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Abstract Ferro-rotational magnet RbFe(SO₄)₂has attracted attention for its stable ferro-rotational phase and electric-field-controlled magnetic chirality. This work presents the multiferroic properties andH–Tphase diagram of RbFe(SO₄)₂, which have been underexplored. Our measurements of magnetic susceptibility, ferroelectric polarization, and dielectric constant under various magnetic fields reveal four distinct phases: (I) a ferroelectric and helical magnetic phase below 4 K and 6 T, (II) a paraelectric and collinear magnetic phase below 4 K and above 6 T, (III) a paraelectric and non-collinear magnetic phase below 4 K and above 9 T, and (IV) a paraelectric and paramagnetic above 4 K. This study clarifies the multiferroic behavior andH–Tphase diagram of RbFe(SO₄)₂, providing valuable insights into ferro-rotational magnets. 

Abstract We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS₄in bulk, few-, and single-layer form. Analysis of the mode pattern reveals aC2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS₃where phonons shift as 1/size²and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density, and size of the van der Waals gap.