Abstract
An interesting sequence of deformations from the NiAs structure type occurs in simple 3d transition-metal phosphides. Whereas for d' and dZ electron counts the NiAs structure type is preferred, for d3 to d6 electron counts a distortion to the MnP type structure occurs, breaking the triangular (39 nets of atoms and involving the formation of metal-metal and phosphorus-phosphorus zigzag chains. Going to a d7 electron count we find a double pairing distortion in the NiP structure with metal and phosphorus pairs. The first deformation is driven by a second-order Jahn-Teller-like distortion, splitting a strong peak in the density of states of the NiAs structure. The distortion from the MnP to the NiP structure type is traced to bond formation between pairs of metal and phosphorus atoms, the driving force being the introduction of an energy gap close to the Fermi level.
The hexagonal NiAs structure is one of the most common AB structures in transition-metal chemistry. It has a unique position in crystal chemistry, being intermediate between predominantly ionic compounds with a NaCl structure and intermetallic compounds having the CsCl structure., Over 100 compounds crystallize with the NiAs or related structures, and with very few exceptions all contain transition-metal atoms in combination with metalloid or B-group atoms.) The more metallic phases have many properties characteristic of intermetallic phases: opacity, metallic lustre, and conductivity. The great variety of electrical and magnetic properties originating from the variable composition makes them interesting materials in metallurgy.
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