Optical Properties of Filled Skutterudite ThFe4P12

Full-Potential Linearized Augmented Plane Wave plus local orbital method (FPLAPW +lo) calculations are performed for ThFe₄P₁₂ in the filled skutterudite in order to investigate the optical properties and to show the origin of the different optical transitions. It is found that the band gap is indirect for ThFe₄P₁₂. Then the contributions of the different transition peaks are analyzed from the imaginary part of the dielectric function. In contrast to recent experimental expectations, our calculations are in good agreement with experimental reflection spectra and ε₁(ω) spectrum.     

Direct and indirect optical transitions in Zn3P2

Absorption measurements were made on single crystals of Zn₃P₂ at temperatures of 300, 80 and 5 K, and photo-voltage spectral responses-were measured at 300 K for Au- and In---Zn₃P₂ contacts. Interband absorption was interpreted as a process involving three mechanisms: (1) indirect transitions from the valence band at the Γ point, (2) either excitations from acceptor level to the conduction band at the Γ point, or second indirect transitions associated with the creation of excitons, and (3) band-to-band direct transitions at the Γ point. The effect of the lighting configuration on spectral PV plots is also discussed, and the origin of two peaks of PV responses is interpreted as being in accordance with optical data. The indirect energy gap has been estimated as 1.315eV at 300 K and 1.335 eV at 80 and 5 K, and the direct one as 1.505, 1.645 and 1.685 eV at 300, 80 and 5 K, respectively.     

Structural and optical properties of thermally evaporated Bi2Te3 films

Bi₂Te₃ films were prepared by thermal evaporation technique. X-ray diffraction analysis for as-deposited and annealed films in vacuum at 150 °C were polycrystalline with rhombohedral structure. The crystallite size is found to increase as the film thickness increases and has values in the range 67–162 nm. The optical constants (the refractive index, n, and absorption index, k) were determined using transmittance and reflectance data in the spectral range 2.5–10 μm for Bi₂Te₃ films with different thicknesses (25–99.5 nm). Both n and k are independent on the film thickness in the investigated range. It was also found that Bi₂Te₃ is a high refractive index material (n has values of 4.7–8.8 in the wavelength range 2.5–10 μm). The allowed optical transitions were found to be direct optical transitions with energy gap  eV. The optical conductivities σ₁ = ƒ(hν) and σ₂ = f(hν) show distinct peaks at about 0.13 and 0.3 eV, respectively. These two peaks can be attributed to optical interband transitions.     

On the energy transfer from Tb to Eu in LiTb1−xEuxP4O12

The undoped stoichiometric materials LiTbP₄O₁₂ and LiEuP₄O₁₂ show effective green Tb³⁺ and red Eu³⁺ luminescence, respectively. Both systems are fully mixable, which offers the possibility of examining the energy transfer from Tb³⁺ to Eu³⁺ in the complete system of LiTb_1−xEu_xP₄O₁₂. As the distance of the rare earth ions is large enough to exclude exchange interaction in these materials, only multipolar interaction is to be expected. Dealing only with forbidden f–f transitions it is possible to produce both Tb³⁺ and Eu³⁺ luminescence at the same time having a suitable Eu³⁺ concentration.     

Dielectric and modulus behavior of LiFe1/2Ni1/2VO4 ceramics

The polycrystalline sample of LiFe_1/2Ni_1/2VO₄ was prepared by a standard solid-state reaction technique and confirmed by X-ray diffractometry. LiFe_1/2Ni_1/2VO₄ has orthorhombic crystal structure whose dielectric and electric modulus properties were studied over a wide frequency range (100 Hz–1 MHz) at different temperatures (296–623 K) using a complex impedance spectroscopy (CIS) technique. The frequency and temperature dependence of dielectric constant (ε_r) and tangent loss (tan δ) of LiFe_1/2Ni_1/2VO₄ are studied. The variation of ε_r as a function frequency at different temperatures exhibits a dispersive behavior at low frequencies. The variation of the ε_r as a function of temperature at different frequencies shows the dielectric anomaly in ε_r at 498 K with maximum value of dielectric constant 274.49 and 96.86 at 100 kHz and 1 MHz, respectively. Modulus analysis was carried out to understand the mechanism of the electrical transport process, which indicates the non-exponential type of conductivity relaxation in the material. The activation energy calculated from electric modulus spectra is 0.38 eV.     

Evidence for pinning by (Sr,Ca)2CuOy particles in partial-melting processed bulk Bi2Sr2CaCu2O8+δ ceramics

Flux distributions of partial-melting processed Bi₂Sr₂CaCu₂O_8+δ ceramics are obtained using magneto-optic imaging. In remanent states (μ₀H_a=0 T), large amounts of trapped flux are observed along (Sr,Ca)₂CuO_y particles embedded in the Bi₂Sr₂CaCu₂O_8+δ matrix. Despite the relatively large size of these particles (up to 30 μm), the pinning effect is similar to that of Y₂BaCuO₅ particles in melt-processed YBa₂Cu₃O_7−δ. Furthermore, we discuss how the pinning capability of non-superconducting particles of different sizes and densities will show up in magneto-optic images.     

Material design of new lithium ionic conductor, thio-LISICON, in the Li2S–P2S5 system

A new lithium ionic conductor of the thio-LISICON (LIthium SuperIonic CONductor) family was found in the binary Li₂S–P₂S₅ system; the new solid solution with the composition range 0.0≤x≤0.27 in Li_3+5xP_1−xS₄ was synthesized at 700 °C and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x=0.065 in Li_3+5xP_1−xS₄ showed the highest conductivity value of 1.5×10⁻⁴ S cm⁻¹ at 27 °C with negligible electronic conductivity and the activation energy of 22 kJ mol⁻¹ which is characteristic of high ionic conduction state. The extra lithium ions in Li₃PS₄ created by partial substitution of P⁵⁺ for Li⁺ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li₃PS₄ (x=0.0 in Li_3+5xP_1−xS₄) and Li₄P_0.8S₄ (x=0.2 in Li_3+5xP_1−xS₄); this conductivity behavior is similar to other thio-LISICON family with the general formula, Li_xM_1−yM_y′S₄ (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conduction behavior and the structure considerations.     

间隙原子H,B,C对LaFe_(11.5)Al_(1.5)化合物磁性和磁热效应的影响

研究了金属化合物LaFe_(11.5)Al_(1.5)H_x(x=0,0.12,0.6,1.3),LaFe_(11.5)Al_(1.5)B_y(y=0.1,0.2,0.3)和LaFe_(11.5)Al_(1.5)C_z(z=0.1,0.2,0.3,0.4,0.5)的磁性、结构和磁热效应.金属化合物样品均形成了良好的NaZn_(13)型单相结构.基于固相-气相反应或者固相-固相反应引入间隙H,B,C原子后,磁性基态从反铁磁态变为铁磁态,饱和磁化强度(M_s)和居里温度(T_c)均呈升高趋势.值得注意的是:随着B和C含量的增加,化合物的相变性质由弱一级相变过渡至二级相变;而随着H含量的增加,相变性质却从二级相变过渡至弱一级相变.同时,化合物LaFe_(11.5)Al_(1.5)H_x,LaFe_(11.5)Al_(1.5)B_y和LaFe_(11.5)Al_(1.5)C_z均呈现出相当大的磁熵变.在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(1.3),LaFe_(11.5)Al_(1.5)B_(0.1)和LaFe_(11.5)Al_(1.5)C_(0.2)的最大磁熵变分别达到12.3,9.6和10.8 J/kg·K.此外,在0—5 T的外磁场作用下,LaFe_(11.5)Al_(1.5)H_(0.6)的制冷能力达到259.2 J/kg,LaFe_(11.5)Al_(1.5)B_(0.1)的制冷能力达到116.4 J/kg,而LaFe_(11.5)Al_(1.5)C_(0.1)的制冷能力达到230.4 J/kg.     

Magnetic properties of YCo5 (70%wt)+Y2Co17 (30%wt) nanocomposite powders at low temperatures

Nanostructured YCo₅ (70%wt)+Y₂Co₁₇ (30%wt) composite powders were prepared by mechanical milling and subsequent annealing at 1073 K for 1.5 min. The average grain size D of the YCo₅ and Y₂Co₁₇ phases, obtained from XRD data, was 14 and 12 nm, respectively. The temperature dependence of the magnetic properties was studied by DC magnetization measurements at temperatures T ranging from 3 to 300 K. Hysteresis loops (H_max=70 kOe) show that both the coercivity H_C and the squareness σ_r/σ_max are temperature-dependent. The coercivity increases from 12 kOe at room temperature to 18 kOe at T=3 K. The observed enhanced remanence (σ_r/σ_max>0.5) indicates that a strong exchange coupling is present at all temperatures used in this study. The maximum magnetization σ_max changes little with temperature and has a value of about 70% of the effective saturation magnetization of the title compound.     

Surface termination, composition and reconstruction of Fe3O4(001) and γ-Fe2O3(001)

We address a current controversy concerning the nature of the surfaces of Fe₃O₄(001) and γ-Fe₂O₃(001) grown on MgO(001) by molecular beam epitaxy. Despite recent claims to the contrary, we show that γ-Fe₂O₃(001) unambiguously exhibits a (1×1) surface net, in contrast to Fe₃O₄(001), which assumes a R45 reconstruction. In addition, we present high-energy-resolution Fe 2p and O 1s core-level photoelectron spectra obtained at both normal and grazing emission for γ-Fe₂O₃(001) and Fe₃O₄(001). These spectra show that the Fe₃O₄(001) surface has a higher Fe(III)/Fe(II) ratio than the bulk, and that the asymmetry in the O 1s line shape for Fe₃O₄(001) is due to final state effects rather than the presence of a surface oxygen or hydroxyl species.     

Crystal structure and phase transitions of [(C2H5)4N]6Bi8Cl30

A new [(C₂H₅)₄N]₆Bi₈Cl₃₀ crystal of the family of alkylammonium halogenobismuthates was grown. X-ray diffraction studies showed that the crystals are monoclinic, space group C2/m with a = 20.117(5), b = 12.682(3), c = 20.396(5) Å, β = 93.03(3), Z =2. The lattice consists of (C₂H₅)₄N⁺ cations and a new type of Bi₈Cl⁶⁻₃₀ anion. Dielectric studies revealed two closely-lying structural phase transitions around 241 K (on cooling). They were interpreted as due to a freezing of the rotational motions of tetraethylammonium cations.     

Ab initio study of structural, electronic and optical properties of MnHg(SCN)4 and FeHg(SCN)4

 The structural, electronic and optical properties of MnHg(SCN)₄ and FeHg(SCN)₄ were studied by means of quantum-mechanical calculations based on the density-functional theory and pseudopotential method. The lattice constants can be compared with the experimental values when the effects of temperature are considered. The peaks of partial density of states of S, C, N and Hg of FeHg(SCN)₄ have a tendency of shifting to the higher energy levels relative to those of MnHg(SCN)₄. The distributions of the 3d electronic states in the transition metal atoms show quite large difference and decide different optical properties. We found that absorptional peaks of FeHg(SCN)₄ lag behind those of MnHg(SCN)₄ and the peak in the infrared range has a higher absorptional intensity, which are in accord with the experimental results. By analyzing the distributions and transitions of the 3d electronic states, we explained the different absorption phenomena.     

Shaped crystal growth and scintillating properties of Yb:(Gd,Lu)3Ga5O12 solid solutions

Shaped single crystals of (Lu_xGd_1−x)₃Ga₅O₁₂ (0.0x1.0) and (Yb_0.05Lu_xGd_0.95−x)₃Ga₅O₁₂ (0.0x0.9) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃Ga₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect-host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Er3+/Yb3+共掺NaYF4/LiYF4微米晶体的上转换荧光特性

采用水热法成功制备了Er³⁺/Yb³⁺共掺杂的NaYF₄和LiYF₄微米晶体. 通过X射线衍射仪和环境扫描电子显微镜对样品的晶体结构及形貌进行表征. 实验结果表明: 六方相NaYF₄微米晶体为棒状结构, 而四方相LiYF₄微米晶体则为八面体结构. 在近红外光980 nm激发下, NaYF₄:Yb³⁺/Er³⁺和LiYF₄:Yb³⁺/Er³⁺ 微米晶体均展现出很强上转换荧光发射. 且NaYF₄:Yb³⁺/Er³⁺微米晶体的荧光发射强度大约是LiYF₄:Yb³⁺/Er³⁺微米晶体的2倍, 但红绿比明显较低. 根据荧光光谱, 并借助激光光谱学及发光动力学深入探讨基质变化及表面修饰剂乙二胺四乙二酸(EDTA)对荧光特性的影响. 实验结果发现: 影响荧光强度的主要因素是基质环境的局域对称性, 而导致不同红绿比则是由于样品表面较多的EDTA分子所引起. Er³⁺掺杂的NaYF₄和LiYF₄ 微米晶体呈现出很强的绿光发射可被应用于全色显示, 荧光粉和微光电子器件中.     

Optical constants of Tl4Ga3InSe8 layered single crystals

The optical properties of Tl₄Ga₃InSe₈ layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 500–1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 1.94 and 2.20 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K revealed that the rate of change of the indirect band gap with temperature is γ=−4.1×10⁻⁴ eV/K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.03 eV. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.10 eV, 23.17 eV, 6.21×10¹³ m⁻² and 2.58, respectively. From X-ray powder diffraction study, the parameters of monoclinic unit cell were determined.     

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH2CH2CH2)3N)]Sb4S7

The chromium(II) antimony(III) sulphide, [Cr((NH₂CH₂CH₂)₃N)]Sb₄S₇, was synthesised under solvothermal conditions from the reaction of Sb₂S₃, Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction, elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2₁/n with a=7.9756(7), b=10.5191(9), c=25.880(2) Å and β=90.864(5)°. Alternating SbS₃³⁻ trigonal pyramids and Sb₃S₆³⁻ semi-cubes generate Sb₄S₇²⁻ chains which are directly bonded to Cr(tren)²⁺ pendant units. The effective magnetic moment of 4.94(6)μ_B shows a negligible orbital contribution, in agreement with expectations for Cr(II):d⁴ in a ⁵A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides.     

Magnetic phase transition in (Fe,Mn)65Ni35 alloys

Magnetization measurements on the Fe₆₀Mn₅Ni₃₅ and Fe₅₀Mn₁₅Ni₃₅ alloy samples were carried out in the temperature range 80T300 K and in magnetic fields up to 8 kOe. The Fe₆₀Mn₅Ni₃₅ was found to order ferromagnetically with a Curie temperature, T_c, above 300 K. From the temperature dependence of the spontaneous magnetization, M_s, it was concluded that the magnetic behavior of Fe₆₀Mn₅Ni₃₅ follows Wohlfarth theory of weak itinerant ferromagnet. The Fe₅₀Mn₁₅Ni₃₅ sample exhibits a magnetic phase transition from ferromagnetism to paramagnetism at T_c=242 K. The critical amplitudes and critical exponents (β, γ and δ) have been determined by using Arrott plots, Kouvel–Fisher method and scaling plots of the reduced magnetization and reduced magnetic field. The values of β, γ and δ are discussed and compared with the results obtained for various theoretical models and also with the experimentally determined values for related systems obtained by others.     

Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromagnetic insulating (FMI) Ln_0.85Ca_0.15MnO₃ (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θ_D/2, θ_D being the Debye temperature) dc conductivity (σ_dc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σ_dc data at low temperature (T<θ_D/2).     

Effect of a magnetic field on MnAs0.88P0.12 below 80 K

MnAs_0.88P_0.12 has been studied by powder neutron diffraction in external magnetic fields up to 15.2 kOe and temperatures down to 4.2 K. MnAs_0.88P_0.12 takes the MnP type atomic arrangement and exists in para-, ferro- and different (essentially) helimagnetic states. The observation of a double 000^± satellite at 4.2 K < T 70 K adds further evidence to the chain of arguments for distinction between the helimagnetic states H'_a (4.2 K < T < T_S,1 ≈ 70 K) and H_a (T_S,2 ≈ 180 K < T < T_N = (243 ± 5) K). External magnetic fields at 4.2 K < T < 70 K evoke a new magnetic state, which is also characterized by a satellite doublet, and is tentatively designated H'_a,fan.     

UV-induced two-photon absorption in BiB3O6 single crystals

We show that BiB₃O₆ (BiBO) crystals, well known for their excellent second harmonic generation (SHG) properties, may also be of interest for third-order optical phenomena, particularly for two-photon absorption (TPA). Photoinduced TPA measurements were performed under illumination of excimer Xe–F laser (λ = 217 nm) as a photoinducing (pumping) beam. It created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for a polarization of the pumping light directed along crystallographic axis b. Quantum chemical simulations indicate on substantial contribution of UV-induced electron–phonon anharmonicity to the observed TPA. The obtained values of TPA coefficients indicate a possibility of using BiBO crystals as UV-operated optical limiters in a wide spectral range.