Competing charge and spin instabilities and giant magnetoelastic coupling in CrSb

Electronic, magnetic, and lattice degrees of freedom are often strongly intertwined in quantum materials, giving rise to competing ordered states and emergent collective phenomena. Understanding how these interactions evolve across symmetry-breaking transitions is essential for identifying new mechanisms of electronic ordering. Here, we show that the antiferromagnet chromium antimonide provides an ideal platform to investigate this interplay and uncover short-range charge-order fluctuations at the M point of the Brillouin zone, q* = [Formula: see text], persisting well above the Néel temperature, T_N. These fluctuations collapse upon entering the magnetically ordered phase, revealing a direct competition between charge and spin order parameters. At T_N, the phonon dispersion at q* develops a pronounced Kohn-like anomaly, signaling strong electron-phonon coupling in the vicinity of the magnetic transition. Below T_N, exchange striction markedly renormalizes the associated soft phonon mode by ∼6 milli-electron volts, corresponding to the largest spin-phonon coupling reported to date. First-principles calculations show that the relevant phonon branch is unstable at the harmonic level only in the absence of magnetic order, highlighting the remarkable sensitivity of the lattice to magnetic symmetry breaking. The suppression of charge correlations upon magnetic ordering, together with the abrupt reconstruction of the phonon spectrum at T_N, demonstrates an exceptional coupling between charge, spin, and lattice degrees of freedom in chromium antimonide. With its chemically simple structure and symmetry-protected altermagnetic state, chromium antimonide emerges as a model system for exploring giant spin-phonon coupling, competing electronic instabilities, and emergent broken symmetries in altermagnetic materials.