Temperature-dependent Mössbauer and dielectric studies of Mg0.95Mn0.05Fe1.0Ti1.0O4

The effect of tetravalent Ti⁺⁴ substitution in Mg_0.95Mn_0.05Fe₂O₄ on its magnetic and electrical properties has been studied using X-ray diffraction, Mössbauer spectroscopy, isothermal dc magnetization and dielectric measurements. X-ray diffraction studies have shown the structural transformation from cubic to tetragonal with the Ti⁺⁴ substitution. The Mössbauer spectra of Mg_0.95Mn_0.05Fe_1.0Ti_1.0O₄ recorded in the temperature range 20-300 K shows the presence of the magnetic as well as quadrupole interactions. The isothermal hysteresis loop infers that the system exhibits a ferrimagnetic ordering at room temperature. The Zero-field-cooled (ZFC) and field-cooled (FC) magnetization studies support ferrimagnetic ordering of Mg_0.95Mn_0.05Fe_1.0Ti_1.0O₄ at room temperature. Signatures of ferroelectric transition have been observed in the temperature range 200-300 K from dielectric measurements. The observed magnetic and dielectric behaviour indicate that this material exhibits multiferroic behaviour.     

Li2B4O7晶体的晶格振动光谱

在室温下测量了Li₂B₄O₇单晶的各种振动类的偏振Raman散射谱和该晶体粉末样品的红外吸收谱(200—4000cm^-1)。根据LO-TO劈裂的实验结果,计算出该晶体极化模的有效电荷和振子强度。通过与BBO和LBO晶体的结构和B—O伸缩振动模频率比较,得出:Li₂B₄O₇晶体可能有较大的非线性光学系数。 关键词：     

In0.53Ga0.47As/In0.52Al0.48As量子阱中的正磁电阻效应

在低温强磁场条件下,对In_0.53Ga_0.47As/In_0.52Al_0.48As量子阱中的二维电子气进行了磁输运测试.在低磁场范围内观察到正磁电阻效应,在高磁场下这一正磁电阻趋于饱和,分析表明这一现象与二维电子气中的电子占据两个子带有关.在考虑了两个子带之间的散射效应后,通过分析低磁场下的正磁电阻,得到了每个子带电子的迁移率,结果表明第二子带电子的迁移率高于第一子带电子的迁移率.进一步分析表明,这主要是由两个子带之间的 关键词： 二维电子气 正磁电阻 子带散射     

Lattice dynamics of ND4I and NH4I crystals

The phonon dispersion curves and one phonon density of states for ND₄I and NH₄I crystals in the sodium chloride structure have been calculated by making use of the deformation dipole model of Hardy incorporating general short range forces out to second neighbours. Since the electronic configuration of both the compounds is identical, we have used the input data of NH₄I to calculate the model parameters for both the crystals. The results have been compared in the three principal symmetry directions with the neutron scattering measurements available only in case of ND₄I. The theoretical results show a reasonably good agreement with the experimental data.     

Li4SiO4-Li3VO4赝二元系和Li4GeO4-Li4SiO4-Li3VO4赝三元系中新的锂离子导体

本文在室温到300℃的温度范围内研究了Li₄SiO₄-Li₃VO₄和Li₄GeO₄-Li₄SiO₄-Li₃VO₄体系中的离子导电性,发现γ_II相固溶体Li_3+xV_1-xSi_xO₄是好的锂离子导体。所研究的成分中Li_3.3V_0.7Si_0.3O₄的离子电导率最高,室温下为1×10^-5Ω^-1·cm^-1,在42—192℃的电导激活能为0.36eV,电子电导率可以忽略,因而这是迄今所发现的最好的锂离子导体之一。粗略确定了Li₄GeO₄-Li₄SiO₄-Li₃VO₄三元系中电导率高的范围,发现在Li_3.5V_0.5Ge_0.5O₄中Si部分取代Ge可以使电导率进一步提高,Li_3.5V_0.5Ge_0.4Si_0.1O₄的室温电导率可达1.3×10^-5Ω^-1·cm^-1,电导激活能为0.40eV。 关键词：     

Observation of electromagnetic resonances in crystals of K2Pt (CN)4 Br0.3.3H2O at microwave frequencies

Electromagnetic dielectric resonances in the range 8–11 GHz have been observed in millimeter sized crystals of K₂Pt(CN)₄Br_0.3.3H₂O. This observation results directly from the existence of large dielectric constants; values for the longitudinal and transverse dielectric constants at 4°K are estimated to be ?6 >～ 3000 and ?⊥ ?4.     

Electron spin lifetimes in Hg0.78Cd0.22 Te and InSb

We have made direct pump–probe measurements of spin lifetimes in long wavelength narrow-gap semiconductors at wavelengths between 4 and 10 μm and from 4 to 300 K. In particular, we measure remarkably long spin lifetimes, τ_s300 ps, even at 300 K for epilayers of degenerate n-type InSb. In this material the mobility is approximately constant between 77 and 300 K, and we find that τ_s is approximately constant in this temperature range. In order to determine the dominant spin relaxation mechanism we have investigated the temperature dependence of τ_s in non-degenerate lightly n-type Hg_0.78Cd_0.22Te of approximately the same band gap as InSb, and find that τ_s varies from 356 ps at 150 K to 24 ps at 300 K. Our results, both in magnitude and temperature dependence of τ_s, imply that the Elliott–Yafet model dominates in these materials.     

High temperature phase transition in NH4H2PO4

NH₄H₂PO₄ (ADP) has been investigated by infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and d.c. conductivity in the temperature range of 25–180° C. Sharp reversible changes were observed in the region from 400 to 500 cm^?1 of the infrared spectra in the temperature range of 138–174° C. Similar and supportive data were obtained with DSC, TGA and DC conductivity measurements. The results clearly suggest a high temperature phase transition for ADP before its melting point.     

New Li+ ion conductors in the system Li4SiO4−Li3AsO4

In the system Li₄SiO₄-Li₃AsO₄, Li₄SiO₄ forms a short range of solid solutions containing up to 14 to 20% Li₃AsO ₄, depending on temperature, and γ-Li₃AsO₄ forms a more extensive range of solid solutions containing up to ≈55% Li₄SiO₄. The Li₄SiO₄-Li₃AsO₄ phase diagram has been determined and is of binary eutectic character. The ac conductivity of polycrystalline samples was measured over the range 0 to at least 300°C for nine different compositions. The two solid solution series have much higher conductivity than the pure end-members; maximum conductivity was observed in the γ-Li₃AsO₄ solid solutions containing ≈40 to 55% Li₄SiO₄, with values of $\text{≈2×10}{}^{\mathrm{?6}}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 20°C rising to ≈0.02 Ω^?1 cm^?1 at 300°C. These values are comparable to those found in the system Li₄SiO₄-Li₃PO₄. The variation with composition of the Arrhenius prefactor and activation energy has been interpreted in terms of the mechanisms of conduction. Li₃AsO₄ is a poor conductor essentially because the number of mobile Li⁺ ions is very small. This number, and hence the conductivity, increases dramatically on forming solid solutions with Li₄SiO₄, by the creation of interstitial Li⁺ ions. At ≈40 to 55% Li₄SiO₄, the number of mobile Li⁺ ions appears to be optimised. An explanation for the change in activation energy of conduction at ≈290°C in Li₄SiO₄ and at higher temperatures in Li₄SiO₄ solid solutions is given in terms of order-disorder of the Li⁺ ions.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

Static and dynamic magnetic characteristics of BaCo0.5Mn0.5Ti1.0Fe10O19

The effect of Mn²⁺Co²⁺Ti⁴⁺ substitution on microwave absorption has been studied for BaCo_0.5Mn_0.5Ti_1.0Fe₁₀O₁₉ ferrite-acrylic resin composites in frequency range from 12 to 20 GHz. X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer, AC susceptometer and vector network analyzer were used to analyze the structural, magnetic and microwave absorption properties. The results showed that the magnetoplumbite structures for all samples have been formed. Based on microwave measurement on reflectivity, BaCo_0.5Mn_0.5Ti_1.0Fe₁₀O₁₉ may be a good candidate for electromagnetic compatibility and other practical applications at high frequency.     

Microstructural changes in Fe-doped Gd5Si2Ge2

Gd₅Si₂Ge₂-based alloys can exhibit a giant magnetocaloric effect (GMCE); this gives them the potential for use in cooling technologies. It has also been reported that a small addition of iron reduces the hysteresis losses in Gd₅Si₂Ge₂-based alloys, thus increasing the net refrigerating capacity. In this investigation, we have been the first to look at the effect on the microstructure and magnetic properties of Gd₅Si₂Ge₂ resulting from a wide range of substitutions of Si by Fe. The macrostructures of the arc-melted buttons revealed some very unusual surface morphologies, and the analytical results revealed a gradual substitution of the Gd₅(Si,Ge,Fe)₄-type phase by a Gd₅(Si,Ge,Fe)₃-type phase and the presence of three grain-boundary phases, two of which contain substantial amounts of iron. The magnetic measurements indicated that larger amounts of iron reduced the hysteresis losses, but at the same time reduced the Curie temperatures to below lower values that would make the material useful in practice.     

添加Nb对CaCu3Ti4O12陶瓷介电性能的影响

采用固相反应法制备了CaCu₃Ti_4-xNb_xO₁₂(x=0，0.01，0.04，0.08，0.2)陶瓷，样品在x取值范围内形成了连续固溶体.在40Hz—110MHz频率范围对样品进行了介电频谱分析，实验结果表明，与纯CaCu₃Ti₄O₁₂不同，含Nb试样除了在频率大于10kHz范围内出现的德拜弛豫 关键词： 巨介电常数 德拜弛豫 阻挡层电容 等效电路     

Laser-induced voltage effects in Ca3Co4O9 thin films on tilted LaAlO3（001） substrates grown by chemical solution deposition

Laser-induced voltage effects in c-axis oriented Ca3Co4O9 thin films have been studied with samples fabricated on 10 tilted LaAlO3(001) substrates by a simple chemical solution deposition method. An open-circuit voltage with a rise time of about 10 ns and full width at half maximum of about 28 ns is detected when the film surface is irradiated by a 308-nm laser pulse with a duration of 25 ns. Besides, open-circuit voltage signals are also observed when the film surface is irradiated separately by the laser pulses of 532 nm and 1064 nm. The results indicate that Ca3Co4O9 thin films have a great potential application in the wide range photodetctor from the ultraviolet to near infrared regions.     

Preparation and characterization of co-doped (Ce0.80La0.15Al0.05O1.90) and multiple-doped (Ce0.80Sm0.10Gd0.05Al0.05O1.90 and Ce0.80Gd0.10Sm0.05Al0.05O1.90) ceria

Attempts have been made to synthesize a few compositions Ce_0.80La_0.15Al_0.05O_1.90, Ce_0.80Sm_0.10Gd_0.05Al_0.05O_1.90, and Ce_0.80Gd_0.10Sm_0.05Al_0.05O_1.90 by citrate–nitrate auto-combustion method. The aim of the present investigation was to study the effect of co-doping and multiple doping on the ionic conductivity of CeO₂ for its use as solid electrolyte in intermediate temperature solid oxide fuel cells. XRD patterns showed that all the samples have fluorite-type crystal structure similar to undoped ceria. Microstructures of thermally etched samples have been studied by scanning electron microscopy. Contributions of grains σ _g and grain boundaries σ _gb to the total conductivity σ _T, have been determined using impedance analysis. Impedance measurements were made in the frequency range 1 Hz–1 MHz and temperature range 250–500 °C. Our experimental results show that multiple doping is more effective than co-doping for improving the oxide ion conductivity of ceria in the materials investigated in the present study.     

Semiconducting properties and band structure of polycrystalline Bi4Mo20O62 compounds

Transport coefficient measurements (electrical conductivity, thermoelectric power and Hall effect) have been performed on compacted bars of Bi₄Mo₂₀O₆₂ in polycrystalline form over the temperature range (130–500 K) Experimental results are discussed in comparison to those obtained recently on Sb₄Mo₂₀O₆₂ They are interpreted on the basis of the same _p-type semiconductor model with two inverted deep levels near the midgap Conduction mechanisms are governed by acoustical-phonon scattering of the carrriers The top of the valence band is assumed to be formed from the d_xy orbitais of some of the Mo atoms leading to narrow bonding bands, while the donor and acceptor levels may be formed from the nonbonding d_xy orbitais of some of the Mo atoms of the distorted octahedron framework EPR measurements are discussed on the basis of this model and in comparison to the EPR results for Sb₄MO₂₀O₆₂ It is assumed that more than one narrow energy level (localized d states located below the valence band) contributes to the area of the line The XPS spectrum obtained on Bi₄Mo₂₀O₆₂ is also discussed.     

Charge ordering and structural transitions in Pr0.5Sr0.41Ca0.09MnO3

The structural and magnetic transitions in Pr_0.5Sr_0.41Ca_0.09MnO₃ have been investigated by neutron diffraction and electron microscopy. Two structural transitions, Imma to I4/mcm and I4/mcm to Pmmn, are observed by decreasing the temperature. Two magnetic transitions, from a paramagnetic insulating to a ferromagnetic metallic and from a ferromagnetic metallic to an antiferromagnetic insulating states at T_C=250 K and T_N=180 K, respectively, are also observed. The structures of these three forms have been determined from neutron powder diffraction data. The first important result concerns the low temperature antiferromagnetic CE type and charge ordered structure, which has been refined in the Pmmn space group, without any constraint. This structure is completely long range ordered, with two Mn-sites, Mn³⁺ in tetragonally elongated octahedra, and Mn⁴⁺, off-centered in nearly regular octahedra. The second important point concerns the abrupt character of the structural transition from the I4/mcm to the Pmmn structure, without any appearance of incommensurability. The magnetic and transport properties of this compound are compared with those of Pr_0.5Sr_0.5MnO₃.     

Heat capacity and phase transitions in NH4LiSO4, Csx(NH4)1?xLiSO4, and RbLiSO4

The heat capacity of NH₄LiSO₄, RbLiSO₄ and Cs_x(NH₄)_1−x LiSO₄ crystals and its behavior over a broad temperature range including the phase transition regions were studied. The entropy changes corresponding to structural transformations in these crystals were found not to be characteristic of straightforward ordering of structural blocks. The results obtained are discussed in terms of phenomenological theory and model concepts. __________ Translated from Fizika Tverdogo Tela, Vol. 47, No. 4, 2005, pp. 696–704. Original Russian Text Copyright ? 2005 by Flerov, Kartashev, Grankina.     

Zero field splittings of Gd3+IN Nd2(SO4)3 · 8H2O and Sm2(SO4)3 · 8H2O crystals at 273 K

The electron paramagnetic resonance (EPR) of Nd₂(SO₄)₃ · 8H₂O and Sm₂(SO₄)₃ · 8H₂O doped with Gd³⁺ has been carried out at 273 K and the spin-Hamiltonian parameters are deduced. The zero field splittings have been computed and compared with those observed directly by Bogle and Symmons. It is found that the discrepancy in the zero field splittings. between computed and directly observed values falls within the range of linewidths of directly observed values.     

Magnetic and transport properties of Nd1.67Sr0.33NiO4

Nd_1.67Sr_0.33NiO₄ polycrystals have been prepared by modified sol–gel method and subsequent annealing. X-ray diffraction analysis, electrical resistivity, magnetic susceptibility and thermal magnetisation have been measured. Rietveld analyses show a tetragonal or pseudo-tetragonal K₂NiF₄-type structure. The resistivity measurements present a change in conduction mode close to 230 K, which corresponds to the charge ordering temperature. Below this temperature, the material adopts a variable range hopping conduction mode; and above, the conduction follows adiabatic thermal activated mode. The magnetic measurements show paramagnetic behaviour in the range of 80–300 K. Moreover, the magnetic susceptibility data show a sign of the charge ordering transition about 230 K.