Theoretical Study On The Structure And Physical Property Of 3dTMP₄ And TMPS Under High Pressure

Transition metal phosphides exhibit excellent physical and chemical properties and are widely interested in the field of electrocatalysis.Pressure can effectively shorten the distance between atoms,regulate the crystal structure and electronic structure of materials,and introduce novel properties which are not available under ambient pressure.Here,by the first-principles calculation based on density functional theory,we researched on the crystal structure,electronic structure and mechanical properties of TMP₄(TM=Cr,Mn,Fe)under high pressure.Firstly,combining the CALYPSO code and first-princples calculations,we found three high-pressure phases for Cr P₄ with space group of I4/mcm,P3₂21 and C2 at the specific pressure at 6.0,42.8 and 55.8 GPa,respectively;four high-pressure phases for Mn P₄ with space group of P-31c,I4/mcm,P3₂21 and P4/nbm at the specific pressure at 20.7,46.3,77.9 and 95.2 GPa,respectively;and one high-pressure phase for Fe P₄ with space group of P4/nbm at the specific pressure at 49.7 GPa.Secondly,both the band structure and states of density curves show that the I4/mcm-and P3₂21-Cr P₄are semiconductors with indirect band gap of 0.33 e V and 1.28 e V respectively,while the other high-pressure phases of Cr P₄,Mn P₄ and Fe P₄ are all metallic.Thirdly,from the results of the electronic structure,strong covalent interaction can be found between the neighboring P atoms,whereas the interactions between TM and P atoms are mainly ionic with part of covalent interaction.Fourth,as the pressure increases,phase transition occurs,TMP₄ transforms from semiconductor to metal,and the covalent interaction between TM-P and P-P in the structure gradually weakens.Elemental doping is an efficient method to improve the activity of catalysts.The transition metal phosphorous sulfide(TMPS)can be regarded as the addition of half of phosphorus to the transition metal disulphide.Hitherto,few researches have been reported on the phases of TMPS under high pressure.Therefore,we systematically explored the stable structures of TMPS(TM=Fe,Co,Ni)between 0-200GPa,and discussed their crystal structures,electronic structures and elastic properties.By a wide structural search,we found the high-pressure phases with the space group of I-4m2 and I4₁md for Fe PS at84.3 and 161.1 GPa,the high-pressure phase with the space group of P2₁/m for Co PS at130.5 GPa and the high-pressure phases with the space group of Pca2₁,C2/m and C2/c for Ni PS at 34.6,53.5 and 76.8 GPa,respectively.All of the proposed high-pressure phases are metallic.Ni PS of Pca2₁ structure type is expected to be synthesized experimentally in the future.In addition,a new structure of Co PS was found to be stable at ambient conditions with Pca2₁ symmetry,Pca2₁-Co PS was a semiconductor with an indirect band gap of 1.25 e V,which was wider that of the known P2₁/c-Fe PS phase was a semiconductor(0.56 e V).By analyzing the electronic structure,a certain degree of covalent interaction can be found between TM and P/S atoms in the TMPS structure.The bonding of P-S and P-P are extremely strong covalent.Significant electrons are accumulated around the S atoms,building distinctive S atomic layers,which play a key role for the metallic character.