【作者】 徐丹；

【导师】 崔啟良；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2018， 博士

【摘要】 压力是决定物质结构和性质的基本参量之一。高压可以调控原子间距,改变晶体结构和电子结构,诱发常规条件下难以发现的新现象、新效应,形成高压相,是发展新概念、新理论和探索新物质的重要手段。三元金属磷氮化物作为潜在离子导体、超硬和发光材料近年来受到了广泛关注。常压下,这类材料普遍以PN₄四面体为基本结构单元,其多变的连接方式使材料形成了丰富的晶体结构类型。由于三元金属磷氮化物多在高温高压条件下合成,其高压结构相变与电子性质也受到越来越多的关注,其中P的高压配位化学是一个关注的焦点。目前,实验上5配位P在高压多晶体γ-P₃N₅和γ-HP₄N₇中已经实现了;理论研究表明6配位P可以存在于δ’-P₃N₅和spinel-type尖晶石的高压相中,然而实验研究中尚未得到证实。近年来,晶体结构预测技术日趋成熟,可以仅根据其化学组分就能准确的预测出在特定压力下材料的结构信息,因此该技术与第一性原理结合已成为材料高压相变研究的重要工具,也为我们探索三元金属磷氮化物的P配位化学问题提供了技术支撑。APN₂（A=Cu和Li）能够由二元氮化物P₃N₅和A₃N在高温高压下合成,常压相具有类β-方石英结构（Pearson标记:tI16）,此前的高压理论研究尚未发现其存在压致结构相变。本文首先采用基于群智能理论的CALYPSO晶体结构搜索方法与第一性原理相结合,在0-160 GPa压力区间内对CuPN₂的结构相变与电子性质进行了系统地研究。我们发现压力可以导致CuPN₂在34 GPa和120 GPa出现相变,即tI16→hR4/oC16→hR4’。如预期的一样,两个高压相之中P都以6配位形式存在;而且还发现两次结构相变时Cu的配位数出现了随压力的增加先降低后增加的奇异压致行为。即低压tI16结构中Cu原子采取4配位数环境,34 GPa高压时hR4和oC16结构中的Cu原子采取2配位数环境,达到120 GPa更高压力时hR4’结构的Cu原子采取6配位数环境。根据Wilson相变理论,绝缘体在足够大的压强下都可以实现价带和导带的重叠,从而可转变为不满带,即成为导体。材料的带隙随压力的增加而减小是一个普遍规律。然而我们在CuPN₂中却发现了反常的压致带隙变化行为—带隙随压力的增加先减小再增大。即tI16→hR4/oC16结构相变时带隙降低～73%;hR4/oC16→hR4’结构相变时带隙增加了～149%。为了理解这一反常的压致带隙变化行为,我们分析了Cu-N距离随压力的变化关系。发现它是随着压力增加先减小再增加的:tI16→hR4/oC16结构相变时Cu-N的距离降低了～7%;hR4/oC16→hR4’结构相变时Cu-N的距离增加了～19%。这一变化趋势与带隙变化趋势一致,表明Cu-N的距离对于能隙的变化选择起到关键作用。此外,电子结构计算显示tI16,hR4/oC16,hR4’结构都是间接带隙半导体。本文利用第一性原理计算方法结合CALYPSO结构搜索系统研究了LiPN₂在0-300 GPa压力范围内的相稳定性、结构和电子性质。预测了三个压力诱导的相变,相变压力点分别为44 GPa、136 GPa和259 GPa,其相变序列为tI16→hR4→cF64→oP8。三种高压多晶型都是6配位,这与四重配位tI16结构大不相同。hR4和cF64结构由紧密排列的PN₆和LiN₆八面体通过边共享连接而成,而oP8结构由边和面共享的PN₆和LiN₆八面体构成,其中N位于三角棱柱的中心。声子计算显示所有结构动力学稳定。电子特性分析表明在0-300 GPa压力范围内LiPN₂是半导体,并且P-N和Li-N键分别是共价键和离子键。本文的研究结果表明三元氮磷金属化合物配位化学可能比预想的更复杂,需要理论工作者与实验工作者相结合进行更加深入的研究。本文研究结果为高压碱金属氮化磷酸盐的高压研究提供了新的思路。更多还原

【Abstract】 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:γ-P₃N₅ andγ-HP₄N₇,and sixfold coordination environment for P have also been reported for hypotheticδ’-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 theβ-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.更多还原