Giant atomic displacement induced by built-in strain in metastable Mn${}_3$O${}_4$

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 post-spinel 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 \emph{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 Å 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 Mn${}^{3+}$ ion.

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