First-principles Study Of High-pressure Phase Transitions In FePS₃ And FePSe₃

Transitional metal phosphorous trichalcogenides MPX₃,where M represents a transition metal such as V,Cr,Mn,Fe,Co,Ni,and X denotes either S or Se,are a kind of semiconductor materials with initial ionic conductivity and a host of ferroic properties.This kind of material with adjustable two-dimensional magnetic,ferroelectricity,fine chemical stability and light absorption performance,has been widely used in two-dimensional spintronic devices,field effect transistors,visible and ultraviolet photodetectors,photo-electrochemical reactions,hydrogen(H₂)storage,and Li-ion batteries,etc.On the other hand,as an effective method to regulate the physical properties of materials and synthesize new materials,high pressure has been widely used in the condensed matter physics.There are not many high pressure studies about MPX₃,and there are still some doubts about the high pressure experiments on FePX₃.For example,how many phase transition does FePX₃ undergo in the pressure range of 0 to 35 GPa?Do the high pressure structures proposed by Haines et al.have a transition from high-spin to low-spin?What are the structural characteristics of the high pressure phases of FePSe₃?In order to be able to explain the above questions,we use first-principles calculations in this paper to study the phase transitions of crystal structure,magnetic,and the bandwidth-controlled Mott transitions of FePS₃ and FePSe₃ under high pressure.At ambient pressure,FePS₃ and FePSe₃ have been confirmed to be the C2/m and R3 phase,and they will undergo two structural phase transitions within the high pressure range of 0?35 GPa.For FePS₃,the B-I phase(C2/m)changes to the B-II phase(C2/m)at 5 GPa,and then further turns into the B-III phase(P31m)at17 GPa;For FePSe₃,its phase transition sequence is similar to that of FePS₃.The ambient-pressure phase(R3)will transform into the B-II phase(C2/m)under a pressure of 6 GPa,and then into the B-III phase(P31m)at around 15 GPa.When the second structural phase transition of FePS₃ and FePSe₃ occurs,it is accompanied by large volume collapse,antiferromagnetic-nonmagnetic transition(spin-crossover)and insulator-metal transition,in line with the phenomena observed in the two high-pressure experiments.Our phonon dispersion calculations show that the above high-pressure structures are all dynamically stable within a specific pressure range.Then we conducted a more comprehensive structure search for FePS₃ through the RG² code based on the combination of space group and graph theory.Through enthalpy,phonon density of states(PDOS)and X-ray diffraction(XRD)simulations,we found that the B-II and B-III phases are still the most stable one within a specific pressure range.According to the high pressure experimental research of MPX₃and the calculation results of this paper,it can be predicted that the most of MPX₃ compounds will undergo a quasi-isomorphic phase transition(B-I?C2/m?B-II?C2/m)under high pressure,and there will be magnetic phase transition and Mott transition.