Mechanical and thermodynamic properties of the monoclinic and orthorhombic phases of SiC_2N_4 under high pressure from first principles

First principles calculations are preformed to systematically investigate the electronic structures, elastic and thermodynamic properties of the monoclinic and orthorhombic phases of Si C2N4 under pressure. The calculated structural parameters and elastic moduli are in good agreement with the available theoretical values at zero pressure. The elastic constants of the two phases under pressure are calculated by stress–strain method. It is found that both phases satisfy the mechanical stability criteria within 60 GPa. With the increase of pressure, the degree of the anisotropy decreases rapidly in the monoclinic phase, whereas it remains almost constant in the orthorhombic phase. Furthermore, using the hybrid density-functional theory, the monoclinic and orthorhombic phases are found to be wide band-gap semiconductors with band gaps of about 2.85 e V and 3.21 e V, respectively. The elastic moduli, ductile or brittle behaviors, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in both phases are also investigated in detail.     

Origin of the 420 nm Absorption Band and Effect of Doping Fluorine in PbWO4 Crystals

The electronic structures for three types of PbW04 (PWO) crystals, the perfect PWO, the PWO containing lead vacancy (PWO-Vpb) and fluorine doped PWO crystal (F^-：PWO), are systematically studied within the framework of density functional theory. The computational results show that the Pb 6s state situates below the valence band so that Pb^2 ions are unable to trap holes forming Pb^3 or Pb^4 to compensate for VPb^2-. The hole-trappers in PWO-Vpb are O^2- ions. Two of the longer-bond O^2- ions share a hole forming O2^3-, and four of the longer-bond oxygen ions trap two holes forming an associated color centre [O2^3--Vpb-O2^3-], which may be the origin of the 42Onto absorption band. It is also concluded that the doping of F^- would reduce the band gap and F^- ions substituting for O^2- can effectively restrict the formation of [O2^3--Vpb-O2^3-] and weaken the 42Onm absorption band and hence enhance the scintillation property of PWO.     

四方Mo2B基本物性的第一性原理研究

采用基于密度泛函理论的第一性原理方法,对新近发现的四方Mo2B在0～40 GPa压力范围内的物性进行了研究,研究内容包括弹性、各向异性、脆性延展性、硬度、理论强度、热容、热膨胀系数等基本物理性质.研究发现在0～40 GPa压力范围内,四方Mo2B晶体均满足力学稳定性条件,并且表现出较强的各向同性.同时发现该晶体具有较高的抗压缩能力,但抗剪切能力较差,抗压和抗剪切能力会随压力增加而增长.此外还发现四方Mo2B为韧性材料,且压力越大韧性越强.计算显示它的维氏硬度为14.3 GPa,限制其作为超硬材料的应用.通过应力-应变的计算发现其不同方向的理想拉伸及剪切强度都比较低.此外,还利用准谐德拜模型研究了四方Mo2B的热膨胀系数以及定容热容等热力学性质.     

Elastic and thermodynamic properties of vanadium nitride under pressure and the effect of metallic bonding on its hardness

By the particle-swarm optimization method, it is predicted that tetragonal P42mc, 141md, and orthorhombic Amm2 phases of vanadium nitride （VN） are energetically more stable than NaCl-type structure at 0 K. The enthalpies of the predicted three new VN phases, along with WC, NaC1, AsNi, CsCl type structures, are calculated each as a function of pressure. It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa. For the considered seven crystal- lographic VN phases, the structures, elastic constants, bulk moduli, shear moduli, and Debye temperatures are investigated. Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaC1 phase. The Debye temperatures of VN predicted three novel phases, which are all higher than those of the remaining structures. The elastic constants, thermodynamic properties, and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria. Moreover, the effect of metallic bonding on the hardness of VN is also investigated, which shows that VNs in P42mc, 141md, and Amm2 phases are potential superhard phases. Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.     

Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF 4 crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory,using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV,which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.     

boldmath Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF₄ crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.