Altermagnetism and Room-Temperature Metal-to-Insulator Transition in CsCr2S2O

Altermagnetism (AM) has been theoretically predicted to arise widely in T2OCh2 (T = 3d transition metal; Ch = S, Se, Te) layers, offering opportunities for unconventional spin-dependent electronic phenomena. However, experimental realization of AM in this family remains scarce. Recently, spin-split electronic bands have been observed using surface-sensitive photoemission spectroscopy in the 1221-type, vanadium-based compounds KV2Se2O and Cs1-δV2Te2O, where the T2OCh2 layer and alkali metal are alternately stacked. Nevertheless, their altermagnetic spin splitting is compensated between adjacent layers and therefore vanishes in the bulk. Here, we report AM in a new 1221-type chromium-based compound, CsCr2S2O. It undergoes antiferromagnetic ordering at 326 K, followed by a metal-to-insulator transition (MIT) at 305 K. The antiferromagnetism (AFM) adopts a C-type configuration, satisfying the spin space symmetry required for AM. Across the MIT, a structural modulation with propagation vector q = (1/2, 1/2, 0) emerges, accompanied by Cr charge disproportionation and magnetic modulation, while preserving the altermagnetic spin splitting. Our comprehensive experiments and density functional theory (DFT) calculations reveal the cooperative interplay among lattice distortion, magnetic ordering, and electronic correlations underlying the Verwey-type MIT. The Verwey-type transition between metallic and insulating altermagnetic states in CsCr2S2O offers a unique opportunity to explore multifunctional charge- and spin-based applications and to study emergent collective electronic states associated with AM.