Structural And Electronic Properties Of MPN₂?M=Li,Cu? Under Pressure:a First-Principles Study

Pressure is a fundamental variable to determine structures and properties of materials.High pressure can adjust interatomic spacing,as well as change the crystal structure and the electronic structure.It can also induce new effects and form new phases that we can't find at ambient environment.High pressure is hence a significant tool to create new theories and explore new materials.In recent years,ternary metal phosphorus nitrides have attracted considerable attention for their potential use in ion conductors,superhard materials and luminescent materials.At ambient pressure,PN₄tetrahedron is commonly found in these compounds,whose interconnection results in different crystal structure types.The structural transition and the electronic properties of these compounds have received increasing interest as they are always synthesized at high temperatures and high pressures,and the coordination chemistry of phosphorus at high pressure have especially generated great interest.To date,pentacoordinate P has been experimentally realized in high-pressure polymorphs:?-P₃N₅ and?-HP₄N₇,and sixfold coordination environment for P have also been reported for hypothetic?'-P₃N₅ and spinel-type BeP₂N₄,which has not been verified experimentally.In recent years,with the rapid development of global optimization algorithms and first-principles methods,it has become possible to determine or predict the atomic structure through theoretical calculations with only information about the chemical composition and external conditions.The effective searching method in combination with first-principles calculations has already been a useful tool for high-pressure research,and it provides a great convenience for us to study the coordination chemistry of phosphorus in ternary phosphorus nitrides.APN₂?A=Cu,Li?can be synthesized by performing reaction of the binary nitrides P₃N₅ and Li₃N at high temperatures and high pressures,and it adopts the?-cristobalite type structure?tI16?.Pressure-induced phase transitions have not been found in Preliminary investigations.The search for structures and electronic properties of CuPN₂ in 0-160 GPa was based on the global minimization of free energy surfaces using ab initio total energy calculations and the particle-swam-optimization scheme as implemented in the CALYPSO code.Firstly,two pressure-induced structural phase transitions were found at 34 and 120 GPa respectively,with a sequence as tI16?hR4/oC16?hR4?.As expected,P atoms are six-fold coordinated in both high-pressure structures.Interestingly,coordination numbers of Cu have an unusual change.With pressure increasing,the coordination numbers first decreased and then increased.Cu atom adopts a two-fold coordinated environment in hR4 and oC16 structures at 34 GPa,which is lower than both of the low-pressure tI16 structure?with four-fold Cu atom?and the hR4?structure at 120 GPa?with six-fold Cu atom?.In terms of Wilson theory of phase transition,the valence and conduction bands of the insulator can be overlapping at sufficiently high pressure.It means that the insulator has changed into a conductor.As a rule,the energy gap of materials decreases with pressure increasing.However,we found an unusual change of the energy gap in CuPN₂ structural transitions,i.e.decreasing in the first while increasing in the second.The energy gap exhibits remarkably large decrease of?73%and an abnormal increase of?149%in the two structural transitions,respectively.To understand the unusual behavior,we analyzed the Cu-N environment which plays a crucial role in determining the energy gap.The Cu-N distance in the structural transitions decrease in the first and increase in the second.A decrease of?7%and an increase of?19%were found in the tI16?hR4/oC16 and hR4/oC16?hR4?transitions,respectively,which are clearly correlated to the changes of the energy gaps,indicating that the Cu-N distance plays a crucial role in determining the energy gap.Besides,electronic structure calculations reveal that all of those structures are semiconductors with indirect energy gap.We explored the phase stability,structural and electronic properties of LiPN₂extensively,over a wide pressure range of 0–300 GPa by using the effective CALYPSO structure searching method in combination with first-principles calculations.Three pressure-induced phase transitions tI16?hR4?cF64?oP8were predicted at 44,136 and 259 GPa,respectively.All of the three high-pressure polymorphs have six-fold coordination environment,which is substantially different from the four-fold coordinated tI16 structure.hR4 and cF64 structures consist of close-packed PN₆ and LiN₆ octahedra connected by edge-sharing,whereas,the oP8structure is built up from edge-and face-sharing PN₆ and LiN₆ octahedra with N lying in the center of trigonal prisms.Phonon calculations showed that all of the three high-pressure polymorphs are dynamically stable.Electronic property analysis reveals that LiPN₂ is a semiconductor from 0 to 300 GPa,and P-N and Li-N bonds are covalent and ionic,respectively.The current work indicates that the coordination chemistry of metal elements in ternary phosphorus nitrides may be much more complex than what it looks at the first sight.And it is worth further researching by both theoretical and experimental workers.The current results provide new insight and guidance for further studying alkali metal nitridophosphates at high pressures.