Lattice symmetry breaking and strain control of single- q magnetism in Na 3 Co 2 SbO 6

Distinguishing single- q from multi- q magnetism constrains microscopic spin Hamiltonians. Here we determine the ground state of the candidate Kitaev magnet Na 3 Co 2 SbO 6 through its coupling to the lattice. High-resolution neutron diffraction reveals a spontaneous monoclinic-to-triclinic distortion at the magnetic transition, in contrast to the absence of detectable lattice-symmetry breaking in triple- q Na 2 Co 2 TeO 6. Together with strain-selective magnetic responses, this identifies single- q order in Na 3 Co 2 SbO 6 over the proposed symmetry-preserving double- q state. Uniaxial compression along a low-symmetry in-plane direction achieves the domain selection that magnetic-field training could not, revealing the intrinsic field-induced phase boundaries of the dominant domain. These results establish strain as a symmetry-conjugate field for magnetic domains and highlight magnetoelastic coupling as a route to diagnose, manipulate, and potentially tune magnetic states.