Giant atomic displacement at a magnetic phase transition in metastable Mn₃O₄

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 postspinel compound Mn₃O₄ at 210 K. Powder neutron diffraction measurements reveal a magnetic structure in which Mn3 ⁺ spins align antiferromagnetically along the edge-sharing 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 Å, even though the crystal symmetry remains unchanged. This "giant" response is due to the coupled effect of built-in strain in the metastable postspinel structure with the orbital realignment of the Mn3⁺ ion.