New High-T_c Charge-Transfer Multiferroicity in the Quasi-2D Antiferromagnet CrSbS₃

Low-dimensional magnets, particularly 2D systems, offer a rich platform for realizing unconventional multiferroic mechanisms, especially when multiple polarization channels coexist. In the quasi-2D antiferromagnet CrSbS₃, which crystallizes in the centrosymmetric orthorhombic space group Pnma and orders magnetically at T_N ≈ 90 K, two independent polarization mechanisms are uncovered. A weak spontaneous polarization (P₁) emerges near T_E ≈ T_N at zero field, driven by partial Cr charge transfer. Synchrotron XRD refinements reveal anomalies in bond valences and average Cr─S bond length, XAS detects subtle modifications in local electronic states, and Raman spectroscopy uncovers anomalous phonon softening, indicating a charge-transfer-driven reconstruction of the lattice and phonon. In addition, second-harmonic generation (SHG) measurements exhibit a clear enhancement below T_E, directly confirming inversion-symmetry breaking and supporting the emergence of the spontaneous polarization P₁. Further, neutron diffraction reveals a C-type antiferromagnetic structure described by the magnetic space group Pnm'a, which breaks inversion symmetry and induces an additional polarization (P₂) under applied magnetic fields. Critical exponent analysis (β ≈ 0.23(2)) highlights quasi-2D magnetism. These results establish CrSbS₃ as a rare high-T_c/T_N (∼90 K) type-II multiferroic, where charge, spin, lattice, and phonon degrees of freedom cooperatively produce robust ferroic responses in a 2D framework.