典型含硼功能材料的第一性原理高压研究
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摘要
硼在元素周期表中处于金属与非金属的分界线上，并且是第三主族唯一的非金属元素。它最外层拥有三个价电子，电子排布为：[He]2s22p1。由于硼的价电子比价轨道（s，px，py，pz）少，所以有人称它为缺电子材料。由于特殊的电子结构以及在元素周期表中的特殊位置，使得硼及其化合物拥有着独特的结构与性质。它既能形成团簇或笼状结构，并以离子键结合形成离子型化合物，又能与碳、氮、氧结合形成空间共价网络结构。这种结构特点往往会给材料带来优良的机械性能，使其广泛应用于工业生产中。例如立方氮化硼作为磨料被广泛应用于切割、打磨工具表面；通过物理或化学气相沉积法，许多金属硼化物往往被用来制作涂层工具的保护层以提高其机械强度；通过离子注入或者离子束沉积，在金属或合金中加入硼原子可以使金属表面强度和显微硬度都得到显著提高，从而替代金刚石用来制作涂层刀具；硼与塑料或铝合金结合，是有效的中子屏蔽材料；硼钢在反应堆中可以用作控制棒；硼纤维经常被用于制造复合材料。因此，硼及其化合物长期以来一直是科学研究领域的热点之一。
另外，由于硼的缺电子特性，使得它在有供电子元素存在的化合物中能够与氢结合，形成稳定的含氢化合物，并在合适的温度和压力条件下，吸放氢分子。因此，它还是构成新型固态储氢材料的重要元素之一。例如，把Ca(BH_4)_2加热到800K时，可以放出9.0wt%的氢气，并形成CaH_2和CaB_6，而它们在623K和10MPa的氢压下添加催化剂，又可以得到将近57%的Ca(BH_4)_2。又例如，氨硼烷（NH_3BH_3）拥有高达19.6wt%的含氢质量百分比，远远超出美国能源部提出的9wt%的2015年预期目标。当温度升高到773K时，氨硼烷可以放出所有的氢，并生成六角氮化硼，可以说是目前最具前景的固态储氢材料之一。此外，还有许多轻金属硼氢化物（例如，LiBH_4，NaBH_4，Mg(BH_4)_2，Ca(BH_4)_2等等）都具有较高的质量和体积储氢密度。例如，LiBH_4的质量储氢密度高达18.4wt%，体积储氢密度也达到了121kg H_2/m~3；NaBH_4也具有10.6wt%质量储氢密度，它们都是潜在的固态储氢材料。
虽然硼化物结构多样且性质优良，但是仍然有许多物理问题没有弄清楚。例如，结构与物性之间，结构、物性与外界环境之间的关系问题都是长期困扰我们，并且一直没有很好解决的重要物理问题，同时也是当前功能型含硼化合物的研究中亟待解决的物理问题。本文以两类典型的含硼功能材料（超硬MnB_2和高含氢Mg(BH_4)_2）为研究对象，采用第一性原理赝势平面波方法，对其高压行为以及合成条件做了深入系统的探讨。本论文的研究结果不仅对这两种材料的实验合成具有指导意义，而且对同类型的含硼功能材料研究也具有借鉴意义。
本文的第一个研究课题是超硬MnB_2的合成条件及其与内部原子振动特性关系的研究。2009年，S. Aydin等人的理论计算发现，ReB_2结构MnB_2的结合能低于已经合成的AlB_2结构的MnB_2，是其常温常压下的基态结构。而且，用A. im ek的理论模型估计的维氏硬度达到了43.9GPa，可能是潜在的超硬材料。但是，在至今大约三年的时间内，无论是用电弧熔融法还是高温高压法，都没有合成出这种超硬的MnB_2相。因此，为了得到其合成条件，我们应用基于简谐近似的第一性原理晶格动力学方法计算了ReB_2型和AlB_2型MnB_2的两相相图。我们的计算结果表明，常压下，ReB_2结构的MnB_2只有在1020K以下才能够被合成出来，当温度高于这个温度时，它就会转变成AlB_2结构的MnB_2。并且压力效应会使这个转变温度降低，当压强高于38GPa时，就只有AlB_2结构的MnB_2可以被合成了。而早先实验的合成温度都高于1020K，这时合成出来的MnB_2都只能落于亚稳的AlB_2结构相中，所以都只有AlB_2结构的MnB_2被合成。因此，要想合成ReB_2结构的MnB_2，就必须要合理控制温度，同时，降低合成压力。进一步分析表明，MnB_2高温下的热力学稳定性很大程度上由Mn的振动频率决定。ReB_2结构的MnB_2中较强的B-Mn相互作用导致其Mn的振动频率较AlB_2结构高，进而抬高其吉布斯自由能，这是导致ReB_2结构MnB_2高温下热力学不稳定的重要原因。因此，在高于1020K的温度下，没有ReB_2结构的MnB_2被合成。
本文的另一个研究课题是高含氢Mg(BH_4)_2的高压行为与压缩特性的研究。在众多轻金属硼氢化物中，Mg(BH_4)_2拥有14.8wt%理论质量储氢密度，是很有希望的轻金属固态储氢材料。但是，H. W. Li等人2007年的实验结果显示，Mg(BH_4)_2的放氢反应动力学性能很差，其放氢温度很高，而且放氢过程是不可逆的，这都限制了其在储氢方面的应用。为了克服这个困难，人们尝试在Mg(BH_4)_2中添加Ti等催化剂以提高其吸氢和放氢性能。球磨技术是在氢化物中添加催化剂的常用方法。但是在球磨过程中，其局域压强会达到几万大气压，这样高的压力往往使这些氢化物发生结构相变，使实验结果不可控。另一方面，由于材料在压缩过程中往往伴随着不可逆的体积坍缩，所以高压方法是合成高体积储氢密度（Volumetric Hydrogen Density, VHD）材料的有效手段之一。这激发起了人们研究Mg(BH_4)_2高压下物理特性（例如，结构相变、压缩特性以及成键方式等等）的兴趣。2009年，L. George等人的实验结果显示，Mg(BH_4)_2在2.5GPa和14.4GPa分别发生了两次结构相变。其中第一次相变是不可逆的，这表明相变后的结构可以保持到常压。高压相变往往会伴随着高体积储氢密度结构的出现。因此，Mg(BH_4)_2的这个不可逆的结构相变让人们看到了合成高体积储氢密度Mg(BH_4)_2并在常压下应用的可能性。但是，他们并没有给出相变后Mg(BH_4)_2的晶体结构以及与相变相关的高压行为。
为了深刻理解Mg(BH_4)_2的高压行为，本文采用第一性原理平面波赝势方法，在0~10GPa的压强范围内，对前人提出的晶体结构、成键特征、压缩特性做了系统的高压研究。我们发现理论上认为的常压下亚稳的I4_1/amd结构和不稳定的P-3m1结构在高压下却能稳定存在。焓差曲线的计算表明，基态的F222结构在0.7GPa的时候会转变成I4_1/amd结构，然后在6.3GPa的时候又会转变成P-3m1结构。而实验上已经合成的P6_122结构（α相）转变到I4_1/amd结构的转变压力为1.2GPa。进一步分析表明，I4_1/amd结构和P-3m1结构都是常压下的高含氢相。他们常压下的体积储氢密度分别为146.4kg H_2/m~3和134.0kg H_2/m~3。声子谱的计算表明，I4_1/amd结构的动力学稳定的压强范围为0~4GPa；而P-3m1结构在高于1GPa的压强下都满足晶格动力学稳定条件。因此，我们认为I4_1/amd结构可以通过高压方法得到，然后保持到常压；而P-3m1结构就只能以高压相形式存在。能带结构的计算表明，这两个结构在此压强区间内一直是离子晶体，并且是拥有5eV带隙的绝缘体。另外，我们还发现，这两个结构都具有各向异性的压缩特性，它们的c轴都很容易被压缩，特别是P-3m1结构的c轴和体积。这表明，可以通过采用沿c轴压缩的办法来提高他们的体积含氢量。另一方面，我们还发现这两种结构的压缩各向异性是由其内部静电场的各向异性所造成的。通过旋转[BH_4]ˉ基团，计算得到的静电势垒值印证了这一观点。
Boron locates on the boundary between metal and nonmetal in periodic table ofelements. In addition, it is the only nonmetal element in group IIIA elements. It hasthree valence electrons and its electronic configuration shells are [He]2s22p1.Some people call boron as electron-deficient material, because it has fewer valenceelectrons (2s22p1) than the number of stable orbitals (s, px, py, pz) in the valenceshell. Due to the special electronic configuration and special location in periodictable of elements, boron and borides usually have novel structures and properties.They can form cluster or cage structures combined by ionic bonds, and also theycan combine with C, N or O to form space covalent net structure which alwaysaccompany with excellent mechanical properties and be widely used in industryproductions. For examples, cubic boron nitride powders are widely used asabrasives; metal borides are used for coating tools through chemical vapordeposition or physical vapor deposition; implantation of boron ions into metals andalloys, through ion implantation or ion beam deposition, results in a spectacularincrease in surface resistance and micro-hardness, and these borides are analternative to diamond coated tools, and their surfaces have similar properties tothose of the bulk boride; boron combined with plastic or aluminum alloy is a goodkind of neutron shielding material; boron steel can be used as control bars in nuclearreactors; boron fiber is always used in production of composite materials. Therefore,studies of boron and borides are one of the hot areas of the researches in a long time.
On the other hand, due to the electron-deficient property, boron can combinewith hydrogen in the compound with electron-donor elements, and form stablehydrides which can absorb/desorb hydrogen under appropriate temperature andpressure conditions. Thus, it is one of the important elements composing the newtype solid hydrogen storage materials. For examples, experimentally, Ca(BH_4)_2canapproximately release9.0mass%of hydrogen, when it is heated to800K, andforms CaH_2and CaB_6. Adversely, with additives, approximately57%of theCa(BH_4)_2is obtained by rehydrogenation at623K in a hydrogen pressure of10MPa. Ammonia borane (NH_3BH_3) is also an important kind of potential hydrogenstorage material, which contains19.6wt%hydrogen and access the exceed morethan twice of the DOE’s (U.S. Department of Energy)2015target. Ammoniaborane can dehydrogenate absolutely and form h-BN, when temperature is higherthan773K. Moreover, many other light metal borohydrides (e.g. LiBH_4, NaBH_4,Mg(BH_4)_2and Ca(BH_4)_2) are also attractive most interests due to their highgravimetric and volumetric hydrogen densities compared to other complex hydrides.LiBH_4has a gravimetric hydrogen density of18.4wt%and a volumetric hydrogendensity of121kg H_2/m~3; Sodium borohydride (NaBH_4) is also a potential hydrogenstorage material and has a theoretical hydrogen storage capacity of10.6wt%. All ofthem are potential solid hydrogen storage materials.
Though borides have novel structures and excellent properties, there are stillmany unsolved scientific problems about this kind of materials. For example, therelationship between structures and physical properties or between structures,physical properties and synthesis condition have both puzzled people for a long timeand haven’t been solved completely. So, both of them are also the urgent problemsto be solved in the area of boron-containing functional materials. In this thesis, wedeeply and systematically studied the high pressure behaviors and synthesis conditions of two types of boron-containing functional materials (superhard MnB_2and high hydrogen-containing Mg(BH_4)_2) by means of first-principles plane wavepseudopotential method. These calculated results not only have guiding significancefor synthesis of these two materials, but also can be used for reference in researchesof other kinds of boron-containing functional materials.
First part of this thesis is the study of synthesis condition of superhard MnB_2and its relationship with lattice vibrations. In2009, a superhard MnB_2withReB_2-type structure has been predicted as the ground state because of the lower freeenergy than the synthesized AlB_2-type structure. By using the A. im ek’stheroretical hardness model, the predicted hardness of ReB_2-type structure reachesto43.9GPa. However, it has not been synthesized successfully for about two yearsno matter by high temperature and high pressure (HTHP) method or by arc-meltingmethod. To obtain the accurate synthesis condition, the P-T phase boundarybetween AlB_2-type and ReB_2-type MnB_2has been completed by first-principleslattice dynamics calculations within quasi-harmonic approximation (QHA). Ourresults show that the ReB_2-type MnB_2can be synthesized only below1020K atambient pressure. Pressure effect makes their transition temperature decrease. Ifpressure is higher than38GPa, only AlB_2-type MnB_2can be obtained. Thesynthesis temperatures of previous experiments (either HTHP or arc-meltingmethod) are all above1020K, so that only AlB_2-type MnB_2can be synthesized.Therefore, it is essential to control the temperature accurately for synthesizing theReB_2-type MnB_2. On another hand, pressure should be controlled as low as possible.Further analyses show that the thermodynamic stability of MnB_2at hightemperature mostly depends on the vibration frequency of Mn atoms. The strongerinteractions between Mn and B in the ReB_2-type MnB_2induce the vibrationfrequencies of Mn atoms shift to higher and increasing of the Gibbs free energy,causing the thermodynamics instability of ReB_2-type MnB_2at high temperature. Therefore, there is no ReB_2-type MnB_2synthesized at the temperature higher than1020K.
The other part of this thesis is the study of high pressure behaviors andcompression properties of high hydrogen-containing Mg(BH_4)_2. Among hydrides,Mg(BH_4)_2is a promising lightweight solid-state hydrogen storage material with atheoretical hydrogen capacity of14.8wt%. However, in2007, Li et al’s experimentsuggested that Mg(BH_4)_2desorbed hydrogen at extraordinarily high temperature andwas actually irreversible. These limitations are due to its poor kinetic property. Thus,to improve the hydrogenation and dehydrogenation properties of Mg(BH_4)_2, catalystsuch as Ti are explored. Ball-milling is often used for adding catalyst into the hydrideto enhance the dehydrogenation. The local stresses can exceed several gigapascals(GPa) in this process, which may induce structural transitions of the hydrides and theuncontrolled experimental results. On the other hand, high pressure experiment is aneffective method of synthesizing hydrogen storage materials with high volumetrichydrogen densities (VHDs), because compressions on materials are usuallyaccompanied by irreversible volume collapse. These all attract people’s interestsabout exploring the important physical properties (e.g. structural phase transition,compressibility and bond characterization et. al.) of Mg(BH_4)_2under high pressure. In2009, L. George et al found that Mg(BH_4)_2has two high pressure phase transitionswhile compress to2.5GPa and14.4GPa and the first phase transition is irreversible.It means that people can synthesize a kind of Mg(BH_4)_2with high VHDs by usingthis phase transition. But many aspects of phase transition in Mg(BH_4)_2are stillunknown and need to be solved for better understanding of its high pressurebehaviors.
For understanding high pressure behaviors of Mg(BH_4)_2, the previously proposedtheoretical and experimental structures, bond characterization and compressibility ofMg(BH_4)_2in a pressure range from0to10GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases ofmeta-stable I4_1/amd and unstable P-3m1proposed recently are extra stable andcannot decompose under high pressure. Enthalpy calculation indicates that the groundstate of F222structure will transfer to I4_1/amd at0.7GPa, and then to P-3m1structure at6.3GPa. And the experimental P6_122structure (α-phase) transfers toI4_1/amd at1.2GPa. Furthermore, both I4_1/amd and P-3m1can exist as highvolumetric hydrogen density phases at ambient pressure. Their theoretical volumetrichydrogen densities reach146.351and134.028g H_2/L at ambient pressurerespectively. The calculated phonon dispersion curve shows that the I4_1/amd phase isdynamically stable in a pressure range from0to4GPa and the P-3m1phase is stableat pressures higher than1GPa. So the I4_1/amd phase may be synthesized under highpressure and retained to ambient pressure. Energy band structures show that both ofthem are always ionic crystalline and insulating with a band gap of about5eV in thispressure range. In addition, they each have an anisotropic compressibility. The c-axisof these structures is easy to compress. Especially, the c axis and volume of P-3m1phase are extraordinarily compressible, showing that compressing alone c axis canincrease the volumetric hydrogen content for both I4_1/amd and P-3m1structures. Onthe other hand, it is found that the anisotropic compressibility of these two phases isthe resuls of their anisotropic inner electrostatic fields. The calculated electrostaticpotential barrier of [BH_4]ˉrotation can support these views.
另外，由于硼的缺电子特性，使得它在有供电子元素存在的化合物中能够与氢结合，形成稳定的含氢化合物，并在合适的温度和压力条件下，吸放氢分子。因此，它还是构成新型固态储氢材料的重要元素之一。例如，把Ca(BH_4)_2加热到800K时，可以放出9.0wt%的氢气，并形成CaH_2和CaB_6，而它们在623K和10MPa的氢压下添加催化剂，又可以得到将近57%的Ca(BH_4)_2。又例如，氨硼烷（NH_3BH_3）拥有高达19.6wt%的含氢质量百分比，远远超出美国能源部提出的9wt%的2015年预期目标。当温度升高到773K时，氨硼烷可以放出所有的氢，并生成六角氮化硼，可以说是目前最具前景的固态储氢材料之一。此外，还有许多轻金属硼氢化物（例如，LiBH_4，NaBH_4，Mg(BH_4)_2，Ca(BH_4)_2等等）都具有较高的质量和体积储氢密度。例如，LiBH_4的质量储氢密度高达18.4wt%，体积储氢密度也达到了121kg H_2/m~3；NaBH_4也具有10.6wt%质量储氢密度，它们都是潜在的固态储氢材料。
虽然硼化物结构多样且性质优良，但是仍然有许多物理问题没有弄清楚。例如，结构与物性之间，结构、物性与外界环境之间的关系问题都是长期困扰我们，并且一直没有很好解决的重要物理问题，同时也是当前功能型含硼化合物的研究中亟待解决的物理问题。本文以两类典型的含硼功能材料（超硬MnB_2和高含氢Mg(BH_4)_2）为研究对象，采用第一性原理赝势平面波方法，对其高压行为以及合成条件做了深入系统的探讨。本论文的研究结果不仅对这两种材料的实验合成具有指导意义，而且对同类型的含硼功能材料研究也具有借鉴意义。
本文的第一个研究课题是超硬MnB_2的合成条件及其与内部原子振动特性关系的研究。2009年，S. Aydin等人的理论计算发现，ReB_2结构MnB_2的结合能低于已经合成的AlB_2结构的MnB_2，是其常温常压下的基态结构。而且，用A. im ek的理论模型估计的维氏硬度达到了43.9GPa，可能是潜在的超硬材料。但是，在至今大约三年的时间内，无论是用电弧熔融法还是高温高压法，都没有合成出这种超硬的MnB_2相。因此，为了得到其合成条件，我们应用基于简谐近似的第一性原理晶格动力学方法计算了ReB_2型和AlB_2型MnB_2的两相相图。我们的计算结果表明，常压下，ReB_2结构的MnB_2只有在1020K以下才能够被合成出来，当温度高于这个温度时，它就会转变成AlB_2结构的MnB_2。并且压力效应会使这个转变温度降低，当压强高于38GPa时，就只有AlB_2结构的MnB_2可以被合成了。而早先实验的合成温度都高于1020K，这时合成出来的MnB_2都只能落于亚稳的AlB_2结构相中，所以都只有AlB_2结构的MnB_2被合成。因此，要想合成ReB_2结构的MnB_2，就必须要合理控制温度，同时，降低合成压力。进一步分析表明，MnB_2高温下的热力学稳定性很大程度上由Mn的振动频率决定。ReB_2结构的MnB_2中较强的B-Mn相互作用导致其Mn的振动频率较AlB_2结构高，进而抬高其吉布斯自由能，这是导致ReB_2结构MnB_2高温下热力学不稳定的重要原因。因此，在高于1020K的温度下，没有ReB_2结构的MnB_2被合成。
本文的另一个研究课题是高含氢Mg(BH_4)_2的高压行为与压缩特性的研究。在众多轻金属硼氢化物中，Mg(BH_4)_2拥有14.8wt%理论质量储氢密度，是很有希望的轻金属固态储氢材料。但是，H. W. Li等人2007年的实验结果显示，Mg(BH_4)_2的放氢反应动力学性能很差，其放氢温度很高，而且放氢过程是不可逆的，这都限制了其在储氢方面的应用。为了克服这个困难，人们尝试在Mg(BH_4)_2中添加Ti等催化剂以提高其吸氢和放氢性能。球磨技术是在氢化物中添加催化剂的常用方法。但是在球磨过程中，其局域压强会达到几万大气压，这样高的压力往往使这些氢化物发生结构相变，使实验结果不可控。另一方面，由于材料在压缩过程中往往伴随着不可逆的体积坍缩，所以高压方法是合成高体积储氢密度（Volumetric Hydrogen Density, VHD）材料的有效手段之一。这激发起了人们研究Mg(BH_4)_2高压下物理特性（例如，结构相变、压缩特性以及成键方式等等）的兴趣。2009年，L. George等人的实验结果显示，Mg(BH_4)_2在2.5GPa和14.4GPa分别发生了两次结构相变。其中第一次相变是不可逆的，这表明相变后的结构可以保持到常压。高压相变往往会伴随着高体积储氢密度结构的出现。因此，Mg(BH_4)_2的这个不可逆的结构相变让人们看到了合成高体积储氢密度Mg(BH_4)_2并在常压下应用的可能性。但是，他们并没有给出相变后Mg(BH_4)_2的晶体结构以及与相变相关的高压行为。
为了深刻理解Mg(BH_4)_2的高压行为，本文采用第一性原理平面波赝势方法，在0~10GPa的压强范围内，对前人提出的晶体结构、成键特征、压缩特性做了系统的高压研究。我们发现理论上认为的常压下亚稳的I4_1/amd结构和不稳定的P-3m1结构在高压下却能稳定存在。焓差曲线的计算表明，基态的F222结构在0.7GPa的时候会转变成I4_1/amd结构，然后在6.3GPa的时候又会转变成P-3m1结构。而实验上已经合成的P6_122结构（α相）转变到I4_1/amd结构的转变压力为1.2GPa。进一步分析表明，I4_1/amd结构和P-3m1结构都是常压下的高含氢相。他们常压下的体积储氢密度分别为146.4kg H_2/m~3和134.0kg H_2/m~3。声子谱的计算表明，I4_1/amd结构的动力学稳定的压强范围为0~4GPa；而P-3m1结构在高于1GPa的压强下都满足晶格动力学稳定条件。因此，我们认为I4_1/amd结构可以通过高压方法得到，然后保持到常压；而P-3m1结构就只能以高压相形式存在。能带结构的计算表明，这两个结构在此压强区间内一直是离子晶体，并且是拥有5eV带隙的绝缘体。另外，我们还发现，这两个结构都具有各向异性的压缩特性，它们的c轴都很容易被压缩，特别是P-3m1结构的c轴和体积。这表明，可以通过采用沿c轴压缩的办法来提高他们的体积含氢量。另一方面，我们还发现这两种结构的压缩各向异性是由其内部静电场的各向异性所造成的。通过旋转[BH_4]ˉ基团，计算得到的静电势垒值印证了这一观点。
Boron locates on the boundary between metal and nonmetal in periodic table ofelements. In addition, it is the only nonmetal element in group IIIA elements. It hasthree valence electrons and its electronic configuration shells are [He]2s22p1.Some people call boron as electron-deficient material, because it has fewer valenceelectrons (2s22p1) than the number of stable orbitals (s, px, py, pz) in the valenceshell. Due to the special electronic configuration and special location in periodictable of elements, boron and borides usually have novel structures and properties.They can form cluster or cage structures combined by ionic bonds, and also theycan combine with C, N or O to form space covalent net structure which alwaysaccompany with excellent mechanical properties and be widely used in industryproductions. For examples, cubic boron nitride powders are widely used asabrasives; metal borides are used for coating tools through chemical vapordeposition or physical vapor deposition; implantation of boron ions into metals andalloys, through ion implantation or ion beam deposition, results in a spectacularincrease in surface resistance and micro-hardness, and these borides are analternative to diamond coated tools, and their surfaces have similar properties tothose of the bulk boride; boron combined with plastic or aluminum alloy is a goodkind of neutron shielding material; boron steel can be used as control bars in nuclearreactors; boron fiber is always used in production of composite materials. Therefore,studies of boron and borides are one of the hot areas of the researches in a long time.
On the other hand, due to the electron-deficient property, boron can combinewith hydrogen in the compound with electron-donor elements, and form stablehydrides which can absorb/desorb hydrogen under appropriate temperature andpressure conditions. Thus, it is one of the important elements composing the newtype solid hydrogen storage materials. For examples, experimentally, Ca(BH_4)_2canapproximately release9.0mass%of hydrogen, when it is heated to800K, andforms CaH_2and CaB_6. Adversely, with additives, approximately57%of theCa(BH_4)_2is obtained by rehydrogenation at623K in a hydrogen pressure of10MPa. Ammonia borane (NH_3BH_3) is also an important kind of potential hydrogenstorage material, which contains19.6wt%hydrogen and access the exceed morethan twice of the DOE’s (U.S. Department of Energy)2015target. Ammoniaborane can dehydrogenate absolutely and form h-BN, when temperature is higherthan773K. Moreover, many other light metal borohydrides (e.g. LiBH_4, NaBH_4,Mg(BH_4)_2and Ca(BH_4)_2) are also attractive most interests due to their highgravimetric and volumetric hydrogen densities compared to other complex hydrides.LiBH_4has a gravimetric hydrogen density of18.4wt%and a volumetric hydrogendensity of121kg H_2/m~3; Sodium borohydride (NaBH_4) is also a potential hydrogenstorage material and has a theoretical hydrogen storage capacity of10.6wt%. All ofthem are potential solid hydrogen storage materials.
Though borides have novel structures and excellent properties, there are stillmany unsolved scientific problems about this kind of materials. For example, therelationship between structures and physical properties or between structures,physical properties and synthesis condition have both puzzled people for a long timeand haven’t been solved completely. So, both of them are also the urgent problemsto be solved in the area of boron-containing functional materials. In this thesis, wedeeply and systematically studied the high pressure behaviors and synthesis conditions of two types of boron-containing functional materials (superhard MnB_2and high hydrogen-containing Mg(BH_4)_2) by means of first-principles plane wavepseudopotential method. These calculated results not only have guiding significancefor synthesis of these two materials, but also can be used for reference in researchesof other kinds of boron-containing functional materials.
First part of this thesis is the study of synthesis condition of superhard MnB_2and its relationship with lattice vibrations. In2009, a superhard MnB_2withReB_2-type structure has been predicted as the ground state because of the lower freeenergy than the synthesized AlB_2-type structure. By using the A. im ek’stheroretical hardness model, the predicted hardness of ReB_2-type structure reachesto43.9GPa. However, it has not been synthesized successfully for about two yearsno matter by high temperature and high pressure (HTHP) method or by arc-meltingmethod. To obtain the accurate synthesis condition, the P-T phase boundarybetween AlB_2-type and ReB_2-type MnB_2has been completed by first-principleslattice dynamics calculations within quasi-harmonic approximation (QHA). Ourresults show that the ReB_2-type MnB_2can be synthesized only below1020K atambient pressure. Pressure effect makes their transition temperature decrease. Ifpressure is higher than38GPa, only AlB_2-type MnB_2can be obtained. Thesynthesis temperatures of previous experiments (either HTHP or arc-meltingmethod) are all above1020K, so that only AlB_2-type MnB_2can be synthesized.Therefore, it is essential to control the temperature accurately for synthesizing theReB_2-type MnB_2. On another hand, pressure should be controlled as low as possible.Further analyses show that the thermodynamic stability of MnB_2at hightemperature mostly depends on the vibration frequency of Mn atoms. The strongerinteractions between Mn and B in the ReB_2-type MnB_2induce the vibrationfrequencies of Mn atoms shift to higher and increasing of the Gibbs free energy,causing the thermodynamics instability of ReB_2-type MnB_2at high temperature. Therefore, there is no ReB_2-type MnB_2synthesized at the temperature higher than1020K.
The other part of this thesis is the study of high pressure behaviors andcompression properties of high hydrogen-containing Mg(BH_4)_2. Among hydrides,Mg(BH_4)_2is a promising lightweight solid-state hydrogen storage material with atheoretical hydrogen capacity of14.8wt%. However, in2007, Li et al’s experimentsuggested that Mg(BH_4)_2desorbed hydrogen at extraordinarily high temperature andwas actually irreversible. These limitations are due to its poor kinetic property. Thus,to improve the hydrogenation and dehydrogenation properties of Mg(BH_4)_2, catalystsuch as Ti are explored. Ball-milling is often used for adding catalyst into the hydrideto enhance the dehydrogenation. The local stresses can exceed several gigapascals(GPa) in this process, which may induce structural transitions of the hydrides and theuncontrolled experimental results. On the other hand, high pressure experiment is aneffective method of synthesizing hydrogen storage materials with high volumetrichydrogen densities (VHDs), because compressions on materials are usuallyaccompanied by irreversible volume collapse. These all attract people’s interestsabout exploring the important physical properties (e.g. structural phase transition,compressibility and bond characterization et. al.) of Mg(BH_4)_2under high pressure. In2009, L. George et al found that Mg(BH_4)_2has two high pressure phase transitionswhile compress to2.5GPa and14.4GPa and the first phase transition is irreversible.It means that people can synthesize a kind of Mg(BH_4)_2with high VHDs by usingthis phase transition. But many aspects of phase transition in Mg(BH_4)_2are stillunknown and need to be solved for better understanding of its high pressurebehaviors.
For understanding high pressure behaviors of Mg(BH_4)_2, the previously proposedtheoretical and experimental structures, bond characterization and compressibility ofMg(BH_4)_2in a pressure range from0to10GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases ofmeta-stable I4_1/amd and unstable P-3m1proposed recently are extra stable andcannot decompose under high pressure. Enthalpy calculation indicates that the groundstate of F222structure will transfer to I4_1/amd at0.7GPa, and then to P-3m1structure at6.3GPa. And the experimental P6_122structure (α-phase) transfers toI4_1/amd at1.2GPa. Furthermore, both I4_1/amd and P-3m1can exist as highvolumetric hydrogen density phases at ambient pressure. Their theoretical volumetrichydrogen densities reach146.351and134.028g H_2/L at ambient pressurerespectively. The calculated phonon dispersion curve shows that the I4_1/amd phase isdynamically stable in a pressure range from0to4GPa and the P-3m1phase is stableat pressures higher than1GPa. So the I4_1/amd phase may be synthesized under highpressure and retained to ambient pressure. Energy band structures show that both ofthem are always ionic crystalline and insulating with a band gap of about5eV in thispressure range. In addition, they each have an anisotropic compressibility. The c-axisof these structures is easy to compress. Especially, the c axis and volume of P-3m1phase are extraordinarily compressible, showing that compressing alone c axis canincrease the volumetric hydrogen content for both I4_1/amd and P-3m1structures. Onthe other hand, it is found that the anisotropic compressibility of these two phases isthe resuls of their anisotropic inner electrostatic fields. The calculated electrostaticpotential barrier of [BH_4]ˉrotation can support these views.
引文
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