Giant atomic displacement at a magnetic phase transition in metastable Mn3O4

Abstract

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 Mn${}_{3}$O${}_{4}$ at 210 K. Powder neutron diffraction measurements reveal a magnetic structure in which Mn${}^{3+}$ spins align antiferromagnetically along the edge-sharing $a$ axis, with a magnetic propagation vector $k=[1/2,0,0]$. In contrast, the Mn${}^{2+}$ spins, which are geometrically frustrated, do not order until a much lower temperature. Although the Mn${}^{2+}$ 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 postspinel structure with the orbital realignment of the Mn${}^{3+}$ ion.