Effects of Temperature and Pressure on Structural Transformations in CuInP₂S₆ and CuInP2S6-In_4/3P₂S₆ heterostructures

CuInP₂S₆ is a van der Waals gapped material with a high transition temperature (T_C=315) under ambient conditions. Recent work has shown that this material is useful for a variety of applications (e.g. ultrasound transducers, frequency conversion, ferroelectric capacitor, ferroelectric field effect transducer, etc). Under pressure, CuInP₂S₆ sees an increase in its ferroelectric Curie temperature, in contrast to most ferroelectric materials, due to the fact that its unit cell decreases in the ferroelectric state. Additionally, at higher pressures (> 4.0 GPa) the layered structure of CuInP₂S₆ transforms into one with a more 3D nature- depending on the pressure applied, the stacking first changes to a higher symmetry monoclinic and then further transforms to a trigonal system which, in similar FePS₃ and FePSe₃ materials results in interesting changes in the band structure enabling changes from semiconductor/insulator to conductor. We have undertaken a dual Raman and diffraction study to characterize the ferroelectric transition as a function of pressure and temperature in CuInP₂S₆. We have also worked to study the effects of pressure on the electronic structure of the material through (P,T) investigations. Finally, we have incorporated ab initio theoretical investigations of CuInP₂S₆ to explain the evolution of the structure-property relationships in this system.