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

TL;DR

This study uncovers a giant atomic displacement in Mn₃O₄ caused by built-in strain during a coupled magnetic and structural phase transition at 210 K, highlighting the interplay between metastable structure and magnetic ordering.

Contribution

It reveals a large atomic shift driven by built-in strain in a metastable Mn₃O₄ compound during a phase transition, a phenomenon not previously observed in such materials.

Findings

Giant atomic displacement (~0.25 Å) at 210 K phase transition.

Coupled magnetic and structural phase transition involving Mn³⁺ and Mn²⁺ ions.

Built-in strain and orbital realignment induce the large atomic shift.

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 {\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 Mn$^{3+}$ ion.