First-principles Study Of Typical Boron-containing Functional Materials Under High Pressure

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（BH₄）₂canapproximately release9.0mass%of hydrogen, when it is heated to800K, andforms CaH₂and CaB₆. Adversely, with additives, approximately57%of theCa（BH₄）₂is obtained by rehydrogenation at623K in a hydrogen pressure of10MPa. Ammonia borane （NH₃BH₃） 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. LiBH₄, NaBH₄,Mg（BH₄）₂and Ca（BH₄）₂） are also attractive most interests due to their highgravimetric and volumetric hydrogen densities compared to other complex hydrides.LiBH₄has a gravimetric hydrogen density of18.4wt%and a volumetric hydrogendensity of121kg H₂/m³; Sodium borohydride （NaBH₄） 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 MnB₂and high hydrogen-containing Mg（BH₄）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 MnB₂and its relationship with lattice vibrations. In2009, a superhard MnB₂withReB₂-type structure has been predicted as the ground state because of the lower freeenergy than the synthesized AlB₂-type structure. By using the A. im ek’stheroretical hardness model, the predicted hardness of ReB₂-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 AlB₂-type and ReB₂-type MnB₂has been completed by first-principleslattice dynamics calculations within quasi-harmonic approximation （QHA）. Ourresults show that the ReB₂-type MnB₂can be synthesized only below1020K atambient pressure. Pressure effect makes their transition temperature decrease. Ifpressure is higher than38GPa, only AlB₂-type MnB₂can be obtained. Thesynthesis temperatures of previous experiments （either HTHP or arc-meltingmethod） are all above1020K, so that only AlB₂-type MnB₂can be synthesized.Therefore, it is essential to control the temperature accurately for synthesizing theReB₂-type MnB₂. On another hand, pressure should be controlled as low as possible.Further analyses show that the thermodynamic stability of MnB₂at hightemperature mostly depends on the vibration frequency of Mn atoms. The strongerinteractions between Mn and B in the ReB₂-type MnB₂induce the vibrationfrequencies of Mn atoms shift to higher and increasing of the Gibbs free energy,causing the thermodynamics instability of ReB₂-type MnB₂at high temperature. Therefore, there is no ReB₂-type MnB₂synthesized 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（BH₄）₂. Among hydrides,Mg（BH₄）₂is a promising lightweight solid-state hydrogen storage material with atheoretical hydrogen capacity of14.8wt%. However, in2007, Li et al’s experimentsuggested that Mg（BH₄）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（BH₄）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（BH₄）₂under high pressure. In2009, L. George et al found that Mg（BH₄）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（BH₄）₂with high VHDs by usingthis phase transition. But many aspects of phase transition in Mg（BH₄）₂are stillunknown and need to be solved for better understanding of its high pressurebehaviors.For understanding high pressure behaviors of Mg（BH₄）₂, the previously proposedtheoretical and experimental structures, bond characterization and compressibility ofMg（BH₄）₂in a pressure range from0to10GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases ofmeta-stable I4₁/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 I4₁/amd at0.7GPa, and then to P-3m1structure at6.3GPa. And the experimental P6₁22structure （α-phase） transfers toI4₁/amd at1.2GPa. Furthermore, both I4₁/amd and P-3m1can exist as highvolumetric hydrogen density phases at ambient pressure. Their theoretical volumetrichydrogen densities reach146.351and134.028g H₂/L at ambient pressurerespectively. The calculated phonon dispersion curve shows that the I4₁/amd phase isdynamically stable in a pressure range from0to4GPa and the P-3m1phase is stableat pressures higher than1GPa. So the I4₁/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 I4₁/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 [BH₄]ˉrotation can support these views.