Zhao-Yi Zeng

Dynamical stability of Mo under high pressure and high temperature

Considering the phonon-phonon interactions, we obtain the high temperature phonons of Mo under high pressure. The dynamically stable regions of bcc and fcc Mo in the phase diagram are predicted. By comparing the anharmonic free energy, we determine the bcc-fcc boundary. The bcc Mo is the stable phase up to 700 GPa. Around 210 GPa, there is no bcc-fcc phase transition, which is different with the results from quasiharmonic approximation.

High Pressure Structural Instability and Thermal Properties of Rutile TiO2 from First-principles

We report a first-principles calculation to investigate the structural instability of rutile TiO2. The high pressure structural parameters are well reproduced. The calculated phonon dispersion curves agree with experiments at zero pressure. Under compression, we capture a large softening around Г point, which indicates the structural instability. From the high pressure elastic constants, we find that the rutile TiO2 is unstable when the applied pressure is larger than 17.7 GPa. Within the quasi-harmonic approximation, the thermal equation of state, thermal expansion coefficient, bulk modulus, and entropy are well reproduced. The thermal properties confirm the available experimental data and are extended to a wider pressure and temperature range.

Magnetism and Sound Velocities of Iron Carbide (Fe3C) under Pressure

The elastic property and sound velocity of Fe3C under high pressure are investigated by using the spin-polarized generalized gradient approximation within density-functional theory. It is found that the magnetic phase transition from the ground ferromagnetic (FM) state to the nonmagnetic (NM) state occurs at ∼73 GPa. Based on the predicted Hugoniot of Fe3C, we calculate the sound velocities of FM-Fe3C and NM-Fe3C from elastic constants. Compared with pure iron, NM-Fe3C provides a better match of compressional and shear sound velocities with the seismic data of the inner core, supporting carbon as one of the light elements in the inner core.

Robust large gap quantum spin Hall insulators in methyl and ethynyl functionalized TlSb buckled honeycombs

A large bandgap is critical for the applications of quantum spin Hall (QSH) insulators at room temperature. Based on the first-principles calculations, we predict that the methyl and ethynyl functionalized TlSb monolayers, namely, TlSb(CH3)2 and TlSb(C2H)2 films, own QSH states with large bandgaps of 0.13 and 0.272 eV, which possess potential applications at room temperature. For TlSb(CH3)2, the QSH phase arises from the spin-orbit coupling (SOC) induced s-p band inversion, while for TlSb(C2H)2, the QSH phase results from the SOC induced p-p bandgap opening. The QSH effect is further characterized by the Z2 topological invariant and topologically protected edge states. Significantly, the QSH states in TlSb(CH3)2 and TlSb(C2H)2 films are robust against external strain and various methyl/ethynyl coverages, making them especially flexible in the substrate selection. Besides, we find that h-BN is an ideal substrate for TlSb(CH3)2 and TlSb(C2H)2 films to keep QSH states with large bandgaps. Thus, the methyl and ethynyl functionalized TlSb films may be good QSH effect platforms for the design and fabrication of topological electronic devices.A large bandgap is critical for the applications of quantum spin Hall (QSH) insulators at room temperature. Based on the first-principles calculations, we predict that the methyl and ethynyl functionalized TlSb monolayers, namely, TlSb(CH3)2 and TlSb(C2H)2 films, own QSH states with large bandgaps of 0.13 and 0.272 eV, which possess potential applications at room temperature. For TlSb(CH3)2, the QSH phase arises from the spin-orbit coupling (SOC) induced s-p band inversion, while for TlSb(C2H)2, the QSH phase results from the SOC induced p-p bandgap opening. The QSH effect is further characterized by the Z2 topological invariant and topologically protected edge states. Significantly, the QSH states in TlSb(CH3)2 and TlSb(C2H)2 films are robust against external strain and various methyl/ethynyl coverages, making them especially flexible in the substrate selection. Besides, we find that h-BN is an ideal substrate for TlSb(CH3)2 and TlSb(C2H)2 films to keep QSH states with large bandgaps. Thus, the methyl an...

First-Principles Study of Lattice Dynamics, Structural Phase Transition, and Thermodynamic Properties of Barium Titanate

Abstract Lattice dynamics, structural phase transition, and the thermodynamic properties of barium titanate (BaTiO3) are investigated by using first-principles calculations within the density functional theory (DFT). It is found that the GGA-WC exchange-correlation functional can produce better results. The imaginary frequencies that indicate structural instability are observed for the cubic, tetragonal, and orthorhombic phases of BaTiO3 and no imaginary frequencies emerge in the rhombohedral phase. By examining the partial phonon density of states (PDOSs), we find that the main contribution to the imaginary frequencies is the distortions of the perovskite cage (Ti-O). On the basis of the site-symmetry consideration and group theory, we give the comparative phonon symmetry analysis in four phases, which is useful to analyze the role of different atomic displacements in the vibrational modes of different symmetry. The calculated optical phonon frequencies at Γ point for the four phases are in good agreement with other theoretical and experimental data. The pressure-induced phase transition of BaTiO3 among four phases and the thermodynamic properties of BaTiO3 in rhombohedral phase have been investigated within the quasi-harmonic approximation (QHA). The sequence of the pressure-induced phase transition is rhombohedral→orthorhombic→tetragonal→cubic, and the corresponding transition pressure is 5.17, 5.92, 6.65 GPa, respectively. At zero pressure, the thermal expansion coefficient αV, heat capacity CV, Grüneisen parameter γ, and bulk modulus B of the rhombohedral phase BaTiO3 are estimated from 0 K to 200 K.