Massachusetts Institute of Technology Lincoln Laboratory Interpretation of the Magnetic and Crystallographic Properties of Compounds with the B31 Structure

The orthorhombic B31 structure, typified by MnP, is interpreted as a distortion of the hexagonal NiAs (B8j) structure due to metal-metal bonding within the hexagonal basal planes. The number of d-like 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 electronegativity 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 cation-cation separation R 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 splittings 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 > R , it is possible to derive interatomic spin correlations from the Heisenberg exchange Hamiltonian and superexchange theory. With the assumption that the sign of the cation-cation exchange couplings stay the same as R varies through R , it is possible to make sharp predictions of Pauli paramagnetism vs antiferromagnetism vs metamagnetism vs ferromagnetism as a function of electron/cation ratio. The low-temperature spin configuration of metamagnetic 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. This technical documentary report is approved for distribution. Franklin C. Hudson, Deputy Chief Air Force Lincoln Laboratory Office