Temperature-Dependent Magnetization, Raman Scattering, and X-Ray Diffraction Study of Phase Transitions in Layered Multiferroic CuCrP₂S₆

Functional van der Waals layered materials exhibit interesting phenomena such as magnetism and ferroelectricity and have been proposed for use in next-generation nanoscale devices. Metal thiophosphates are an interesting class of these materials that contain a common structural framework where altering the cation can induce different types of ferroic ordering, including ferroelectricity and magnetism. The compound CuCrP₂S₆ is a promising 2D material that evinces multiferroic behavior where the Cu⁺ and Cr⁺³ cations are responsible for antiferroelectric (AFE) and antiferromagnetic ordering, respectively, which are predicted to couple. Here, we use magnetization, X-ray diffraction, and Raman spectroscopy to map out these phase transitions. The AFE phase transition is complex and shows a gradual transition to complete antipolar order with an intermediate quasi-antipolar step. X-ray diffraction studies reveal evidence for negative thermal expansion which we argue is tied to magnetic frustration. This is accompanied by a drastic reduction in rotational and translational mode frequencies of the anion groups in CuCrP₂S₆. Our temperature-dependent structural data provides an important reference for subsequent research into this promising 2D multiferroic material.