We predicted a new ternary hydride Li 2 SiH 6 at high pressures. A systematic structure search in Li 2 SiH 6 compound reveals novel stable phases with intriguing electronic and phonon properties.It is found that Li 2 SiH 6 is dynamically stable from ambient pressure up to 400 GPa with three novel phases: P312, P 3, and P 62m. The calculation of electron-phonon coupling combined with Bardeen-Cooper-Schrieffer's argument indicates that this compound may be a candidate for high T c superconductors under high pressures. In particular, the maximum T c of P 62m-Li 2 SiH 6 at 400 GPa reaches 56 K. These findings may pave the way for obtaining room temperature superconductors in dense hydrogen-rich compounds. INTRODUCTIONThe search for high-temperature superconducting materials has always been a hot topic in the field of condensed matter physics. According to the traditional Bardeen-Cooper-Schrieffer (BCS) theory[1], the superconducting transition temperature is directly proportional to the Debye temperature, and the Debye temperature is inversely proportional to the mass, so lighter elements may have higher superconducting transition temperatures, such as hydrogen. However, solid hydrogen is an insulating molecular crystal under ambient pressure, and there are strong covalent bonds in hydrogen molecules, so it is hard to obtain superconducting hydrogen under ambient pressure. In order to achieve superconductivity, conditions such as external pressure are needed. As a basic thermodynamic variable, pressure can change the distance between atoms of matter, so that atoms can be rearranged, and the crystal and electronic structure can be modulated. High pressure can very effectively shorten the distance between atoms, increase the overlap of adjacent electron orbits, and then change the interaction and electronic structure between atoms/molecules, forming high-pressure new phases with new structures and properties that are difficult to form under conventional conditions. Generally, in a sufficiently high pressure environment, the band gap will be narrowed, and the energy bands will overlap, which can transform the non-metallic state into the metallic state. Wigner and Huntington proposed that insulating hydrogen molecules can be transformed into metallic hydrogen under high pressure, showing a metallic state [2]. Ashcroft proposed chemical preloading [3], that is, non-hydrogen elements in hydrogen-rich compounds have an interaction effect on hydrogen elements in the

High-pressure structural searches of superhydrides CeBeH8 and CeBH8 were performed under ambient pressure up to 300 GPa. We identify Fm3m-CeBeH8 with a superconducting transition temperature Tc of 56 K at 10 GPa. Two more phases with spacegroup R3m and C2/m, were investigated within the increasing pressures. CeBH8 shows a similar phase transition process as CeBeH8 but with higher transition pressures and higher Tc. Fm3m-CeBH8 is predicted to be superconducting above 120 GPa with a maximum Tc of 118 K at 150 GPa. R3m-CeBH8 and C2/m-CeBH8 are dynamically stable above 120 GPa and 100 GPa, respectively. The maximum Tc is 123 K at 195 GPa for R3m-CeBH8, and 115 K at 350 GPa for C2/m-CeBH8. Our work enriches the family of ternary hydrides and may provide a useful guideline for further search for superconducting hydrides at low and moderate pressures.

The electronic structure and magnetic properties of the (Co1-xFex)Tet(CoxFe2-x)OctO4 spinels (x is defined as the degree of inversion) scenario are investigated theoretically from first-principles, using generalized gradient approximation (GGA) method for the systems with strong coulomb correlations, which gives a correct description of the electronic structure. The GGA+U method gives an improved qualitative result compared with the GGA not only for the excited-state properties such as energy gaps but also for the ground-state properties such as magnetic moments and crystal parameters. The nominal valence of the transition metal elements and the ground state structure have been established based on the study of variation of the cation distribution (x=0.0, 0.25, 0.5, 0.75 and 1.0) over the tetrahedral and octahedral sites. The site-preference calculation on bulk systems indicates that Co2+ ions strongly prefer the octahedral B sites, and the electronic structure and magnetic properties of cobalt ferrites highly depend on the cation distributions even though the chemical composition of the compound does not change. The results are in good agreement with the available experimental data and most of the other theoretical results.

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