Luminescence of Ce3+ in lanthanum silicon oxynitride

The luminescence properties of lanthanum silicon oxynitride(La-Si-O-N) series doped by trivalent Ce ions have been investigated to seek for tunable wavelength-conversion phosphor for white light emitting diode applications.Four compound hosts of LaSiO2N,La4Si2O7N2,La5Si3O12 N,and La2Si6O3N8 were synthesized and examined in this work.Crystallographic examination for the equal amount of Ce 3+ substitution indicated that the covalency degree decreased in a sequence LaSiO2 N > La2Si6O3N8 > La4Si2O7N2 > La5Si3O12 N,not simply in correlation to the ratio of N3/O2.Excitation and emission spectrum measurements showed the main features of Ce3+ luminescence in the series:the centre of gravity of 5d bands depends on crystal-field splitting more strongly than that on covalency of Ce-N bonding;nephelauxetic effect could not be observed clearly for the investigated series;to some extent Stokes shift was dominated by crystal-field splitting rather than Ce-N covalency degree.     

Ab initio study of the electronic structure and elastic properties of Al5C3N

We investigate the electronic structure, chemical bonding and elastic properties of the hexagonal aluminum carbonitride, Al₅C₃N, by ab initio calculations. Al₅C₃N is a semiconductor with a narrow indirect gap of 0.81 eV. The valence bands below the Fermi level (E_F) originate from the hybridized Al p-C p and Al p-N p states. The calculated bulk and Young's moduli are 201 GPa and 292 GPa, which are slightly lower than those of Ti₃SiC₂. The values of the bulk-to-shear-modulus and bulk-modulus-to-c44 are 1.73 and 1.97, respectively, which are higher than those of Ti₂AlC and Ti2AlN, indicating that Al₅C₃N is a ductile ceramic.     

Sr和Ba替代对Eu掺杂Ca2.955Si2O7的结构和发光特性的影响研究

利用固相反应法制备了Sr和Ba替代的Ca_2.955-xM_xSi₂O₇: 0.045Eu²⁺ (M= Sr, Ba, x= 0.1-0.5)系列荧光粉, 利用较大离子半径的Sr和Ba元素替代Eu掺杂Ca_2.955-xM_xSi₂O₇ 中的Ca元素,研究Sr和Ba替代对样品结构和发光特性的影响. X射线衍射测试结果表明,少量Sr和Ba替代不会改变基质的晶体结构, 样品仍然为单斜晶系.未替代前, Ca_2.955Si₂O₇: 0.045Eu²⁺ 样品的发射峰在574 nm左右,随着Sr含量的增加,样品的发射峰发生蓝移; 而Ba含量在x= 0.1-0.4时不会引起发射峰位置的移动, 但x= 0.5样品的发射峰发生蓝移.同等含量的Sr和Ba部分替代样品中的Ca元素, Ba替代样品的光谱强度较强.     

液晶胆甾相中的分子短程关联

向列相液晶的二粒子集团理论被推广应用于研究胆甾相二维模型.手征性分子固定在三维简单立方晶格的格点上,而分子取向限制在二维.理论结果表明,平衡态螺旋波矢依赖于温度的变化,且存在胆甾相到向列相相变.通过考虑分子间短程关联,二粒子集团理论的数值结果较平均场理论更接近Monte Carlo模拟结果.     

Room-Temperature Ferromagnetism in Semiconducting TiO2-δ Nanoparticles

TiO2-δ nanoparticles are synthesized by the sol-gel method and annealed under different reducing atmosphere. The x-ray diffraction patterns show that anatase is the dominant phase with small amounts of the futile phase of TiO2-δ for all the samples. Magnetic measurements indicate that the samples annealed in reducing atmosphere exhibit unprecedented room-temperature ferromagnetism, in particular, the saturation magnetization Ms is up to about 8.6 × 10^-3 emu/g for the sample annealed in H2/Ar mixture. Analysis of the x-ray photoelectron spectroscopy spectra for the samples processed under different conditions indicates that the amounts of Ti^3＋ or Ti^2＋ cations, namely, the concentration of oxygen vacancies, increase with intensifying reducing atmosphere during processing, which shows that ferromagnetism in this material strongly depends on the concentration of oxygen vacancies. The relationships between the ferromagnetism and the crystal structure as well as the grain size in this material are also discussed.     

Infrared Absorption of Phase Separated La0.67-xPrxCa0.33MnO3

Infrared absorption spectra of La0.67-xPrxCa0.33MnO3 （x= 0, 0.18 and 0.36） are experimentally studied in the temperature range 20 -300K. Absorption peak splitting corresponding to the stretching oscillation of the Mn-O bond, together with a shift of peak position, is observed below the Curie temperature. These features weaken and even disappear as the samples are warmed up to the Curie temperature, which indicates that this anomaly may be a result of phase separation in the compounds.