Magnetic phase transitions and magnetoelastic coupling in S=12 Heisenberg antiferromagnets

We combined magneto-infrared spectroscopy and first-principles calculations to unravel the role of spin-phonon coupling in the vicinity of the magnetic field-driven phase transitions in two chemically similar $S=1/2$ Heisenberg antiferromagnets, ${\mathrm{CuF}}_{2}({\mathrm{H}}_{2}\mathrm{O}){}_{2}$(3-Clpy) and $[\mathrm{Cu}{(\mathrm{pyz})}_{2}({\mathrm{HF}}_{2})]{\mathrm{PF}}_{6}$. This comparison resolves questions about the conditions under which the lattice participates in magnetic-field-driven transitions and, at the same time, provides a way to predict how the lattice is likely to support microscopic spin rearrangements.