First-principles Study On The Structure And Properties Of Stannate Pyrochlores And Binary Compounds

Recently,there have been considerable progresses in the studies of the resistance toradiation damage of pyrochlore oxide materials and pressure-induced phase transitionsin binary compounds.In the present paper,the factors,including structural,bonding,electronic properties and defect formation energies,influencing the resistance toradiation damage of lanthanide Ln₂Sn₂O₇(Ln=Sm,Gd,Tb,Ho,Er,Lu,La,Pr,Nd,Y)stannate pyrochlore oxides have been studied systematically using first principlecalculation based on Density Functional Theory.At the same time,the pressure-inducedphase transition in binary compounds(A^NB^8-N,N is the outermost valence electronicnumber of A element) have also been studied systematically.The main results are asfollows:1.A systematic study has been carried out to investigate the resistance to radiationdamage of lanthanide stannate Ln₂Sn₂O₇(Ln=Sm,Gd,Tb,Ho,Er,Lu,La,Pr,Nd,Y)pyrochlore oxides from different aspect,including structural,bonding,electronicproperties and defect formation energies,using the Vienna ab initio simulation package(VASP) based on Density Functional Theory.From the structural analysis,it is foundthat the calculated static lattice parameters agree well with experimental measurements,which indicate the basis group and method used in our calculation are reasonable.TheSnO₆ octahedra have smaller structural deviations from the ideal fluorite structure whenthe 48foxygen positional parameter,x,becomes larger,which will show more radiationresistance to ion beam-induced amorphization for pyrochlores.The present worksuggests that the radiation resistance decrease as the lanthanide Ln³⁺ ionic radiusincreases in stannate pyrochlores Ln₂Sn₂O₇(Ln= Sm,Gd,Tb,Ho,Er,Lu,La,Pr,Nd,Y),sowe prognosticate that Lu₂Sn₂O₇ is the most resistant to ion beam-induced amorphizationand La₂Sn₂O₇ is the lowest resistance.From the analysis of bonding properties,it canalso be seen that the 48f> bond overlap population shows an inregular variationwith increasing lanthanide Ln³⁺ ionic radius,which suggests the covalent 48f>bond is not a primary factor for radiation resistance.From the analysis of defectformation energies,the formation energies of the cation-antisite defects increases continuously from Lu³⁺ to Tb³⁺ and Gd³⁺ to Nd³⁺,indicating a significantly reducedradiation resistance as the lanthanide Ln³⁺ cation ionic radius increases.The results arein consistent with the experimental observations on the resistance behavior of thestannate pyrochlores.Lu₂Sn₂O₇ has the lowest cation-antisite defect formation energy,suggesting it has the strongest radiation resistance.The electronic density of states(DOS) analysis shows that there exists some hybridization between Sn 5p and O 2pstates,which influences obviously the radiation resistance.Electronic band structures ofLn₂Sn₂O₇ pyrochlores present a direct band gap and a obvious semiconducting feature.2.We have systematically studied pressure-induced phase transition in binarycompounds KCl,MgS,CaS,YSb and ScSb using the plane-wave pseudopotentialDACAPO code based on Density Functional Theory.The influences of pressure onstructural,electronic and optical properties have been investigated.The calculatedresults show that the B1 structure is the most stable at 0 GPa and 0 K for these binarycompounds.Under compression,our calculations show that these compounds willundergo B1→B2 structural phase transition.It is found that the B1→B2 transitionalpressure increase as the cation ionic radius decreases for KCl,CaS and ScSb located atthe same period in Period Table due to the increasing covalent bonding between thecation and anion,however,the B1→B2 transitional pressure decrease as the increasingcation ionic radius and lattice constant for MgS and CaS,YSb and ScSb located at thesame family in Period Table,respectively.We also find for the first time that the otherstructural phase transition from our theoretical calculation,that are the B3→B2 andB4→B2 phase transition for MgS and CaS,B2→T1 and B3→T1 phase transition forKCl.These results will provide some theoretical basis for farther experimental research.Pressure make the bond length shorten,volume minish,bulk modulus increase,andelectric charge transfer.The change of electronic property makes the variety of opticalproperty,respectively.We find the absord for light concentrate on ultraviolet radiationfor these binary compounds.