First Principles Study On Structural Phase Transition And Superconductivity Of Barium-Phosphorus-Hydrogen System

Barium is one of the most active elements of alkaline earth metal elements.Barium compounds have excellent physical properties and chemical properties,which make them widely used in industry.In recent years,experimental signature of superconductivity with high superconducting transition temperature has been found in hydrogen-rich compounds PH3 under high pressure.However,further studies show that all of the phosphorous hydrogen compounds under high pressure are unstable.This thesis intends to introduce Ba atoms into the unstable phosphorus hydrogen system in order to obtain a stable Ba-P-H system.Using the first-principles calculation method based on the density functional theory and the crystal structure predict method based on the evolutionary algorithm,this thesis wants to explore new barium phosphides and barium phosphorus hydrogen ternary compounds with specific physical and chemical properties and their potential superconductivity in Ba-P and Ba-P-H systems at high pressure.The main contents of our thesis are as follows.(1)This thesis finds two new stable barium phosphides,Ba2 P and BaP,both of which were dynamically stable at atmospheric pressure.With 3 8)8)symmetry structure Ba2 P shows metallicity,and with Immm-symmetry structure BaP exhibits characteristics of semiconductor properties with a band gap of 0.35 e V.(2)This thesis searches for stable barium phosphides in the pressure range of 50 GPa ~ 200 GPa,that are Ba2 P,BaP2 and BaP.In the range of 0 ~ 200 GPa,both Ba2 P and BaP2 have a structural phase transition,while BaP undergoes three structural phase transitions in the range of 0 ~ 200 GPa.By studying the electronic band structure and the phonon spectra of Ba2 P,BaP2 and BaP at a given pressure,we find that I4/mmm-Ba2 P at 50 GPa,(9(93 8)8)-BaP2 at 50 GPa,Cmmm-BaP at 50 GPa,Imma-BaP at 100 GPa and P4/nmm-BaP at 180 GPa all exhibits metallicity and are dynamically stable.(3)This thesis predicts the stable barium phosphorus hydrogen ternary compounds in the Ba-P-H system in the pressure range of 50 GPa ~ 200 GPa.Our results show that there have three stable barium phosphorus hydrogen ternary compounds under high pressure,BaPH,BaPH6 and BaP2H14.The study on the electronic band structure shows that these stable barium phosphorus hydrogen ternary compounds have metallic characteristics.Using Allen-Dynes modified Mc Millan formula,we predict that Amm2-BaPH6 is a superconductor with a superconducting transition temperature of 31.7 K at 100 GPa.However,Pnma-BaPH and Ima2-BaP2H14 are weak superconductors with superconducting transition temperatures of 0 K(at 50 GPa)and 0.8 K(at 200 GPa).Further studies show that the higher superconducting transition temperature of Amm2-BaPH6 at 100 GPa stems from the strong electronphonon coupling( = 0.70)and the high logarithmic average frequency(7)7)7)7)7)7)= 897.83 K)caused by high-frequency vibrations of H atoms.The theoretical research in this thesis makes up for the research vacancy of barium phosphorus system under high pressure,and also enriches the structures of barium phosphorus system.The theoretical study of Ba-P-H system under high pressure shows that the introduction of the third element can make the originally unstable P-H system become stable.In addition,the predicted BaPH6 is a superconductor with a higher superconducting transition temperature,which also enriches the family of ternary superconducting hydrides.