Magnetostructural correlations in antiperovskite manganese nitrides were investigated systematically for stoichiometric and solid solution Mn3Cu1−xAxN (A = Co, Ni, Zn, Ga, Ge, Rh, Pd, Ag, In, Sn or Sb). This class of nitrides is attracting great attention because of their giant negative thermal expansion, which is achieved by doping Ge or Sn into the A site as a relaxant of the sharp volume contraction on heating (spontaneous volume magnetostriction ωs) because of the magnetovolume effects. The physical background of large ωs and mechanism of how the volume contraction becomes gradual with temperature are central concerns for the physics and applications of these nitrides. An entire dataset of thermal expansion, crystal structure and magnetization demonstrates that the cubic triangular antiferromagnetic state is crucial for large ωs. The intimate relationship between ωs and the magnetic structure is discussed in terms of geometrical frustration related to the Mn6N octahedron and magnetic stress concept. The results presented herein also show that ωs depends on the number of d electrons in the A atom, suggesting the important role of the d orbitals of the A atom. Not all the dopants in the A site, but the elements that disturb the cubic triangular antiferromagnetic state, are effective in broadening the volume change. This fact suggests that instability neighboring the phase boundary is related to the broadening. The relation between the gradual volume change and the local structure anomaly is suggested by recent microprobe studies.

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Plastics have beneficial features of light weight and high workability, but their large thermal expansion emerges as a shortcoming for which reduction is desired in highly advanced industries. The negative thermal expansion (NTE) of antiperovskite manganese nitrides is greater than -30 ppm/K in a (coefficient of linear thermal expansion), which is several to ten times as large as that of conventional NTE materials. The giant NTE of the manganese nitrides can compensate or control large thermal expansion of plastics by forming composites. We attempt to make the fine grained NTE manganese nitrides for thermal expansion control of light transmitting plastics. In the fine graining process, NTE of manganese nitrides changed because of a change in the chemical composition. A technique to control NTE and grain size simultaneously is necessary for the manganese nitride filler.