锡酸盐烧绿石及二元化合物结构与性能的第一性原理研究

1、锡酸盐烧绿石及二元化合物结构与性能的第一性原理研究        近年来，对烧绿石材料抗辐照性能以及二元化合物高压相变的研究受到了广泛的关注。本论文利用基于密度泛函理论的第一性原理研究了结构、键合和电子性质以及缺陷形成能几种因素对镧系锡酸盐烧绿石氧化物材料Ln_2Sn_2O_7(Ln=Sm，Gd，Tb，Ho，Er，Lu，La，Pr，Nd，Y)抗辐照性能的影响。同时对二元化合物ANB(8-N)(N是A元素的最外层价电子数)在高压下的结构相变行为进行了系统的研究。主要结论如下：1利用基于密度泛函理论的平面波赝势程序VA

2、SP首次系统地研究了结构、键合和电子性质以及缺陷形成能几个方面对镧系锡酸盐烧绿石氧化物材料Ln_2Sn_2O_7(Ln=Sm，Gd，Tb，Ho，Er，Lu，La，Pr，Nd，Y)抗辐照性能的影响。从结构分析来看，计算得到的晶格常数与实验值吻合很好，表明我们所采用的基组和计算方法是可靠的。氧位置参数x值越大，八面体SnO_6对萤石结构的偏离就越小，材料更趋近于萤石结构，抗辐照能力更强，我们预测该系列氧化物的抗辐照性能随镧系Ln(3+)离子半径的增大逐渐减小，其中，Lu_2Sn_2O_7烧绿石抗辐照性能最强，La_2Sn_2O_7的抗辐照性能最差。从键结合分析来看，Sn-O_(48f)键的布居值随

3、镧系Ln(3+)离子半径的增大呈现出无规律的变化，说明Sn-O_(48f)的共价键合不是影响其抗辐照性能的主要因素。从缺陷形成能的分析来看，从Lu(3+)到Tb(3+)以及从Gd(3+)到Nd(3+)，随着镧系Ln(3+)阳离子半径的增加，阳离子反位缺陷形成能逐渐增加，抗辐照性能逐渐减弱，这与实验现象是吻合的。Lu_2Sn_2O_7有最低的阳离子反位缺陷形成能，暗示具有最强的抗辐照能力。电子态密度分析表明，Sn 5p态和O 2p态间存在轨道杂化，这种杂化强弱对材料的抗辐照性能有明显的影响，能带分析显示锡酸盐烧绿石系列材料属于直接带隙型材料，表现出明显的半导体特征。2利用基于密度泛函理论的平面波

4、赝势程序DACAPO计算了KCl、MgS、CaS、YSb和ScSb几种二元化合物的高压相变行为。模拟了压强对B1构型和B2构型的力学、电子和光学性质的影响。计算结果表明，在零温零压下这些化合物的B1构型是最稳定的，随着压强的增大，会发生从B1B2构型的转变。首次发现，对于同周期的三种化合物KCl、CaS和ScSb，随着阳离子半径的逐渐减小，阴阳离子间共价键合逐渐增强，发生B1B2构型的相变压强也就越大。但同族化合物MgS和CaS，YSb和ScSb，随着阳离子半径的增加，晶格常数增大，B1B2构型的相变压强减小，阳离子对其相变压强的影响非常显著。我们也从理论上首次预测了这些化合物的其他高压结构相

5、变行为，即MgS和CaS的B3B2和B4B2的结构转变以及KCl化合物的B2T1和B3T1的结构相变。这将为进一步的实验研究提供一定的理论依据。压强促使了化合物的键长缩短，体积减小，体积模量增大，电荷发生转移，电子性质的变化导致光学性质也相应的发生了变化。研究发现，这几种二元化合物对光的吸收主要集中在紫外光区Recently,there have been considerable progresses in the studies of the resistance toradiation damage of pyrochlore oxide materials and pressure-i

6、nduced 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 Ln_2Sn_2O_7(Ln=Sm,Gd,Tb,Ho,Er,Lu,La,Pr,Nd,Y)stannate pyrochlore oxides have been s

7、tudied systematically using first principlecalculation based on Density Functional Theory.At the same time,the pressure-inducedphase transition in binary compounds(ANB(8-N),N is the outermost valence electronicnumber of A element) have also been studied systematically.The main results are asfollows:

8、1.A systematic study has been carried out to investigate the resistance to radiationdamage of lanthanide stannate Ln_2Sn_2O_7(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 in

9、itio 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.TheSnO_6 octahedra have sm

10、aller 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 Ln(3+) io

11、nic radiusincreases in stannate pyrochlores Ln_2Sn_2O_7(Ln= Sm,Gd,Tb,Ho,Er,Lu,La,Pr,Nd,Y),sowe prognosticate that Lu_2Sn_2O_7 is the most resistant to ion beam-induced amorphizationand La_2Sn_2O_7 is the lowest resistance.From the analysis of bonding properties,it canalso be seen that the bond overl

12、ap population shows an inregular variationwith increasing lanthanide Ln(3+) ionic radius,which suggests the covalent 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 L

13、u(3+) to Tb(3+) and Gd(3+) to Nd(3+),indicating a significantly reducedradiation resistance as the lanthanide Ln(3+) cation ionic radius increases.The results arein consistent with the experimental observations on the resistance behavior of thestannate pyrochlores.Lu_2Sn_2O_7 has the lowest cation-a

14、ntisite 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 ofLn_2Sn_2O_7 pyr

15、ochlores 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 onstruc

16、tural,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 B1B2 structural phase transition.It is found that the

17、 B1B2 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 B1B2 transitional pressure decrease as the increasingcation ionic radius and lattice

18、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 B3B2 andB4B2 phase transition for MgS and CaS,B2T1 and B3T1 phase transition forKCl.These results will provide some theoretical basis for farther experimental research.Pressure make the bond length shorten,volum