INTERPRETATION OF THE MAGNETIC AND CRYSTALLOGRAPHIC PROPERTIES OF COMPOUNDS WITH THE B31 STRUCTURE

摘要

The orthorhombic B31 structure, typified by MnP, is interpreted as a distortion of the hexagonal NiAs (B8]) structure due to metal-metal bonding within the hexagonal basal planes. The number of d—Iike electrons per cation is known from the chemical formula provided the Fermi level lies in an energy gap between filled and empty bands of broad-band states, as is generally the case where there is a large electro negativity difference between cation and anion. It is argued that in stoichiometric materials, with an integral electron/cation ratio, it is possible to define operationally a critical catenation separation Rc such that the d-like states must be treated as collective states if R < R , may be treated as localized states if R > R . An empirical value for R is presented for transition-metal oxides. Since the cubic component of the ligand fields and the intra-atomic exchange give splitting that are larger than the widths of d-like bands, it is possible to construct schematic one-electron energy diagrams for various electron/cation ratios. From a knowledge of R , it is possible to distinguish localized from collective d-like states that are simultaneously present. These diagrams are used to obtain the spin-only contribution to the atomic moment. For the case R > Rc , it is possible to derive interatomic spin correlations from the Heisenberg exchange Hamiltonian and super exchange theory. With the assumption that the sign of the cation cation exchange couplings stay the same as R varies through Rc , it is possible to make sharp predictions of Pauli par magnetism vs. an tiferromagnetism vs met magnetism vs ferromagnetism as a function of electron/cation ratio. The low-temperature spin configuration of met magnetic MnP is predicted to be a strongly distorted spiral propagating along the orthorhombic c-axis with spins lying mostly in the b-c plane. It is also noted that the B20 structure of FeSi can be interpreted as a distortion of the zinc-blende structure due to metal-metal bonding.