystalline and magnetic structures are in good agreement with experiment. Consequently, also the other magnetic moments are rotated out of their collinear orientation. The noncollinear magnetic structure is due to the fact that the MnIV atoms arranged on triangular faces of the coordination polyhedra are not entirely nonmagnetic-their frustrated antiferromagnetic coupling leads to the formation of a local spin structure reminiscent of the Néel structure of a frustrated triangular antiferromagnet. The crystal structure of α − Mn is essentially a consequence of these conflicting tendencies-it may be considered as a topologically close-packed intermetallic compound formed by strongly magnetic (MnI, MnII) and weakly magnetic (MnIII) or even nearly nonmagnetic (MnIV) atoms. Short interatomic distances produced by strong bonding tend to quench magnetism. Members of a group typically have similar properties and electron configurations in their outer shell. It is shown that the strange properties of Mn arise from conflicting tendencies to simultaneously maximize according to Hund’s rule the magnetic spin moment and the bond strength, as expected for a half-filled d band. Mn Manganese 25 54.938 Glossary Group A vertical column in the periodic table.
In this paper we present an ab initio spin-density functional study of the structural and magnetic properties of α − Mn. The magnetic phase transition is coupled to a tetragonal distortion of the crystalline structure. At a Néel temperature of T N = 95 K, a transition to a complex noncollinear antiferromagnetic phase takes place. While most metallic elements adopt a simple crystal structure and order magnetically-if at all-in a simple ferromagnetic or antiferromagnetic configuration, the stable phase of manganese at ambient conditions, paramagnetic α − Mn, adopts a complex crystal structure with 58 atoms in the cubic cell. Manganese is an element with outstanding structural and magnetic properties.
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