Structural Design Of Superhard And Iron-based Superconducting Materials

Superhard and superconducting materials are two major types of importantfunctional materials having various applications. Structural design of new superhardand superconducting materials is important issue in materials science and condensedmatter physics. The development of the computer technology, the improvement ofsolid state theory, the development of the structure prediction method, and theprogress of high-pressure technology, together provide theoretical and technologicalsupports for design of these materials.Superhard materials have important applications in geology and mining, cutting,precision instruments manufacturing, and modern cutting-edge science due to theirexcellent mechanical properties. Diamond and cubic BN(c-BN) are two kind ofwidely used superhard materials. However, the thermal and chemical inertness ofdiamond are poor. It tends to transformed into graphite or react with iron at higherthan750°C, which limits its application in cutting iron containing materials at hightemperature. Comparing with diamond, c-BN has better thermal and chemicalinertness. However, the hardness of c-BN (about66GPa) is much lower thandiamond. Therefore, searching for new superhard materials possessing both highhardness and thermal/chemical inertness is a key issue needed to be solved.Encouragingly, a number of new superhard or hard compounds have beensuccessfully synthesized experimentally, such as BC2N、BC5、B6O、OsN2、OsB2and WB4. However, the experimental challenge on synthesis still remains, since thetemperature and pressure are difficult to control. Therefore, the blind experimentalsynthesis requires a large amount of manpower and materials resources. The lowefficiency and little rate of success will naturally block the industrial synthesis inbatch. Thus, the theoretical design of new superhard materials is essential as requeststo improve the feasibility and effectiveness of experimental synthesis and/or findsome extricable approaches.The first part of this work is based on this background to develop an effectivemethodology for designing superhard materials. The approach requires only thechemical compositions and certain external conditions (here, pressure) to predict thehardness vs. energy map, from which the energetically favorable superhard or hardstructures are readily accessible. These obtained energetically favorable superhard orhard structures can be the important candidates for theoretical study or experimentalsynthesis. In contrast to the traditional ground-state structure prediction method wherethe total energy was solely used as the fitness function, here we adopted hardness asthe fitness function in combination with CALYPSO method for structural searching,and the first-principles methods for energy calculation to construct the hardness vs.energy map by seeking a proper balance between hardness and energy for a bettermechanical description of given chemical systems. In the hardness calculations, weadopted and improved the earlier hardness model based on bond strength (proposedby im nek et al) by applying the Laplacian matrix to account for the weak Van derWaals bonds in the highly anisotropic and molecular systems, which enable auniversal theory. We benchmarked our approach in typical superhard systems, such aselemental carbon, the binary B-N, and ternary B-C-N compounds. Nearly all theexperimentally known structures have been successfully predicted at zero pressure,such as the diamond, hexagonal-diamond, graphite, c-BN and h-BN. Most of thepreviously proposed superhard carbon structures have been reproduced, such as theZ-carbon, chiral-framework carbon, F-carbon, bct4-carbon, and T12-carbon. It is also encouraging that we have also discovered several new meta-stable superhard carbonstructures. The hardness values of these new meta-stable superhard structures arecomparable to that of diamond and the energies are more stable than graphite undercertain pressure. The results suggested that our approach is reliable.Superconductors has many applications in various filed due to their properties ofzero electrical resistance and expulsion of magnetic fields under certain criticaltemperature. Searching the high-temperature superconductors and exploring thesuperconducting mechanism are the hot topics in the field. In2008, the iron-basedsuperconductors were discovered, which becomes a new class of high-temperaturesuperconductor after the copper-oxide based superconductors. These materials allshare a common layered structure based on FeX4(X=P, As, S, Se and Te) tetrahedrallayers, which is a key parameter determine the physical property of iron-basedmaterials. It is even found that the maximum Tcusually occurs when the FeX4is closeto the regular tetrahedron. It is well known that high pressure can change theinter-atom distance and modify the electronic properties of solid without introducingany chemical complexity. Therefore we can adopt high pressure method to alter theFeX4tetrahedron structure, change the electronic properties around Fermi energy, andfinally modify the superconductivity.The second part of this work is to use the “111” type iron-based superconductors(LiFeAs, NaFeAs and LiFeP) as representation to design their high-pressure structuresby CALYPSO method and investigate their high-pressure electronic properties byfirst-principle method. Before the high pressure studies, we have noticed that theparent compound of LiFeAs is superconducting without chemical doping, and thecommon low-temperature antiferromagnetic and phase transition did not detectedpreviously. These characters of LiFeAs are unique and different from NaFeAs and the“1111” or “122” type of iron-based superconductors. Does this mean that LiFeAs istotally different from the other iron-based superconductors and has some distinct properties at low-temperature? This inspired our research interests. We thus carriedthe first-principle studies on the magnetic, structural and electronic properties ofLiFeAs at zero temperature and pressure. The details are as follows:(1) We have calculated the total energy of several magnetic states (paramagnetic,ferromagnetic and antiferromagnetic) of P4/nmm structure for LiFeAs by thefirst-principle methods. It was found that the striped antiferromagnetic structure withFe layer antiferromagnetically coupled along the z axis has the lowest energy.Moreover, the striped antiferromagnetic tetragonal P4/nmm structure was distorted,which finally induced the phase transition to the orthorhombic Cmma structure. Thesimilar phenomenon was also found in NaFeAs, which is consistent with theexperimental discovery of structural and magnetic phase transition at low temperature.From the density of the states, it was found that the Fermi level of thestriped-antiferromagnetic Cmma phase shifted to the valley of the pseudogap, whichfurther demonstrates that the Cmma structure is more stable at zero temperature.These results suggest that LiFeAs has similar magnetic, structural and electronicproperties to NaFeAs at low temperature.(2) Under high pressures, NaFeAs has been discovered experienced anisostructural phase transition at about3GPa with Tcreaching a maximum about31K.At~20GPa, a clear structural phase transition was identified, however the new phaseremained structurally unknown. In LiFeAs and LiFeP, Tc has been observed todecrease linearly with increasing pressure, but no phase transitions were reported uptoabout20GPa and2.75GPa, respectively. Are there some other unknownhigh-pressure properties in these materials? In this work, we found that although thethree compounds are chemically similar, they adopted distinct structural phasetransitions under high pressure: P4/nmm-P-3m1-I4mm-P63/mmc for LiFeAs,P4/nmm-Cmcm-I4mm for LiFeP, and P4/nmm-Cmcm-P-3m1for NaFeAs. Thehigh pressure orthorhombic Cmcm phase preserved the structural features of FeX4(X=As,P) tetrahedral layers present in the ambient-pressure P4/nmm structure.However, the FeX4tetrahedrons in the Cmcm phase were clearly distorted, leading tochanges in the electronic behavior around the Fermi level. Under higher pressures, theFeX4layered structural features were no longer persistent and high-densitythree-dimensional FeAs5/FeAs6structures were stabilized in other P-3m1, I4mm,P63/mmc phases. It is interesting that the tetragonal I4mm structure turn out to be asemiconductor, which is totally different from the other structures and uncommon tosee in the iron-based materials. Such property might closely relate to the newpyramidal FeAs5units in the I4mm phase. These results will greatly expand ourunderstanding of iron-based superconductors, and shed light on the high-pressureproperties of the other type of iron-based materials.