Superconductivity in the ternary borides CeOs3B2 and CeRu3B2: Magnetic susceptibility and specific heat measurements

The compounds CeOs₃B₂ and CeRu₃B₂ show superconductivity with T_c's of 3.5 and 1.1 K respectively. The magnetic susceptibility of both these compounds may be described as the sum of a Curie-Weiss term dominating at low temperature and a large temperature independent term. The heat capacity measurements yield the value of electronic specific heat coefficient γ of 40.1 and 15.6 mJ/mol K² for Os and Ru compounds, respectively. These compounds appear to be conventional bulk superconductors in which Ce is in a strongly mixed valent state.     

Crystal structure and magnetism of UFe3B2

The structure of the UFe₃B₂ compound has been refined down to R=0.022 and wR₂=0.052 from single crystal X-ray diffraction data. This uranium boride crystallizes in the CeCo₃B₂ type-structure (P6/mmm space group no. 191, Z=1, ρ=10.79 g/cm³), with lattice parameters at room temperature a=0.5052(1) nm, c=0.3002(1) nm and V=0.664(1) nm³. Magnetization measurements made between 2 K and 800 K suggested that UFe₃B₂ is an antiferromagnet with a rather high Néel temperature of T_N=268±5 K. No other magnetic transitions were observed down to the lowest studied temperature.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

Luminescent properties of SrAl2B2O7: Ce, Tb

By using metal nitrates as starting materials, SrAl₂B₂O₇: Tb³⁺ and SrAl₂B₂O₇: Ce³⁺, Tb³⁺ powder phosphors were prepared by sol-gel method. X-ray diffraction (XRD), photoluminescence excitation and emission, as well as kinetic decays were employed to characterize the resulting samples. The results show that energy transfers from Ce³⁺ to Tb³⁺ ions. The emission intensity of Tb³⁺ ions in SrAl₂B₂O₇ could be greatly intensified when Ce³⁺ ions are doped into SrAl₂B₂O₇: Tb³⁺. The decay times of SrAl₂B₂O₇: Tb³⁺ were prolonged when Ce³⁺ ions were doped. The doping of Ce³⁺ ions not only improved the luminescent intensity, but also made the materials gets stable luminescent properties.     

New red phosphors BaZr(BO3)2 and SrAl2B2O7 doped with Eu for PDP applications

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) characteristics of Eu³⁺ ion doped borate phosphors; BaZr(BO₃)₂:Eu³⁺ and SrAl₂B₂O₇:Eu³⁺ are studied. The excitation spectra show strong absorption in the VUV region with the absorption band edge at ca. 200 nm for BaZr(BO₃)₂:Eu³⁺ and 183 nm for SrAl₂B₂O₇:Eu³⁺, respectively, which ensures the efficient absorption of the Xe plasma emission lines. In BaZr(BO₃)₂:Eu³⁺, the charge transfer band of Eu³⁺ does not appear strongly in the excitation spectrum, which can be enhanced by co-doping Al³⁺ ion into the BaZr(BO₃)₂ lattices. The luminescence intensity of BaZr(BO₃)₂:Eu³⁺ is also increased by Al³⁺ incorporation into the lattices. The PL spectra show the strongest emission at 615 nm corresponding to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺ in both BaZr(BO₃)₂ and SrAl₂B₂O₇, similar to that in YAl₃(BO₃)₄, which results in a good color purity for display applications.     

On the ferromagnetism of AlFe2B2

Based on extensive Mossbauer effect (ME) and magnetization measurements, the orthorhombic AlFe₂B₂ was characterized as a ferromagnet (FM) because this character is evident as an onset of a FM transition at T_c=320 K and characteristic magnetizations isotherms below T_c. At liquid helium temperatures, the magnetization saturates to μ_sat≈1μ_B per Fe atom; a value which is half the one reported for the iron metal indicating a relatively more filled 3d band. The ME analysis revealed a hyperfine field H(0) of 88(2) kOe, an isomer shift (relative to Fe) of 0.50(2) mm/s, and a quadrupole parameter of 0.02 mm/s: all parameters extrapolated to zero Kelvin. The itinerant character of the magnetic moment will be discussed.     

Adsorption of NH3 and NO2 molecules on C48B6N6 heterofullerene: A DFT study on electronic properties

Adsorption of NH₃ and NO₂ molecules on the external surface of C₄₈B₆N₆ heterofullerene is investigated using DFT method. Attachment of NH₃ and NO₂ on C₄₈B₆N₆ heterofullerenes are compared with the bare C₄₈B₆N₆ model optimized at the B3LYP/6-31G^? level. The high surface binding energies indicates that ammonia undergoes chemical adsorption and could be compatible with the long recovery time but C₄₈B₆N₆ should be good NO₂ sensors with quick response as well as short recovery time. Total (TDOS) and partial (PDOS) density of state calculations is also considered to elucidate the difference in the NH₃ and NO₂ gas detection mechanism of C₄₈B₆N₆. The overlap population density of state (OPDOS) indicated that the chemical adsorption is due to the overlap of atomic orbitals below the Fermi level. The calculated results suggest that the C₄₈B₆N₆ heterofullerene is a suitable sensor material for NO₂ and is an ideal material for elimination and filtering of ammonia.     

Magnetic and crystallographic study of Tb0.75Y0.25Co3B2

Tb_0.75Y_0.25Co₃B₂ was studied as a function of temperature by neutron powder diffraction, ac susceptibility and SQUID magnetization measurements. The solid solution, which is of hexagonal symmetry and is paramagnetic at 300 K, undergoes a magnetic Co–Co ordering transition at ∼150 K, and a second magnetic Tb–Tb ordering transition at ∼17 K. The latter induces a spin-reorientation transition, in which the magnetic axis rotates from the c-axis toward the basal plane. The component of the magnetic axis, which is perpendicular to c, leads to a crystal symmetry reduction from hexagonal to monoclinic. The observed magnitude of the magnetic moment of the Tb ion is 1.5 μ_B, unusually small relative to the free ion and parent compound (TbCo₃B₂) values. These magnetic and crystal properties are discussed and compared with what was previously published for the parent compound.     

Directional frustration of magnetic moments in (U0.50Dy0.50)Ni2B2C

Polycrystalline (U_0.50Dy_0.50)Ni₂B₂C solid solution was prepared and found by X-ray diffraction to crystallize in BCT LuNi₂B₂C-type structure (space group I4/mmm) of the end compounds UNi₂B₂C and DyNi₂B₂C. AC susceptibility and magnetization show paramagnetic behavior down to 6.5 K, with the values θ=−5(5) K and μ_eff=7.7(1) μ_B, compatible with those of the end compounds, and indicate possible cooperative phenomena at lower temperatures. The observed paramagnetism, at variance with antiferromagnetic ordering in (Pr_0.50Dy_0.50)Ni₂B₂C, is attributed to a directional frustration of the magnetic moments on the (U,Dy) site.     

Luminescent properties of Eu-activated Sr3B2SiO8: A red-emitting phosphor for white light-emitting diodes

Single-phased Sr₃B₂SiO₈:Eu³⁺ phosphor was prepared by a solid-state method at 1020 °C. The luminescence spectra showed that Sr₃B₂SiO₈:Eu³⁺ phosphor can be effectively excited by near ultraviolet light (393 nm) and blue light (464 nm). When excited at 393 or 464 nm Sr₃B₂SiO₈:Eu³⁺ exhibited the main emission peaks at 611 and 620 nm, which resulted from the supersensitive ⁵D₀→⁷F₂ transition of Eu³⁺. The luminescence intensity of Sr₃B₂SiO₈:Eu³⁺ at 611 and 620 nm reached the maximum when the doping content of Eu³⁺ was 4.5 mol%. Its chromaticity coordinates (0.646, 0.354) were very close to the NTSC standard values (0.67, 0.33). Thus, Sr₃B₂SiO₈:Eu³⁺ is considered to be an efficient red-emitting phosphor for long-UV InGaN-based light-emitting diodes.     

A novel blue-emitting phosphor: BaAl2B2O7: Pb

Pb²⁺ doped BaAl₂B₂O₇ materials were prepared by a solution combustion synthesis. The phase of the synthesized materials was determined using the powder X-ray diffraction. The photoluminescent properties of Pb²⁺ doped BaAl₂B₂O₇ materials were investigated using spectrofluorometer at room temperature. The emission and excitation bands of BaAl₂B₂O₇: Pb²⁺ were observed at 423 and 266 nm, respectively. The dependence of the emission intensity on the Pb²⁺ concentration for BaAl₂B₂O₇: Pb²⁺ was investigated. The Stokes shifts of BaAl₂B₂O₇: Pb²⁺ was calculated to be 13 953 cm⁻¹.     

Magnetic and structural characteristics of the R3Ni7B2 compounds (R = rare earth)

Magnetic and structural behaviour and phase relationships of materials of composition R₃Ni₇B₂ (R = Nd-Lu) were investigated. Detailed X-ray analysis yields that two hexagonal structures are encountered. For the heavy rare earth (Gd-Lu) the compounds crystallize in the CeNi₃ structure. The space group is P6₃/mmc and each unit cell contains two formula units. The R₃Ni₇B₂ where R = Nd-Sm (including Yb₃Ni₇B₂) crystallize in the CeCo₄B structure. The space group is P6/mmm and each unit cell contains one formula unit. The detailed crystal structures are discussed. The magnetic measurements show that Yb₃Ni₇B₂ and Lu₃Ni₇B₂ are Pauli paramagnetic. Sm₃Ni₇B₂ is ferromagnetically ordered with a huge intrinsic magnetic hardness. The magnetization at the coercive field at low temperatures is extremely time dependent. The R₃Ni₇B₂ which crystallize in CeNi₃ structure are antiferromagnetic at low temperatures. All Mossbauer and magnetization experimental results can be explained assuming an antiferromagnetic exchange interaction in both 2(c) and 4(f) crystallographic sites and a ferromagnetic interaction between these sites.     

Electronic structure and magnetic properties of Gd3Co11B4 compound

The electronic structure of hexagonal Gd₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd₃Co₁₁B₄ compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μ_B/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd₃Co₁₁B₄ compound with those of Nd₃Co₁₁B₄ compound.     

Synthesis and characterization of thermoluminescent Li2B4O7 nanophosphor

Lithium borate (Li₂B₄O₇) is a low Z_eff, tissue equivalent material that is commonly used for medical dosimetry using the thermoluminescence (TL) technique. Nanocrystals of lithium borate were synthesized by the combustion method for the first time in the laboratory. TL characteristics of the synthesized material were studied and compared with those of commercially available microcrystalline Li₂B₄O₇. The optimum pre-irradiation annealing condition was found to be 300 °C for 10 min and that of post-irradiation annealing was 300 °C for 30 min. The synthesized Li₂B₄O₇ nanophosphor has very poor sensitivity for low doses of gamma up to 10¹ Gy whereas from 10¹ to 4.5×10² Gy this phosphor exhibits a linear response and then from 4.5×10² to 10³ Gy it shows supralinearity. Thermoluminescence properties of Li₂B₄O₇ nanophosphor doped with Cu has also been investigated in this paper. It shows low fading and a linear response over a wide range of gamma radiation from 1×10² to 5×10³ Gy. Therefore the synthesized lithium borate nanophosphor doped with Cu may be used for high dose measurements of gamma radiations.     

BaB2O4-K2O和BaB2O4-K2B2O4赝二元系相图的研究

用X射线衍射和差热分析方法研究了BaB₂O₄-K₂O和BaB₂O₄-K₂B₂O₄赝二元系的相平衡关系。BaB₂O₄-K₂B₂O₄属共晶体系,共晶温度为850±3℃,共晶点成分为45mol％K₂O。在Ba 关键词：     

Synthesis and luminescence properties of europium doped YBa3B9O18

Europium (Eu³⁺) doped YBa₃B₉O₁₈ were synthesized by conventional solid state solidification methods. (Y_1−xEu_x)Ba₃B₉O₁₈ formed solid solutions in the range of x=0–1.0. The luminescence property measurements upon excitation in ultraviolet–visible range show well-known Eu³⁺ excitation and emission. The charge transfer excitation band of Eu³⁺ dominates the excitation spectra. The emission spectrum of Eu³⁺ ions consists mainly of several groups of lines in the 550–720 nm region, due to the transitions from the ⁵D₀ level to the levels ⁷F_J (J=0, 1, 2, 3, 4) of Eu³⁺ ions. The dependence of luminescence intensity on Eu³⁺ concentration shows no concentration quenching for fully concentrated EuBa₃B₉O₁₈. Eu³⁺ doped YBa₃B₉O₁₈ are promising phosphors for applications in displays and optical devices.     

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：     

Superconductivity in Ba2/3Pt3B2 with the Kagome lattice

Resistivity, magnetization and low temperature specific heat (down to 0.4 K) of Ba_2/3Pt₃B₂ with the Kagome lattice of the transition metal Pt atoms have been investigated. The magnetization hysteresis loops measured on the sample indicate that it belongs to a type-II superconductor. The specific heat exhibits ΔC/γ_nT|_Tc≈1.7$\Delta C/{\gamma }_{n}T{|}_{T_c}\approx 1.7$, showing a moderate coupling superconductivity. Furthermore, the magnetic field dependence of the electronic specific heat coefficient in the low temperature limit demonstrates a linear relation γ_e∝H${\gamma }_e\propto H$, suggesting an s-wave gap. Surprisingly, the Wilson ratio determined here is about 34, which may be explained by short range magnetic correlation. Compared with the compound LaRu₃Si₂, we intend to attribute the extraordinarily large Wilson ratio to the deficiency of Ba atoms. Further theoretical and experimental efforts are required to clarify this issue.     

MgB2和Mg0.93Li0.07B2的电阻率与霍尔效应研究

测量了MgB₂和Mg_0.93Li_0.07B₂的电阻率ρ(T)与霍尔系数R_H(T)的温度依赖关系.电阻率的测量结果表明,MgB₂和Mg_0.93Li_0.07B₂的正常态电阻率与温度有平方的依赖关系.MgB₂和Mg_0.93Li_0.07B_{2关键词： 电阻率 霍尔效应}     

Magnetic properties of amorphous ferromagnet Fe78Mo2B20

Low-field magnetic susceptibility and the magnetic field dependence of magnetization of Metglas 2605 A (Fe₇₈Mo₂B₂₀) were studied between 300 and 600 K and in fields up to 10kG. It is shown here that for an amorphous ferromagnetic alloy, the various methods of determination of Curie temperature T_c give the same value, which in this case is (564 ± 1) K. The critical exponent γ is 1.7 ± 0.1. Our low-field susceptibility measurements on Metglas 2605 (Fe₈₀B₂₀) gives a T_c of (634 ± 3) K while the reported high-field method value is 647 K. These results are discussed in terms of crystallization temperatures.