Magnetoelastic honeycomb fragmentation in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>VI</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:math>

The unexpected discovery of ordered magnetism in two-dimensional van der Waals materials at the monolayer limit [B. Huang , ] challenges the Mermin-Wagner theorem [N. D. Mermin and H. Wagner, ], which forbids spontaneous breaking of continuous symmetries in two dimensions at finite temperatures. The persistence of static magnetism in low dimensions is fundamentally influenced by magnetic anisotropy which is tied to the local single-ion crystalline electric field. Crucially, spin-orbit coupling connects the structural properties with spin degrees of freedom. We investigate the magnetic single-ion properties in the two-dimensional van der Waals magnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>VI</a:mi> <a:mn>3</a:mn> </a:msub> </a:math> . Utilizing neutron and x-ray diffraction, we map out the symmetry breaking phase transitions in <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>VI</b:mi> <b:mn>3</b:mn> </b:msub> </b:math> and argue for the presence of a single structural transition at <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi mathvariant="normal">T</c:mi> <c:mi>S</c:mi> </c:msub> <c:mo>∼</c:mo> <c:mn>80</c:mn> </c:mrow> </c:math> K, driven by an orbital degeneracy, followed by a ferromagnetic transition at a lower temperature, <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:msub> <e:mi mathvariant="normal">T</e:mi> <e:mi>C</e:mi> </e:msub> <e:mo>∼</e:mo> <e:mn>50</e:mn> </e:mrow> </e:math> K. Through a comparative analysis of samples prepared under varying conditions, we suggest that lower temperature transitions reported near <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mo>∼</g:mo> <g:mn>30</g:mn> </g:math> K are not intrinsic to <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:msub> <h:mi>VI</h:mi> <h:mn>3</h:mn> </h:msub> </h:math> . A group theoretical analysis suggests a structural transition from rhombohedral <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:mrow> <i:mi>R</i:mi> <i:mover> <i:mn>3</i:mn> <i:mo>¯</i:mo> </i:mover> </i:mrow> </i:math> to triclinic <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:mrow> <j:mi>P</j:mi> <j:mover> <j:mn>1</j:mn> <j:mo>¯</j:mo> </j:mover> </j:mrow> </j:math> or <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mi>P</k:mi> <k:mn>1</k:mn> </k:mrow> </k:math> . This transition is significant as it suggests the formation of two distinct crystallographically inequivalent <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:msup> <l:mrow> <l:mi mathvariant="normal">V</l:mi> </l:mrow> <l:mrow> <l:mn>3</l:mn> <l:mo>+</l:mo> </l:mrow> </l:msup> </l:math> sites on the honeycomb lattice, each with distinct spin-orbital properties. Neutron spectroscopy provides evidence for dominant magnetic exchange coupling only between symmetry-equivalent sites in the triclinic unit cell. We suggest this breaks up the low-temperature two-dimensional honeycomb <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:msub> <n:mi>VI</n:mi> <n:mn>3</n:mn> </n:msub> </n:math> lattice into two interpenetrating approximately hexagonal planes resulting in a fragmentated honeycomb. Our findings highlight the critical role of magnetoelastic coupling in determining the magnetic and structural phases in two-dimensional van der Waals magnets.