High-pressure phase transitions of Mg2Ge and Mg2Sn: First-principles calculations

Phase transitions of the anti-fluorite compounds Mg₂Ge and Mg₂Sn under high pressure were investigated using the first-principles plane-wave method within the pseudopotential and generalized gradient approximations. The calculated results show that Mg₂Ge and Mg₂Sn undergo two first-order phase transitions at high pressure and the sequence of the pressure-induced phase transitions is from the anti-fluorite to the anti-cotunnite, and then to the Ni₂In-type structure. The high pressure behaviors of Mg₂Ge and Mg₂Sn are similar to Mg₂Si and the isostructural alkali-metal oxide Li₂O. Moreover, the electronic and optical properties of both the anti-fluorite and the high-pressure phases are presented.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.     

Mg2Si化合物在静水压下的电子输运性能研究

本文基于第一性原理采用全电势线性缀加平面波方法和波尔兹曼理论运算了在静水压下Mg₂Si的电子和热电性能. 研究发现, 对于n型载流子控制Mg₂Si输运性质, 应变达到0.02时, 室温情况下, 热电性能参数得到了明显提高, 其塞贝克系数增幅为26%, 功率因数增幅47%; 高温时, 功率因数增幅45%. 而对于主要载流子为空穴时, 其热电系数最值出现在应变为0.01时. 但其数值与未施加静水压的结构相比提高不多, 表明对于p型Mg₂Si半导体应变对其输运性能的影响不大. 并且结合电子能带结构图解释这些现象.     

Phase transition, structural and thermodynamic properties of Mg2Si polymorphs

The plane-wave pseudo-potential method within the framework of first principles is used to investigate the structural and elastic properties of Mg 2 Si in its intermediate pressure(Pnma) and high pressure phases(P 6 3 /mmc).The lattice constants,the band structures.The bulk moduli of the Mg 2 Si polymorphs are presented and discussed.The phase transition from anti-cotunnite to Ni 2 In-type Mg 2 Si is successfully reproduced using a vibrational Debye-like model.The phase boundary can be described as P = 24.02994 + 3.93 × 10 3 T 4.66816 × 10 5 T 2 2.2501 × 10 9 T 3 + 2.33786 × 10 11 T 4.To complete the fundamental characteristics of these polymorphs we have analysed thermodynamic properties,such as thermal expansion and heat capacity,in a pressure range of 0-40 GPa and a temperature range of 0-1300 K.The obtained results tend to support the available experimental data and other theoretical results.Therefore,the present results indicate that the combination of first principles and a vibrational Debye-like model is an efficient scheme to simulate the high temperature behaviours of Mg 2 Si.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

Mg2Sn电子结构及热力学性质的第一性原理计算

采用基于第一性原理的赝势平面波方法系统地计算了Mg₂Sn基态的电子结构、弹性常数和热力学性质.计算结果表明Mg₂Sn的禁带宽度为0.1198eV.运用线性响应方法确定了声子色散关系和态密度,得出Mg₂Sn的热力学性质如等容比热和德拜温度.计算Mg₂Sn的热导率并与实验数据相比较. 关键词： 第一性原理 电子结构 弹性常数 热力学性质     

Electronic structural, elastic properties and thermodynamics of Mg17Al12, Mg2Si and Al2Y phases from first-principles calculations

Electronic structures, elastic properties and thermal stabilities of Mg₁₇Al₁₂, Mg₂Si and Al₂Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al₂Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young's moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

Raman scattering in Ag-intercalated TiS2

Ag-intercalated TiS₂ has been investigated using electron diffraction and Raman scattering. The energies of the E_g-1 and A_1g modes in 1T-TiS₂ at 300 K were found to be 232 and 336 cm^-1, respectively. In Ag_0.3TiS₂, at an ambient temperature of 4.2 K, modes were observed at 207, 239, 277, 311, and 347 cm^-1. Three of these modes have been associated with the formation of a superlattice at low temperatures. The superlattice formation was observed by electron diffraction and is attributed to an ordering of the silver atoms at interlayer interstices.     

Raman scattering of NiS2

Raman scattering measurements of NiS₂ is done and five optical phonon peaks are found. These peaks are assigned to A_g, E_g and 3T_g phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS₂ and MnS₂.     

Raman scattering of LiGaO2

The Raman spectrum of LiGaO₂ is reported together with a preliminary identification of the optical modes and their temperature dependence between 6 and 700 K. The observed temperature dependence of the lines gave no indication for second order processes. However, at low temperatures additional lines have been observed indicating a phase transition at approximately 65 K.     

Raman scattering in layered compound 2H-WS2

Raman scattering of layer-type compound 2H-WS₂ has been studied at room temperature. The first-order Raman peaks are observed at 27.4, 357 and 423 cm^-1. The low-frequency peak at 27.4 cm^-1 is a rigid-layer mode, from which the interlayer shear force constant is estimated. The central force model is applied to the high-frequency phonons. The interlayer shear force constant is much smaller than the intralayer force constants. We also find several peaks due to the second-order processes.     

Raman scattering from magnons in CoCl2 and FeCl2

One-magnon Raman scattering has been observed in the metamagnets CoCl₂ and FeCl₂. The k = 0 magnon energies are 16 ± 1 cm^-1 at 21 K and 16.4 ± 0.4 cm^-1 at 12 K, respectively and these values are in good agreement with previous AFMR and neutron scattering results. A search for two-magnon scattering in both compounds was unsuccessful, largely because of masking from nearby first-order phonons and a weak temperature dependent broad band at 140 cm^-1 in CoCl₂, which is assigned to two-phonon scattering from acoustic phonons.     

Raman scattering from 1T-TaS2

Raman measurements on the 1T-polytype of TaS₂ are reported. In the commensurate charge density wave state, a large number of Raman-active peaks are observed below 400 cm^-1. Most of these peaks are attributed to k = 0 optic phonons resulting from superlattice formation.     

Spin-dependent phonon Raman scattering in CdCr2Se4

Influences of ferromagnetic ordering on the phonon Raman scattering are studied for CdCr₂Se₄ through the intensity measurements of Raman spectra between 25 and 300 K with various wavelengths of excitation light (488.0–676.4 nm). Spin-dependent enhancements of Raman cross section are observed for optical phonon lines D(168 cm^?1) and F(238 cm^?1) with excitation wavelengths of about 630 and 550 nm, respectively. This kind of phenomenon in spinel-type chalcogen chromites seems to originate in spin-dependent intermediate interactions in the excited states of specific electronic transitions with which the incident or scattered light is resonant.     

Effect of nanocomposite structure on the thermoelectric properties of 0.7-at% Bi-doped Mg2Si nanocomposite

Nanocomposites offer a promising approach to the incorporation of nanostructured constituents into bulk thermoelectric materials. The 0.7-at% Bi-doped Mg₂Si nanocomposites are prepared by spark plasma sintering of the mixture of nanoscale and microsized 0.7-at% Bi-doped Mg₂Si powders. Microstructure analysis shows that the bulk material is composed of nano- and micrograins. Although the nanograin hinders electrical conduction, the nanocomposite structure is more helpful to reduce thermal conductivity and increase the Seebeck coefficient, hence improving thermoelectric performance. A dimensionless figure of merit of 0.8 is obtained for the 0.7-at% Bi-doped Mg₂Si nanocomposite with 50-wt % nanopowder, which is about twice larger than that of the sample without nanopowder.     

The Raman scattering of Zn3As2

The Raman scattering of Zn₃As₂ crystal is presented within 50–420 cm⁻¹ energy range. The group theory method are used to determine symmetry of the allowed lattice modes, and further to discuss the results obtained.     

Resonant Raman scattering of CdCr2Se4

The scattering cross section of the Raman-active phonons at 156 cm^?1 (Eg) and 169 cm^?1 (F2g) in the ferromagnetic semiconductor CdCr₂Se₄ (T_c=130 K) has been measured as a function of incident photon energy between 1.55 and 2.81 eV, both in the ferromagnetic and paramagnetic phases. The resonance curve peaks sharply near 2 eV and shows a broadening for temperatures below the Curie point. The relative line intensities change significantly with photon energy. The results show that the concept of spin-dependent Raman scattering in the ferromagnetic spinels has to be revised in terms of exchange-splitting-induced resonant Raman scattering.     

Indirect exciton luminescense and Raman scattering in CdI2

Intrinsic luminescence and Raman scattering in 4HCdI₂ have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.     

Raman scattering in 3RNbS2

Raman scattering spectra on a layer type compound 3RNbS₂ were measured at 300 and 77 K, and three peaks were observed at 382, 325 and 281 cm^?1 at 300 K. The vibration modes were analyzed in terms of Bromley's nearest neighbour model, and a good agreement with the experiment was obtained.