Structural change and phase coexistence upon magnetic ordering in the magnetodielectric spinel Mn$_3$O$_4$

TL;DR

This study reveals that Mn₃O₄ exhibits phase coexistence of tetragonal and orthorhombic structures below its magnetic transition temperature, driven by lattice mismatch strains, highlighting a common phenomenon in magnetic spinels at low temperatures.

Contribution

The paper demonstrates the coexistence of tetragonal and orthorhombic phases in Mn₃O₄ below 42 K using high-resolution synchrotron X-ray diffraction, emphasizing strain-induced phase coexistence in magnetic spinels.

Findings

Coexistence of tetragonal and orthorhombic phases below 42 K.

Significant lattice mismatch strains drive phase coexistence.

Strain tensors reveal a highly strained matrix accommodating phase nucleation.

Abstract

Cooperative Jahn-Teller ordering is well-known to drive the cubic $Fd\overline{3}m$ to tetragonal $I$4$_1$/$amd$ structural distortion in Mn$_3$O$_4$ at 1170 $^{\circ}$C. Further structural distortion occurs in Mn$_3$O$_4$ upon magnetic ordering at 42 K. Employing high-resolution variable-temperature synchrotron x-ray diffraction we show that tetragonal $I$4$_1$/$amd$ and orthorhombic $Fddd$ phases coexist, with nearly equal fractions, below the N\'eel temperature of Mn$_3$O$_4$. Significant variation of the orthorhombic $a$ and $b$ lattice constants from the tetragonal $a$ lattice constant is observed. Structural phase coexistence in Mn$_3$O$_4$ is attributed to large strains due to the lattice mismatch between the tetragonal $I$4$_1$/$amd$ and the orthorhombic $Fddd$ phases. Strain tensors determined from Rietveld refinement show a highly strained matrix of the $I4_1/amd$ phase that accommodates the nucleated orthorhombic $Fddd$ phase in the phase coexistence regime. A comparison of the deformation observed in Mn$_3$O$_4$ to structural deformations of other magnetic spinels shows that phase coexistence may be a common theme when structural distortions occur below 50 K.