In a joint research between Waseda University and Aoyama Gakuin University, the mechanism of "negative thermal expansion" of the substance "reverse perovskite type manganese nitride" that expands when cooled was elucidated for the first time in the world.
Normally, a substance contracts when cooled and expands when warmed, but it is known that a reverse perovskite-type manganese nitride exhibits a large volume expansion when cooled.However, the physical mechanism of why it expands when cooled has been a mystery for over 40 years.
On the other hand, in the reverse perovskite type manganese nitride, it is also known that the spins of electrons on the manganese ion are aligned with the temperature drop.Therefore, in this research, we focused on the relationship between the "electron spin alignment phenomenon" and the "negative thermal expansion phenomenon".
The research group first found that there are two types of interaction between electron spins, one that attempts to align the spins antiparallel and the other that attempts to align them in parallel.Then, it was found that the interaction between spins is stronger when the volume is expanded and the ions are separated from each other than when the volume is contracted as in a normal substance because of the competition between them.Therefore, when the temperature is lowered and spin alignment occurs, the crystal volume spontaneously expands in order to strengthen the interaction between spins, and a "negative thermal expansion phenomenon" occurs.
This suggests that a substance with competing spin-to-spin interactions is likely to cause a negative thermal expansion phenomenon, and a crystal structure expected as an unknown negative thermal expansion substance has also been proposed.Furthermore, the research group constructed the world's first mathematical model that reproduces the spin alignment of inverted perovskite-type manganese nitrides, and succeeded in completely elucidating the mechanism of the negative thermal expansion phenomenon theoretically.
Negative thermal expansion substances are expected to enable the realization of unprecedented materials whose volume and length do not change even when the temperature changes, by combining them with ordinary substances.
Paper information:[Physical Review Materials] Theory of magnetism-driven negative thermal expansion in inverse perovskite antiferromagnets