Theoretical Structure Design Of Tungsten And Rhenium Nitrides And Complex Hydride Li₂BeH₄ At High Pressures

Finding the global minimum of the free-energy surface for crystal at high-pressure is of great importance in both study of high-pressure phase transition and design of new materials. Currently, the most widely used method in this field is the so called'enumeration method'(EM), which can be briefly described as follows: A number of candidates are proposed based on the known structures of other materials, and the free energy favorable structure is obtained at the end of structural optimization of these candidates. Obviously, EM is a local optimization method and usually fails to reach the global minimum. In contrast, the newly developed"Universal Structure Predictor: Evolutionary Xtallography"(USPEX),"Globle Space-Group Optimization"(GSGO), and"Crystal structure Analysis using Particle Swarm Optimization"(CALYPSO) are global optimization methods for general crystal structure prediction, which are very efficient in finding the global minimum valley of the ab initio free-energy surface and has a success rate of nearly 100 % for crystal with up to tens of atoms in the unit cell, furthermore, which find large sets of competitive local minimums. These methods are taking place of EM gradually and becoming increasingly important in both the study of high-pressure phase transition and design of new materials.There has been considerable interesting in finding new superhard materials (SMs) because they are of primary importance in modern science and technology. SMs of partially covalent compounds of transition metal with light elements (B, C, N and O) have so far not been reported experimentally. However, the recently synthesized pyrite type PtN₂ has been predicted to be SM with theoretical Vickers hardness exceeding 48 GPa, which has triggered worldwide interest in the mechanical properties of 5 d transition metal nitrides.The first part of our work is to find new (super) hard materials in 5 d serious transition metal nitrides MNx (M = W, Re; x = 1, 2), through searching the global minimum of their ab initio free-energy landscape under various pressures as implement in the USPEX code.The NiAs type structure has been found to be the best candidate for WN, which agrees well with the results of the EM calculations in former study. Furthermore, we have found WN₂ energetically favors two hexagonal structures (HEXs) of space group P63/mmc and P-6m2 with very similar enthalpies, which are energetically much superior to previously proposed baddeleyite- and cotunnite-type structures in the pressure range of 0-60 GPa, and stable against decomposition into a mixture of W+N₂ or WN+1/2N₂. The two hexagonal structures are dynamical stable at both ambient and high pressures without phonon softening behaviors in their whole Brillouin Zones.Both two HEXs of WN₂ reveal a stacking of N-W-N"sandwiches"layers consisted of edge sharing WN6 trigonal prisms. The strong hybrid of W 5 d orbital and N 2 p orbital led to directional covalent W-N bonding in the prisms. The strong covalent N-N bonding provides a strengthening effect on these crystals by reinforcing the links of the N-W-N sandwiches layers. The two HEXs possess high calculated bulk (shear) modulus of about 360 (230) GPa, indicating they are ultra-incompressible materials and can withstand shear strain to a large extent. Actually, they are both ultra-hard materials with theoretical Vickers hardness of about 36 GPa exceeding that of SiO₂ (30.6 GPa) andβ-Si₃N₄ (30.3 GPa). The outstanding mechanical properties of the two HEXs can be attributed to the strong covalent N-N and N-W bonding networks over the whole crystals. Both the two HEXs are semiconductor with indirect band gap of about 1.2 eV, and synthesizable at above 30 GPa within our thermodynamic study.We found two structures with pace group R-3m and C2/m energetically surpass the NiAs- and columbite-type structures for ReN and ReN₂, respectively. Several structural characters of the two new phases are worthy of remark: (i) Just like noble metal dinitrides, ReN contains N-N bonds; (ii) in contrast to noble metal dinitrides, one-half of N-N bonds are broken in ReN₂. R-3m and C2/m phases are dynamical stable at both ambient and high pressures without phonon softening behaviors in their whole Brillouin Zones, and synthesizable at high-pressure within our thermodynamic study. Both the new phases reveal metallic behavior with 5-d electrons of Re as major carriers.ReN and ReN₂ possess very large bulk modulus (> 370 GPa) and shear modulus (> 200 GPa). ReN₂ is ultra-hard materials with a Vickers hardness of 25 GPa, which can be attributed to the strong covalent N-Re and N-N bonding network over the whole crystal. However, the hardness of ReN is not yet computable because of the strong metallic Re-Re bonding layers forbid the N-Re and N-N bonding to form 3D networks. In view of ReN has a stacking of"Re-N-N-Re"layers, which is likely to exhibit strong mechanical anisotropy.Recently, Filinchuk et al. had found a phase transition of LiBH4 at 1.2 GPa from the ambient-pressure Pnma structure into a Ama2 structure, which possess BeH4 anions have a nearly square-planar coordination comprising four Li atoms. The high-pressure structure was believed to be a step towards destabilization of LiBH4. Therefore, the exploration of the high-pressure polymorphs of other light metal complex hydrides is fundamental interest and has attracted great attentions.The second part of our work is to obtain the crystal structure of experimental observed dense phase (DP) for lithium beryllium hydrides (Li2BeH4) at pressure above 9.1 GPa, through finding its global minimum of the ab initio free-energy landscape under various pressures as implement in the USPEX code, and then get a glimpse of the hydrogen decomposing efficiency of DP by a quantify study of its chemical bonds.In good agreement with previous experimental data, we found a phase transition at 7.1 GPa from the ambient-pressure polymorphα-Li2BeH4 (phase I) to aβ-Na₂SO₄-type structure (phase II). The second transformation from phase II to a La₂NiO₄-type structure (phase III) occurs at 28.8 GPa. Both phase II and III are dynamical stable when their energy stable. The current results have ruled out previously proposed Cs₂MgH₄-type structure. The two transformations are of first order with remarkable volume collapse of 3.32 % and 5.17 %, respectively, which can be easy detected by X-ray diffraction. The metalized phenomena were not found in both the two new phases up to 40 GPa.Phase II possesses the BeH4 tetrahedrons, similar to that of phase I, but in dramatic contrast to the peculiar BeH4 octahedral layer in phase III. The Mulliken charges reveal an ionic picture for the interaction between Li atoms and BeH4 tetrahedron or BeH4 octahedral layers. The hydrogen decomposing efficiencies of phase II and are not much different from phase I since their Mulliken overlap population (MOP) are about the same ( 0.84), but phase III is likely to be better in view of its MOP is just 0.64. However, phase III is unlikely to be stabilized at ambient pressure in consideration of the high transition pressure. Our results suggest that the dense structure will not be a step towards destabilization of Li2BeH4.