The Chemical Reactions Of Alkali Elements And Fe At High Pressures

Earth’s core accounts for one-third of the planet’s mass and has a central role inEarth’s overall energy budget and dynamics. Although the core has long been known tobe composed mainly of iron, together with some nickel, the identity of the lighterelements that make up about8%of the core’s mass has been an enigma for nearly60years. Better knowledge of the core’s main light elements will shed light on heatflow in Earth’s deep interior, on the origin and growth of the core’s solid inner region,and on the generation and evolution of Earth’s magnetic field. Hence, unmasking theidentity and abundance of the core’s main light elements will also be a mjor stepforward in understanding Earth’s geochemical evolution.Seismic data provide robust constraints on density and sound velocities throughoutthe core. These data reveal that the core is divided into a large, liquid outer core with aradius of about3,500kilometres, within which is embedded a smaller, solid inner corewith a radius of about1,200km. The differences in seismic velocity and densitybetween the liquid outer and solid inner cores are also known, as is the existence ofweak seismic ature is that the abundance of light elements in the inner core seems to beonly about half of that in the outer. All of these observable aspects provide importantcharacteristics that the successful light-element candidates must have. However, directlyprobing potential core materials against these geophysical criteria is challengingbecause of the extreme conditions that exist in the core—pressures of136–364gigapascals and temperatures of about4,000–6,500kelvin.But which element is it? Here we proposed that alkali elements(Li, Na, etc) shouldbe one of the suspects. Using the crystal structure prediction at high pressure, we haveextensively explored the chemical reactions of Iron and alkali elements at high pressureand obtained the significant and innovative results as follows:1. The Earth’s crust contains2.6%sodium by weight, making it the sixth mostabundant element on Earth. Therefore, sodium is one of the most potential candidate for the light elements in the Earth’s core. In my research, The possible existence of stableFe-Na compounds under the pressure of the earth’s core has been investigated withstructural prediction calculations. Only FeNa3and FeNa4are found to bethermodynamically most stable, which displayed complex structures and which havenot been predicted or observed before. In FeNa3, a novel planar structure formed byfused squares and heptagons with Fe atoms chains running down the7-member ringsand electrides in the four member rings was predicted at150GPa. At higher pressurethe3D framework becomes more favorable. A C2/c structure of FeNa4remarkablysimilar to the self-clathrate guest-host structure Ba-IV has been identified from150GPato360GPa. This observation indicates the diversity of chemical bonding of Na at highpressure. Moreover, sodium usually assume to be ‘electropositive’ while formingcompounds with others species. However, analysis of the electronic structure suggeststhat electrons are removed from the Na in the lower pressure planar structure but gainelectrons from the Fe in the3D framework, which is based on hybridization with emptyNa3p and3d orbitals. In this study, we found an unusual and counter-intuitivephenomenon that the supposedly “electropositive” Na atom can either donate or acceptelectrons.2. As the lightest metal and the least dense solid element, lithium’s low reactivitycompared to other alkali metals is due to the proximity of its valence electron to itsnucleus (the remaining two electrons are in lithium’s1s orbital and are much lower inenergy, and therefore they do not participate in chemical bonds). According toMiedema’s rules, the charge densities of alkale metals are much smaller than that of thetransition elements, thus permitting compound formation between them. However,pressure can induce significant changes in atomic and electronic structures, thus tuningthe atomic structure and the very nature of chemical bonds to produce novel materials.By combining theoretical predictions, we found that new materials with differentstoichiometries emerge at high pressures.We here study the phase stabilities andstructural evolution of the Li-Fe inter-metallic compounds under pressure by using anunbiased particle swarm optimization method on crystal structure prediction. Our resultsshow that, three stoichiometric LixFe1-xcompounds (LiFe, Li3Fe and Li3Fe2) that arestable over a range of pressures, and two intriguing structures of Li3Fe with spacegroups P6/mmm and P4/mbm, stable at pressure ranges of45–64GPa and64–100GPa,respectively. These phases possess intriguing crystal structures and peculiar physicalproperties under pressure. Such unusual compounds are likely to find potential applications if synthesized in sufficiently large quantities. While powerfulcomputational methods, such as CALYPSO, are are yet to be developed. This workrepresents a significant step forward in understanding the HP phase diagram of Li-Feintermetallic compound, and also provides a novel way to search for alloys. SubsequentHP synthesis of other alloys might be anticipated to greatly increase the choice ofmaterials for a variety of applications.