Yongliang Guo

Structural Phase Transition of ThC Under High Pressure

Thorium monocarbide (ThC) as a potential fuel for next generation nuclear reactor has been subjected to its structural stability investigation under high pressure, and so far no one reported the observation of structure phase transition induced by pressure. Here, utilizing the synchrotron X-ray diffraction technique, we for the first time, experimentally revealed the phase transition of ThC from B1 to P4/nmm at pressure of ~58 GPa at ambient temperature. A volume collapse of 10.2% was estimated during the phase transition. A modulus of 147 GPa for ThC at ambient pressure was obtained and the stoichiometry was attributed to the discrepancy of this value to the previous reports.

Pressure-induced structural transformations and polymerization in ThC 2

Thorium-carbon systems have been thought as promising nuclear fuel for Generation IV reactors which require high-burnup and safe nuclear fuel. Existing knowledge on thorium carbides under extreme condition remains insufficient and some is controversial due to limited studies. Here we systematically predict all stable structures of thorium dicarbide (ThC2) under the pressure ranging from ambient to 300 GPa by merging ab initio total energy calculations and unbiased structure searching method, which are in sequence of C2/c, C2/m, Cmmm, Immm and P6/mmm phases. Among these phases, the C2/m is successfully observed for the first time via in situ synchrotron XRD measurements, which exhibits an excellent structural correspondence to our theoretical predictions. The transition sequence and the critical pressures are predicted. The calculated results also reveal the polymerization behaviors of the carbon atoms and the corresponding characteristic C-C bonding under various pressures. Our work provides key information on the fundamental material behavior and insights into the underlying mechanisms that lay the foundation for further exploration and application of ThC2.

Evidence of s-wave pairing symmetry in the layered superconductor Li0.68NbO2 from specific heat measurements

A high quality superconducting Li0.68NbO2 polycrystalline sample was prepared by deintercalation of Li ions from Li0.93NbO2. The field-dependent resistivity and specific heat were measured down to 0.5 K. The temperature dependence of the upper critical field is determined. A notable specific-heat jump is observed at the superconducting transition temperature T-c similar to 5.0 K at zero field. Below T-c, the electronic specific heat shows a thermal activated behavior and agrees well with the theoretical result for the BCS s-wave superconductor. It indicates that the superconducting pairing in Li0.68NbO2 has s-wave symmetry.

A First-Principles Study on the Vibrational and Electronic Properties of Zr-C MXenes

Within the framework of density functional theory calculations, the structural, vibrational, and electronic properties of Zrn Cn − 1 (n = 2, 3, and 4) and their functionalized MXenes have been investigated. We find that the most stable configurations for Zr-C MXene are the ones that the terminal groups F, O, and OH locate on the common hollow site of the superficial Zr layer and its adjacent C layer. F and OH-terminated Zr3 C2 and Zr4 C3 have small imaginary acoustic phonon branches around Γ point while the others have no negative phonon modes. The pristine MXenes (Zr2 C, Zr3 C2 and Zr4 C3 ) are all metallic with large DOS contributed by the Zr atom at the Fermi energy. When functionalized by F, O and OH, new hybridization states appear and the DOS at the Fermi level are reduced. Moreover, we find that their metallic characteristic increases with an increase in n. For (Zrn Cn − 1 )O2, Zr2 CO2 is a semiconductor, Zr3C2O2 is a semimetal, and Zr4 C3O2 becomes a metal.

Structure, charge ordering and physical properties of LuFe{sub 2}O{sub 4}

Microstructural properties, phase transitions, and charge ordering of LuFe(2)O(4) have been extensively investigated by means of transmission electron microscopy (TEM) in a large temperature range from 20 to 550 K. The experimental results demonstrate that the LuFe(2)O(4) crystal is commonly modulated by charge ordering (CO), which is often recognizable by superstructure reflections. The (001) twinning domains as a common defect often appear in the LuFe(2)O(4) crystals along the c-axis direction, with the crystals across each (001) boundary rotated by 180 degrees with respect to one another. The in situ cooling TEM observations from 300 K down to 20 K reveal remarkable alternations of the superstructures, suggesting a complex CO process in the present system. Careful analysis shows that the CO in the frustrated ground state is characterized by a modulation with a wave vector of q(1)=(1/3 1/3 2). In situ heating TEM observations from 300 to 550 K clearly reveal that the CO modulation in LuFe(2)O(4) becomes invisible above a critical temperature of about T(C)=530 K. These facts suggest that the CO should be the essential driving force for the structural transitions and ferroelectricity observed in this kind of layered material. Experimental measurements on the ferroelectricity show that the LuFe(2)O(4) material, in general, has a large dielectric constant of about 10 000 at room temperature. In order to understand the properties of low-temperature phase transitions, the magnetization and specific heat from 300 to 4 K have been briefly discussed.