We present x-ray, neutron scattering and heat capacity data that reveal a coupled first-order magnetic and structural phase transition of the metastable mixed-valence post-spinel compound Mn3O4 at 210 K. Powder neutron diffraction measurements reveal a magnetic structure in which Mn3+ spins align antiferromagnetically along the edge-sharing \emph{a}-axis, with a magnetic propagation vector k = [1/2, 0, 0]. In contrast, the Mn2+ spins, which are geometrically frustrated, do not order until a much lower temperature. Although the Mn2+ spins do not directly participate in the magnetic phase transition at 210 K, structural refinements reveal a large atomic shift at this phase transition, corresponding to a physical motion of approximately 0.25 {\AA} even though the crystal symmetry remains unchanged. This "giant" response is due to the coupled effect of built-in strain in the metastable post-spinel structure with the orbital realignment of the Mn3+ ion.
@article{arxiv.1211.1667,
title = {Giant atomic displacement induced by built-in strain in metastable Mn$_3$O$_4$},
author = {Shigeto Hirai and Antonio M. dos Santos and Maxwell C. Shapiro and Jamie J. Molaison and Neelam Pradhan and Malcolm Guthrie and Christopher A. Tulk and Ian R. Fisher and Wendy L. Mao},
journal= {arXiv preprint arXiv:1211.1667},
year = {2012}
}