Pressure dependent Raman,and conductivity studies of the fast ion conductors Cu2HgI4, Ag2HgI4 and Tl2ZnI4

High pressure Raman, electrical conductivity, and optical microscopic studies on the ternary fast ion conductor Cu₂HgI₄ were undertaken to delineate the pressure-temperature phase diagram. In addition, comparison of the pressure dependence of Raman shifts in Cu₂HgI₄ and Tl₂ZnI₄ was used to assist in making vibrational assignments whenever possible.     

Single crystal polarized Raman spectra of the solid electrolyte Cu2HgI4; attempt frequencies for ion motion in Ag2HgI4

The polarized Raman scattering from small single crystals of Cu₂HgI₄ provided assignments for the more prominent Raman features to specific irreducible representations. The E symmetry assignment, mass dependence, and pressure dependence of the 36 cm^?1 band in Cu₂HgI₄ and 24 cm^?1 band in Ag₂HgI₄ indicate that these features approximate the attempt frequency for ion hopping. The unusually high pre-exponential factor in the Arrhenius expression for ion hopping is discussed in light of the observed attempt frequency; we conclude that despite the high activation energy the conduction mechanism is similar to other heavy-metal solid electrolytes.     

Electrical conductivity and composition range of Ag2HgI4 between room temperature and 75°C

The composition range of α- and β-Ag₂HgI₄ near the transformation temperature is determined by X-ray diffraction. The electrical conductivity measured in the α-phase is nearly unaffected by deviation from stoichiometry. This is caused by a compensation of the composition dependences of σ₀ and of the activation energy E_A. From the experimental results the dependences of the entropy and of the Gibbs energy for migration of Ag-ions on temperature and on composition are estimated.     

Theoretical studies on the structural, electronic, and optical properties of Ag2HgSnSe4

We investigate the electronic structure of Ag₂HgSnSe₄ in a wurtzite-stannite structure with the first principles method. This crystal is a direct band-gap compound. In addition the dielectric function, absorption coefficient, reflectivity, and energy-loss function are studied using the density functional theory in the generalized gradient approximation. We discuss the optical transitions between the valence bands and the conduction bands in the spectrum of the imaginary part of the dielectric function at length. We also find a very high absorption coefficient and a wide absorption band for this material. The prominent structures in the spectra of reflectivity and the energy-loss function are discussed in detail.     

Hot stage X-ray diffractometer studies on Ag2PbI4 and Ag4PbI6

Mixtures of AgI and PbI₂ cooled from the melt result in the peritectic formation of a fast ion conducting phase centred about Ag₄PbI₆, which is face centred cubic with a = 6.33(5)A; this phase exhibits high electrical conductivity. On cooling to about 125°C, dissociation occurs to γAgI and PbI₂, accompanied by the transient formation of another phase, centred about Ag₂PbI₄. A modified form of the T-x section of the equilibrium phase diagram at AgI concentrations greater than 60 mole % and below 300°C is proposed.     

Synthesis and transport properties of La2NiO4

La₂NiO₄ compounds were prepared by a modified sol–gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol–gel as precursors. The high-temperature transport properties of the samples were investigated. The band structure, total density of states (DOS), and partial density of states (PDOS) of low-temperature orthorhombic (Bmab) phase and high-temperature tetragonal (I4/mmm) phase for La₂NiO₄ were calculated in order to study the transport properties of the as-obtained samples.     

Far infrared optical properties of proustite (Ag3AsS3)

The infrared reflection and transmission spectra of Ag₃AsS₃ single crystals have been between 20 and 650 cm^-1 at room temperature. Fourteen phonon modes were observable in reflectivity with the dominant restrahlen bands occuring at 360 cm^-1 and 35 cm^-1 for E? c and at 335 cm^-1 and 35 cm^-1 for E⊥ c. These measurements indicate that generation of far infrared radiation by mixing of two laser frequencies using proustite as the down-conversion crystal is not possible.     

Optical properties of new photovoltaic materials: AgCuO2 and Ag2Cu2O3

AgCuO₂ and Ag₂Cu₂O₃ are new types of semiconductor materials. A theoretical study is presented for both the electronic and optical properties of these new photovoltaic materials in the framework of density functional theory (DFT). The calculated cohesive energy is −3.606 eV/atom and −3.723 eV/atom for Ag₂Cu₂O₃ and AgCuO₂, respectively. Electronic calculations indicate that AgCuO₂ is a small band gap semiconductor and Ag₂Cu₂O₃ is metallic in nature. The valency state of Cu is divalent in Ag₂Cu₂O₃ and trivalent in AgCuO₂. The largest absorption coefficient of CuO₂ is 332 244, which is significantly greater than that of CuInSe₂, CdTe, GaAs, etc.     

Pressure dependence of electronic structure and magnetic properties in Fe16N2

The electronic structures and magnetic properties of Fe₁₆N₂ system and their pressure dependence were investigated by using first-principles calculations based on the density functional theory. It has been found that the total magnetic moment in Fe₁₆N₂ system decreases monotonically as increasing pressure from 0 to 14.6 GPa. A phase transition from ferromagnetic (FM) to non-magnetic (NM) occurs with a volume collapse of around 0.008  at 14.6 GPa, The lattice constants a and c for magnetic results decrease monotonically as pressure increasing from 0 to 14.6 GPa, at 14.6 GPa, the lattice constant a decreases sharply, on the contrary, the lattice constant c increases abruptly. We think that the change of microscopic structure of Fe₁₆N₂ is responsible for the phase transition from FM to NM.     

光催化半导体Ag$lt;sub$gt;2$lt;/sub$gt;ZnSnS$lt;sub$gt;4$lt;/sub$gt;的第一性原理研究

通过基于密度泛函理论的第一性原理计算, 对光催化水解半导体Ag₂ZnSnS₄的改性方案做了理论研究. 在与同类化合物的带边位置比较后发现, Cu与Ge共掺杂能够在Ag₂ZnSnS₄中实现禁带宽度和带边位置的双重调节, 从而使其能带结构优化到光催化水解最为理想的状态. 另外, CuGaSe₂ 可与Ag₂ZnSnS₄形成type-Ⅱ型带阶结构, 制备它们的异质结同样可用于提升其光催化水解性能. 关键词： 光催化半导体 2ZnSnS₄')" href="#">Ag₂ZnSnS₄ 带阶 电子结构优化     

Electrical transport properties of Ag3Sn compound

The temperature behaviors of the electrical resistivity in polycrystalline Ag₃Sn bulk samples were investigated experimentally. We found that the temperature dependence of resistivity shows concave function characteristics from 305 down to 26 K, and can be described by a parallel resistor model [H. Wiesmann et al., Phys. Rev. Lett. 38 (1977) 782]. The resistivities of all samples reveal T² behavior from 26 down to . We compared our data to the existing theories in which the T² dependence of resistivity has been proposed. However, we found none of them can describe the experimental data, and the physical mechanism of the T² behavior of resistivity at low temperatures is still enigmatical.     

Electronic structure and optical properties of wurtzite-kesterite Cu2ZnSnS4

As promising light-absorber material for solar cells, Cu₂ZnSnS₄ was found to have another crystal structure (wurtzite-kesterite) in addition to the conventional zinc blende-kesterite structure. Structural flexibility of Cu₂ZnSnS₄ opens up an avenue to develop light-absorber material with novel exciting properties and applications. However, its electronic and optical properties have not been comprehensively studied yet. For this purpose, the method of density functional theory within hybrid functional of PBE0 was adopted to study the structural, electronic, and optical properties of wurtzite-kesterite Cu₂ZnSnS₄ in this Letter. The calculated results suggested that the energy of its band gap is about 1.372 eV and it has obvious optical anisotropy. Furthermore, its crystal structure leads local internal fields that are especially beneficial to suppress the recombination of photoexcited electron–hole pairs.     

Electrical transport and magnetic properties of Nd2(WO4)3

Measurements of the molar magnetic susceptibility (X_m) of a powdered sample of Nd₂(WO₄)₃ in the temperature range 300–900 K, and the electrical conductivity (σ) and dielectric constant (?$\text{)}\text{?}$ of pressed pellets of the compound in the temperature range 4.2–1180 K are reported. X_m obeys the Curie-Weiss law with a Curie constant C= 3.13 K/mole, a paramagnetic Curie temperature θ= ?60 K and a moment of Bohr magnetons, p= 3.49 for the Nd³⁺ ion. The electrical conductivity data can be explained in terms of the usual band model and impurity levels. Both the σ and ?$\text{\$}\text{?}$data indicate some sort of phase transition round 1025 K. The conductivity follows Mott's law $\text{σ = A exp (?B/T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{)}$ in the temperature range $\text{200 < T < 3000}\text{K with}\text{B = 45.00 (}\text{K}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{and}\text{A = 1.38 × 10}{}^{\mathrm{?5}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$. The dielectric constant increases slowly up to 600 K, as is usual for ionic solids. The increase becomes much faster above 600 K, which is attributed to space-charge polarization of thermally generated charge carriers.     

Linear and nonlinear optical properties of Ca3(VO4)2

Single crystals of Ca₃ (VO₄)₂ have been grown by the Czochralski method. The refractive indices show that the crystals are phasematchable between 1.72 μm and 2.94 μm with favourable values for acceptance angle and spectral width of the incident laser beam in the entire phasematching range. The nonlinear optical coefficients have been measured by the Maker fringe method at a fundamental wavelength of 1.064 μm. They are comparable to those of KDP. Thus the crystals are potentially useful for frequency conversion of broadband or tunable laser sources as semiconductor and F-center lasers. They may also be installed in high f-number IR image converters.     

First-principles calculations of electronic, optical and elastic properties of ZnAl2S4 and ZnGa2O4

The optimized crystal structures, band structures, partial and total densities of states (DOS), dielectric functions, refractive indexes and elastic constants for ZnAl₂S₄ and ZnGa₂O₄ were calculated using the CASTEP module of Materials Studio package. Pressure effects were modeled by performing these calculations for different values of external hydrostatic pressure up to 50 GPa. Obtained dependencies of the unit cell volume on pressure were fitted by the Murnaghan equation of state, and the relative changes of different chemical bond lengths were approximated by quadratic functions of pressure. Variations of applied pressure were shown to produce considerable re-distribution of the electron densities around ions in both crystals, which is evidenced in different trends for the effective Mulliken charges of the constituting ions and changes of contour plots of the charge densities. The longitudinal and transverse sound velocities and Debye temperatures for both compounds were also estimated using the calculated elastic constants.     

Ab initio band structure calculations of the low-temperature phases of Ag2Se, Ag2Te and Ag3AuSe2

Ab initio band structure calculations were performed for the low-temperature modifications of the silver chalcogenides β-Ag₂Se, β-Ag₂Te and the ternary compound β-Ag₃AuSe₂ by the local spherical wave (LSW) method. Coordinates of the atoms of β-Ag₂Se and β-Ag₃AuSe₂ were obtained from refinements using X-ray powder data. The structures are characterized by three, four and five coordinations of silver by the chalcogen, a linear coordination of gold by Se, and by metal-metal distances only slightly larger than in the metals. The band structure calculations show that β-Ag₃AuSe₂ is a semiconductor, while β-Ag₂Se and β-Ag₂Te are semimetals with an overlap of about 0.1-0.2 eV. The Ag 4d and Au 5d states are strongly hybridized with the chalcogen p states all over the valence bands. β-Ag₂Se and β-Ag₂Te have a very low DOS in the energy range from about −0.1 to +0.5 eV. The calculated effective mass β-Ag₂Se is about 0.1-0.3 m_e for electrons and 0.75 m_e for holes, respectively.     

Phases and phase transitions of the superionic conductor Ag2HgI4 in the temperature range between 4.2 K and 370 K detected by diffuse reflectance spectrometry

Diffuse reflectance spectra of the superionic conductor Ag₂HgI₄ powder were recorded between 4.2 K and 370 K and transformed into the Kubelka-Munk function. Six parameters of the spectral band of this function, related to the absorption band of the particular phase, were considered. The graph representing the logarithm of the integrated Kubelka-Munk spectral band versus inverse temperature was most informative. The Arrhenius behaviour of this function, in a temperature interval, was related to the existence of a phase with constant lattice structure. The non-Arrhenius behaviour was tentatively attributed to a temperature sensitive phase. Thus fifteen different crystalline phases were found, only four of them having been, up to now, unquestionably admitted. In particular, evidence was given for the existence of two superionically conducting phases and two room temperature phases. All the graphs representing the chosen parameters versus inverse temperature pointed to the existence of two separate sequences of phases: one generated by cooling gradually the β-phase, stable at room temperature, to low temperatures (β sequence), the other by heating the β-phase above the σ → β (T)₊ transition point and cooling gradually the formed α'′-phase (α′ sequence). The concept of photon absorption thermal activation energy was introduced. Its largest value was of the order of magnitude of the electrical conductivity thermal activation energy of Ag₂HgI₄. Estimates of energy band gaps and their evolution with temperature were made. It is suggested that the polymorphism of the substance is the main reason for the hysteresis loop in the superionic conductivity transition region. The low temperature phases manifested luminescence caused by exciton decay.     

First-principles study on the electronic structure and optical properties of T12Cd2（SO4）3 and Rb2Cd2（SO4）3

With the help of ab initio full-potential linearized augmented plane wave （FPLAPW） method, calculating the electronic structure and linear optical properties is carried out for XCd2（SO4）3 （X =Tl, Rb）. The results show that Tl2Cd2（SO4）3 （TlCdS） has a larger band gap than Rb2Cd2（SO4）3 （RbCdS） and the energy bands for RbCdS are more dispersive than those of TlCdS. From their partial densities of states （PDOS）, we have observed that the hybridization between S ionic 2p and O atomic 2p orbitals forms SO4 ionic groups. The remarkable difference between RbCdS and TlCdS is, however, the degree of hybridization between cation （Tl and Rb） and its surrounding oxygen atoms. In the view of quantum chemistry, the strong p-d hybridization indicates the existence of their cation ionic bonds （Cd-O, Rb-O, and Tl-O）. The calculations of TlCdS and RbCdS show their optical properties to be less anisotropic. Their anisotropies in the optical properties mainly occur in a low photon energy region of 5-16 eV.     

Temperature dependence of magnetic properties of NiFe2O4 nanoparticles embeded in SiO2 matrix

Spinel ferrite NiFe₂O₄ nanoparticles (?25 nm) in SiO₂ matrix were prepared by sol–gel method. The phase and average crystallite size of the samples were determined by X-ray diffraction method and the particle size distributions were studied by a transmission electron microscope. Magnetic properties of the samples were investigated with different ferrite particle sizes and at various temperatures down to 10 K. Superparamagnetic properties were observed at room temperature when the particle size is less than 10 nm.In superparamagnetic state, the field dependence of magnetization follows Langevin function which was originally developed for paramagnetism. The effective anisotropy constant K_eff is found to increase significantly with the decrease in particle volume and an order of magnitude higher than that of the bulk samples when the particle size is below 5 nm due to the dominance of surface anisotropy. In case of nanosized systems, the effect of size reduction on the law of approach to saturation has also been studied in detail.