Recent advances in tools for crystal structure analysis enabled us to describe a new phenomenon in structural chemistry, which, to this day, has remained hidden. Here we describe a crystal structure with an incommensurate compositional modulation, Mn0.6Ni0.4As. The sample adopts the NiAs type structure, but in contrast to a normal solid solution, we observe that manganese and nickel separate into layers of MnAs and NiAs with thickness of 2-4 face-shared octahedra. Experimentally, results are obtained by combination of 3D electron diffraction, scanning transmission electron microscopy and neutron diffraction. The distribution of octahedral units between the manganese and nickel layers is perfectly described by a modulation vector q = 0.360(3) c*. An additional periodicity is thus present in the compound. Positional modulation is observed of all elements as a consequence of the occupational modulation.
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