Exotic compositional ordering in Mn-Ni-As intermetallics.

In this work we benefited from recent advances in tools for crystal structure analysis that enabled us to describe an exotic nanoscale phenomenon in structural chemistry. The Mn0.60Ni0.40As sample of the Mn1-xNixAs solid solution, exhibits an incommensurate compositional modulation intimately coupled with positional modulations. The average structure is of the simple NiAs type, but in contrast to a normal solid solution, we observe that manganese and nickel segregate periodically at the nano-level into ordered MnAs and NiAs layers with thickness of 2-4 face-shared octahedra. The detailed description was obtained by combination of 3D electron diffraction, scanning transmission electron microscopy and neutron diffraction. The distribution of the manganese and nickel layers is perfectly described by a modulation vector q = 0.360(3) c*. Displacive modulations are observed for all elements as a consequence of the occupational modulation, and as a means to achieve acceptable Ni-As and Mn-As distances. This extraordinary modulated evolution of magnetic MnAs and non-magnetic NiAs-layers with periodicity at ~10 A level, may provide an avenue for spintronics.