Exemplary Research Of Novel Chemical Reactions At High Pressures

Supervisor: Cademician Guangtian Zou, Prof. Yanming MaPressure as a thermodynamic parameter, has a huge effect on the synthesis andits properties of the matter. Since the chemical reaction or phase change determinedthermodynamic free energy G, which is a function of pressures and temperatures.High-pressure chemical reactions expand another dimension parameter. Thereby theprobability of the chemical reactions has been greatly increased under high pressures.High-pressure often decreases the distance between atoms, induces charge transferbetween atoms, changes the atom’s valence states, and hereby reduces chemicalreaction barrier and promotes the probability of the chemical reactions, which can’toccur under normal pressure. High-pressure chemical reactions have uniqueadvantages in developping condensed matter theory and riching the high-pressurechemistry theory. Therefore, designing new materials with novel properties applyingthe high-pressure technology has become an important method. The researches underextreme conditions of high pressure will get significant progress in terms of newmaterials, new physics, and new chemistry. The purpose of this paper is to exploresome typical new chemical reactions at high pressure (such as inorganic solid-solidreaction of lithium and boron, inert gas xenon and molecules F2and N2) usingCALYPSO method and obtain new materials with novel properties, enriching anddevelopping the high-pressure condensed matter theory. The innovative results areobtained as follows.1. Boron has always been recognized as a complex element, both structurally andelectronically: its crystalline phases are numerous and inevitably complicated, whichis related to its electron deficiency and thus the tendency to form multicenter bonds. Icosahedra and other large polyhedral figure prominently as a structural motif for B.Boron is clearly close to the metal and nonmetal line; it is formally a semiconductorbut becomes metallic when doped, for instance with lithium, and can then exhibitsuperconducting behavior. Using an unbiased structure search method based onparticle-swarm optimization algorithms in combination with density functional theorycalculations, we investigate the phase stabilities and structural changes of variousLi-B systems on the Li-rich regime under high pressures. We identify the formation offour stoichiometric Li borides (Li3B2, Li2B, Li4B and Li6B) having unforeseenstructural features that might be experimentally synthesizable over a wide range ofpressures. It is strikingly found that the B-B bonding patterns of these Li boridesevolve from graphite-like sheets to in turn zig-zag chains, dimers, and eventuallyisolated B ions with the increase of Li contents. These intriguing B-B bondingfeatures are chemically rationalized by the elevated B anionic charges as a result of Liâ†'B charge transfer.2. Xenon filled shell element is a typical inert gas, due to the low chemicalreactivity, it is difficult to form a stable bond with other elements forming compound.Since the chemical bonding of xenon compounds is very interesting, research on inertgas compounds always a theoretical or experimental hot spot. Chemical bonding Xeand alkali (earth) metals, transition elements, III-VII family of elements has beenfound. However, the Xe-Xe chemical bonding has not been found till now. In addition,there is considerable controversy over the xenon hypervalence. The existing forms ofXenon fluorides have been controversial at high pressures. Furthermore, thehigh-pressure phase transition behavior of XeF2between theory and experiment is stillcontroversial. Therefore, xenon fluoride research at high pressures is important. Usingan unbiased structure search method based on particle-swarm optimization algorithmsin combination with density functional theory calculations, we investigate the phasestabilities and structural changes of various Xe-F systems under high pressures. Weidentify the formation of five stoichiometric Xe fluorides (Xe2F, XeF, Xe3F2, XeF2,XeF4and XeF6) having unforeseen structural features that might be experimentally synthesizable over a wide range of pressures. XeF2evolves into the ion phase(XeF)+F-at high pressures. XeF4remains molecular crystal up to200GPa, XeF4,while, XeF6can polymerize with higher coordination number under increasingpressure. The graphite-like Xe layers form is discovered in metallic Xe2F. It isstrikingly found that ionic Xe3F2with semiconductor feature，i.e.[Xe2]δ+[XeF2]δ-(δ=0.6). The novle Xe-Xe covalent bond is found firstly in XeF. It fills the gaps of thechemical bonding between xenon atoms. Sharing its5p electrons, Xe can be formedfluorides compounds with molecular, covalent, ionic and metallic character.3. Noble gase Xe as typical closed-shell systems is inert benchmark exampleswhere the chemical octet rule stems from. It is difficult to form a stable bond withother elements forming compound. Since the chemical bonding of xenon compoundsis very interesting, research on inert gas compounds always a theoretical orexperimental hot spot. Diatomic N2molecules, having the strongest-known tripleN≡N bonds. As a result, N2is chemically inert and hardly to interact with othersubstances at normal conditions. So, they are critical elements, as their abundancesconstrain the models for giant planet formation and the origin of their atmospheres.Due to the inertness of Xe and N2, they can’t react under atmospheric conditions.Therefore, finding new xenon nitrides is important. In addition, Xenon nitrides aspotential polymeric single N-N bond high desitiy energy materials. So, the researchon Xe and N2at high pressures is very important. Using an unbiased structure searchmethod based on particle-swarm optimization algorithms in combination with densityfunctional theory calculations, study the reactivity of Xe and N2under high pressures,unexpectedly, resulted in the prediction of a hexagonal phase XeN6with N-sp3hybridization including two Xe-N and two N-N covalent bonds, stable above146GPaaccessible to the present high-pressure experiment. The N-sp3hybridization of XeN6(N atom has no lone pair of electrons) is different form that of the early discoverednitrides (N atom has a lone pair of electrons). Xe-N covalent bonds are the keyfavoring the formation of XeN6. Besides, Xe bonds unexpectedly with12N atomsand forms a hypervalent state. Newly discovered hypervalent xenon violat the traditional concept of octet rule.