Theoretical Studies On The Structures And Physical Properties Of Manganese Nitrides And Manganese Oxides Under High Pressure

Manganese based light element compounds have long been stuied in the field of condensed matter physics,material science and chemistry due to their excellent magnetic properties,excellent mechanical properties and stable chemical properties.High pressure(HP)is an effective and clean method to obtain new materials and discover new mechanisms.It can lower the energy barriers of some chemical reactions,then synthesize new structures that are difficult to be synthesized at ambient pressure.Furthermore,HP can change hybridization modes,bonding ways,electron structures,and atomic distances,which could affect a material's micro-structures and its mechanical,thermal,optical,electronical,magnetic properties.The high pressure phase might be preserved to ambient conditions,and be advanced multifunctional materials that are difficult to obtain with other methods.Therefore,it is significant to study the behavior of manganese-based light element compounds under HP for condensed matter physics and materials science.In this paper,the crystal structure search combined with the first-principles calculations method were used to systematacially study the structures and properties of the Mn-N system and Mn O₂ under 0-100 GPa.The crystal structure,electronic structure properties,thermodynamic properties,mechanical properties,bonding properties,hardness and superconducting properties are deeply analyzed.The innovative results are as follows:(1)We systematically examined the stoichiometric phase spaces of Mn-N compounds from 0 to 100 GPa with a full consideration of magnetism.The new magnetism considered high pressure phase diagram was created.The superconducting Mn N₄ with a planar N₄ ring was discovered,and the existance of N₄ units makes Mn N₄ a hard material.The mechanical evolution of Mn-N system was studied.The Mn N₄ stable in the pressure range from 40 to 100 GPa.The electronic structures reveal that the N₄ring is driven by the sp² hybridization of nitrogen atoms.This phase with T_c?1.6 K and high bulk modulus B=381 GPa,which make it potentially interesting as a hard superconductive material.Moreover,we re-summarize the phase transition sequence for the Mn N compounds,the semi-conducting NM-zb phase(5 GPa)first transforms to metallic AFM-Ni As(40 GPa),which further transforms to the more stable metallic FM-rs phase.According to Zhong's hardness model,AFM-Ni As-Mn N also have outstanding compressibility,the hardness values over 30 GPa.The mechanical properties show that covalent interaction has a great effect on N-rich structures'hardness and hardly effect on Mn-rich structures in Mn-N compounds.Our work play a guiding role in the synthesis experiments of nitrogen-rich manganese nitrogen compounds.(2)We proposed the newly identified Mn N₂ compounds as a model to solve the long pussling problem of the transition from spin state to superconducting state.Then we explains the relationship between superconducting temperature and pressure from the perspective of crystal structures change.With high pressure regulation,the materials reverse design will be realized.We found that the new Mn N₂,which becomes a superconductor through pressure,is a good model structure to study the relationship between magnetism and superconductivity.The superconducting state can be obtained in two ways:the first is the pressure-induced AFM P2₁/m-Mn N₂ to non-magnetic I4/mmm phase at 30 GPa;while the other is only pressure-induced ferromagnetic I4/mmm phase to magnetic vanished at 14 GPa,the T_c?9.6 K and accompanied with negative compression along the c axis.In the second way,the edge-sharing tetragonal(Mn N₅)pyramids instead of distorted octahedrons(Mn N₆),accompanied by negative compression along the c axis.The d²²_x-y orbital is pushed above the Fermi level because of increasing crystal-field splitting.Thus,the system transforms from a high spin state to a low spin state,driving disappearance of magnetism and emergence of superconductivity.And all of these structures can remain to 0 GPa.We also find the T_c of I4/mmm phase decreases with increasing pressure and can reach 17.6 K at 0 GPa.Remarkably,I4/mmm-Mn N₂ can achieve spin-to-superconducting state mutual transformation by pressure.Moreover,the AFM-P2₁/m-Mn N₂ phase is extremely incompressible with the hardness value above 20 GPa,which is a potential ultra-hard material.This work proves systematic interpretation for the connection between magnetism and superconductivity,which might give good clues to design new materials.(3)The high pressure phase transition sequence of Mn O₂ was determined,and the relationship between the band gap change of Mn O₂ and octahedral strain under pressure were also studied in this work.It was found that the competition between octahedral bond length and bond angle,and the connection mode of octahedron had an effect on the band gap.The study found that Pa?phase can be adjusted to the Shockley-Queiser limit.Based on the first principle calculation,Mn O2 is systematically studied in the pressure range of 0-100 GPa.The phase transition sequence is AFM-P4₂/mnm?AFM-Pnnm?AFM-Pa(?),and the two high-pressure structures can be stabilized to ambient pressure.The The phase transition sequence of Mn O₂ has been determined:AFM-P4₂/mnm phase transforms to the AFM-Pnnm phase at 9.9 GPa,and then to a cubic-type structure AFM-Pa(?)phase at 44.7 GPa.We found that the competition between bond length and bond angle results in expansion or shrinkage of its bandgap within in the Pnnm phase with increasing pressure,because of the interaction of Mn-d and O-p states.Futher,we found the similar pressure induced bandgap evolution in the Pnnm phases of Si O₂,Ge O₂,and Pb O₂.The different types of octahedral connectivity in the Pnnm and Pa(?)phases lead to a notable pressure-induced bandgap enlargement.In the Pa(?)phase,the bandgap can be tuned to 1.34 e V with pressure to meet the Shockley-Queisser limit.Moreover,the two high-pressure phases could be quenched to ambient pressure.In this study,new mechanism for pressure-induced bandgap enlargement is proposed,and a new method for finding suitable bandgaps for semiconductor materials is provided.