Sodium-23 MAS NMR study on nuclear quadrupole interaction in NA(1-x)Ag(x)NO2.

Ag-impurity effects on the first- and second-order quadrupole interaction (QI) at ²³Na site in an isomorphic mixed system, Na_1−xAg_xNO₂ (x=0, 0.0084, 0.026, 0.079, 0.094, 0.16), have been investigated by employing ²³Na (I=3/2) magic angle spinning nuclear magnetic resonance (MAS NMR) technique. The central transition (CT) and satellite transition (ST) are simultaneously observed with this system. From the spectral analysis, the quadrupole parameter and its distribution width are obtained as a function of Ag concentration. From the intensity loss of CT MAS centerband and of the envelope function of ST MAS sidebands due to impurities, the range of their influence on the second- and first-order QI is estimated. The estimated ranges contain the second and first neighbouring Na sites from the resonating ²³Na nucleus for the first- and second-order QI, respectively.     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

NiS2-xSex在x=1.00附近的反铁磁量子相变

针对NiS_2-xSe_x系统在x=1.0０附近发生的反铁磁量子相变,制备了一系列NiS_2-xSe_x(x=0.96, 0.98, 1.00, 1.05, 1.10和1.20)多晶样品,对其结构、磁性质和电阻率进行了系统的观测.结果发现：样品磁化率-温度关系呈现典型的强关联电子系统特征;与铜氧化物超导体相类似,它们的电阻率-温度关系在很宽的温 关键词： 量子相变 反铁磁自旋涨落 2-xSe_x体系')" href="#">NiS_2-xSe_x体系     

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Mixed ion effects in Na/Ag-NaZr2(PO4)3

The ionic conductivity against the ionic ratio showed a shallow minimum in Na/Ag-NaZr₂(PO₄)₃ with a three-dimensional diffusion path. The ionic conductivity of the mixed ion system Na/Ag was calculated by means of the path probability method (PPM) using a two-dimensional honeycomb lattice model, resulting from the weak interaction between two different ions.²³Na MAS NMR spectra showed the second-order quadrupole interaction with the electric field gradient. It is concluded that the mixture of two ions in NaZr₂(PO₄)₃ with a three-dimensional diffusion path led to the conductivity minimum due to the ordered distribution of Ag and Na ions. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

19F-decoupling of half-integer spin quadrupolar nuclei in solid-state NMR: application of frequency-swept decoupling methods

In solid-state NMR studies of minerals and ion conductors, quadrupolar nuclei like ⁷Li, ²³Na or ¹³³Cs are frequently situated in close proximity to fluorine, so that application of ¹⁹F decoupling is beneficial for spectral resolution. Here, we compare the decoupling efficiency of various multi-pulse decoupling sequences by acquiring ¹⁹F-decoupled ²³Na-NMR spectra of cryolite (Na₃AlF₆). Whereas the MAS spectrum is only marginally affected by application of ¹⁹F decoupling, the 3Q-filtered ²³Na signal is very sensitive to it, as the de-phasing caused by the dipolar interaction between sodium and fluorine is three-fold magnified. Experimentally, we find that at moderate MAS speeds, the decoupling efficiencies of the frequency-swept decoupling schemes SW_f-TPPM and SW_f-SPINAL are significantly better than the conventional TPPM and SPINAL sequences. The frequency-swept sequences are therefore the methods of choice for efficient decoupling of quadrupolar nuclei with half-integer spin from fluorine.     

过渡族元素掺杂BaTiO3-Tb1-xDyxFe2-y层状复合材料中的磁电效应

用溶胶-凝胶法制备1.0%mol Mn,Cr,Co掺杂 BaTiO₃(BTO)粉体,在1350℃下烧结成多晶陶瓷样品.X射线衍射和差示扫描量热分析表明,室温下掺杂BaTiO₃具有四方钙钛矿结构;居里点和相变潜热随Cr,Mn,Co掺杂逐渐降低.将掺杂BaTiO₃与Tb_1-xDy_xFe_2-y(TDF)胶合制成双层磁电复合材料,并研究了Cr:BTO-TDF,Mn∶BTO-TDF,Co:BTO-TDF层状复合材料中的磁电效应.实验表明,在340×80 A·m^-1偏置磁场下, Cr:BTO-TDF的横向磁电电压系数达到最大值586 mV·cm^-1·(80 A·m^-1)^-1.在400×80 A·m^-1偏置磁场下,Mn∶BTO-TDF和Co:BTO-TDF的横向磁电电压系数的最大值分别为480 mV·cm^-1·(80 A·m^-1)^-1和445mV·cm^-1·(80 A·m^-1)^-1.研究表明掺杂BaTiO₃-TDF层状复合材料中具有较强的磁电耦合.作为无铅压电材料,掺杂BaTiO₃制备的磁电效应器件颇具应用前景. 关键词： 磁电效应 双层复合材料 3')" href="#">掺杂BaTiO₃ 1-xDy_xFe_2-y')" href="#">Tb_1-xDy_xFe_2-y     

组合结构化合物Ho2Ni7-xFex(x=0-3.0)的晶体结构、结构转变和磁性

在金属间化合物的结构演变中,原子尺寸因素起着重要的作用.由于密堆积效应,不同原子半径比的元素往往形成不同的结构.而自由电子填充于原子构成的晶体结构的间隙中,它对化合物的结构也有影响.基于组合结构化合物Ho2Ni7-xFex,结合原子尺寸与自由电子对晶体结构的不同影响,文章探讨一种单位体积内自由电子浓度的经验方法来判断Ho2Ni7-xFex化合物中两种异构体间的转变.随着Fe含量的增加,Ho2Ni7-xFex化合物先结晶成Gd2Co7型三方结构,然后结晶成Ce2Ni7型六方结构.利用Rietveld精修技术和磁测量,获得了化合物的晶体结构参数和饱和磁化强度.化合物晶胞常数随Fe含量增加而增加,饱和磁化强度则随之减少(dMs/dx=-2).分析结果表明,单位体积内自由电子浓度更高,化合物形成三方结构,反之则形成六方结构.     

Temperature calibration for high-temperature MAS NMR to 913 K: 63Cu MAS NMR of CuBr and CuI, and 23Na MAS NMR of NaNbO3

The solid-state phase transitions of CuBr, CuI and NaNbO₃ can be readily observed using ⁶³Cu and ²³Na high-temperature magic-angle spinning nuclear magnetic resonance spectroscopy. Temperature has large, linear effects on the peak maximum of ⁶³Cu in each solid phase of CuBr and CuI, and there is large jump in shift across each phase transition. The ²³Na MAS NMR peak intensities and the line widths in NaNbO₃ also clearly show its high-temperature transition to the cubic phase. These data can be used to calibrate high-temperature MAS NMR probes up to 913 K, which is two hundred degrees higher than the commonly-used temperature calibration based on the chemical shift of ²⁰⁷Pb in Pb(NO₃)₂.     

Dispersive optical constants of a-Se100−xSbx films

Thin films of amorphous Se_100−xSb_x (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (E_o), dispersion energy (E_d) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, E_g^opt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.     

Al and Si MAS NMR investigations of cordierite glass, μ- and α-cordierite

²⁷Al and ²⁹Si Magic-Angle Spinning NMR results are reported for conventionally prepared glass of cordierite stoichiometry (2MgO · 2Al₂O₃ · 5SiO₂), the metastable high-quartz solid solution (μ-cordierite) and the high-temperature polymorph of cordierite (α-cordierite). Both, ²⁷Al two-dimensional (2D) quadrupole nutation experiments and ²⁷Al satellite transition spectroscopy (SATRAS) have been applied to identify two different tetrahedrally-coordinated aluminium sites (AlO₄). SATRAS has been used to extract the quadrupole interaction parameters and their distribution, the isotropic chemical shifts and the relative populations of the different Al sites. Both, the ²⁷Al and ²⁹Si NMR results, lead to the conclusion that a perfect Si/Al disorder does not exist in these investigated cordierite samples.     

Proton Dynamics in Letovicite, (NH4)3H(SO4)2: A H and N NMR Spectroscopic Study

The improper ferroelastic phase letovicite (NH₄)₃H(SO₄)₂ has been studied by ¹H MAS NMR as well as by static ¹⁴N NMR experiments in the temperature range of 296–425 K. The ¹H MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appears just below the phase transition temperature which is due to the acidic protons in the paraelastic phase. The lowering of the second moment M₂ for the ammonium protons takes place in the same temperature range as the formation of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T_c. The static ¹⁴N NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensional ¹H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase.     

Synthesis and magnetic properties of Fe100-xMox alloy nanowire arrays

The Fe100-xMox(13≤x≤25) alloy nanowire arrays are synthesized by electrodeposition of Fe 2+ and Mo 2+ with different ionic ratios into the anodic aluminum oxide templates.The crystals of Fe100-xMox alloy nanowires gradually change from polycrystalline phase to amorphous phase with the increase of the Mo content and the nanowires are of amorphous structure when the Mo content reaches 25 at%,which are revealed by the X-ray diffraction and the selected area electron diffraction patterns.As the Mo content increases,the magnetic hysteresis loops of Fe100-xMox alloy nanowires in parallel to the nanowire axis are not rectangular and the slopes of magnetic hysteresis loops increase.Those results indicate that the magnetostatic interactions between nanowires and the magnetocrystalline anisotropy both have significant influences on the magnetization reversal process of the nanowire arrays.     

Al satellite transition spectroscopy (SATRAS) of polycrystalline aluminium borate 9Al2O3 · 2B2O3

²⁷Al NMR spectra of polycrystalline aluminium borate 9Al₂O₃ · 2B₂O₃ have been measured at 104, 130 and 156 MHz. The parameters of the quadrupole interaction and the isotropic chemical shifts have been obtained by fitting the CT/MAS pattern and consideration of the inner satellite transitions m = 3/2 ↔ 1/2 and m = −1/2 ↔ −3/2. The gain in spectral resolution concerned with the observation of the MAS lines of the inner satellites leads to complete separation of the signals of AlO₆, AlO₅ and AlO₄ polyhedra. Also signals of structural groups of one and the same coordination number can be distinguished. Experimental and theoretical lineshape calculations are compared.     

Electron-correlation-induced band renormalization and Mott transition in Ca1-xSrxVO3

We present the local density approximate+Gutzwiller results for the electronic structure of Ca_1-xSr_xVO₃. The substitution of Sr²⁺ by Ca²⁺ reduces the bandwidth, as the V—O—V bond angle decreases from 180° for SrVO₃ to about 160° for CaVO₃. However, we find that the bandwidth decrease induced by the V—O—V bond angle decrease is smaller as compared to that induced by electron correlation. In correlated electron systems, such as Ca_1-xSr_xVO₃, the correlation effect of 3d electrons plays a leading role in determining the bandwidth. The electron correlation effect and crystal field splitting collaboratively determine whether the compounds will be in a metal state or in a Mott-insulator phase.     

Ti1－xCrxO2±δ体系的相关系、晶体结构和磁性能研究

采用溶胶凝胶法制备了Ti_1-xCr_xO_2±δ体系系列样品.利用扫描电子显微镜(SEM)，X射线光电子能谱(XPS)，粉末X射线衍射分析(XRD)方法研究了Ti_1-xCr_xO_2±δ系列样品的颗粒尺寸、形貌、组分化学态、相关系和固溶区范围；并利用超导量子干涉磁强计对样品的磁性能进行了研究.采用Rietveld结构精修的方法研究了Cr的不同掺杂量对TiO₂晶体结构的影响，研究表明，1000℃烧结的样品的固溶区范围是x=0—0.03，为金红石单相；随着Cr掺杂量的增加，金红石相晶胞参数规律性地减小；当x>0.03，为金红石相和CrO₂相两相共存.综合XRD和磁性测量结果，500℃烧结的样品的固溶区范围是x=0—0.02，为锐钛矿单相；随着Cr掺杂量的增加，锐钛矿相晶胞参数规律性地减小；当x≥0.04，为锐钛矿相和绿铬矿相(Cr₂O₃)两相共存.XPS实验结果表明，500℃和1000℃退火的样品中Cr都是以Cr⁺³和Cr⁺⁶两种化学态存在，1000℃烧结的样品中可能有更多的Cr³⁺转化为Cr⁶⁺.根据M-H和M-T曲线的测试结果发现，本文500℃烧结的Ti_1-xCr_xO_2±δ体系样品当x=0—0.02时，为室温铁磁性.当x≥0.04时，由铁磁相和顺磁相所组成，在低温下有较强的铁磁性；室温下主要是顺磁相，铁磁相只占据很小的体积分数. 关键词： 1-xCr_xO_2±δ体系')" href="#">Ti_1-xCr_xO_2±δ体系 相关系 固溶区 磁性能     

Double metal-insulator peaks and effect of Sm substitution on magnetic and transport properties of hole-doped La0.85Ag0.15MnO3

La_0.85−xSm_xAg_0.15MnO₃ (x=0−0.2) ceramics were prepared using the conventional solid-state synthesis method to investigate the effect of Sm³⁺ substitution on magnetic and electrical transport properties. Magnetic susceptibility versus temperature measurements showed all samples exhibit ferromagnetic to paramagnetic transition with Curie temperature, T_c decreasing from 283 K (x=0) to 164 K (x=0.2) with increasing Sm³⁺. The observed slope in susceptibility, χ′ versus temperature curves below T_c indicates the possible presence of FM and AFM phases in the metallic region. In addition, a deviation from the Curie-Weiss law above T_c in 1/χ′ versus T curves indicates the existence of a Griffith's phase in the x=0.05−0.2 samples due to the Sm³⁺ ion substitution. The Griffith temperature, T_G was found to decrease from 295 K (x=0.05) to 229 K (x=0.2). Electrical resistivity measurements of the samples in zero field showed transition from metallic behavior to insulating behavior as the temperature was increased. For x=0, two metal-insulator, MI transition peaks were observed at T_p1=282 K and at T_p2=250 K. Both peaks shifted to lower temperatures with the increase in Sm³⁺. The relative resistivity of the first peak to the second peak decreases with increasing Sm³⁺ for x>0.05 while at x=0.2 the T_p1 peak was strongly suppressed. Magnetoresistance, MR was observed to weaken with Sm³⁺ substitution. The metallic region of the ρ(T) curve of the x=0−0.15 samples was fitted to the model of electron-electron and electron-magnon scattering while the insulating region was fitted to the variable range hopping, VRH model. The resistivity behavior indicated that the substitution of Sm³⁺ weakened the double exchange process and enhanced the Jahn-Teller effect. Our results indicated that the T_p1 peak is strongly related to the double-exchange mechanism while the T_p2 peak is suggested to originate from magnetic inhomogeneity.     

Phonon-assisted intersubband transitions in wurtzite GaN/InxGa1-xN quantum wells

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite GaN/In_xGa_1-xN quantum wells is presented. The quantum-confined Stark effect induced by the built-in electric field and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled Schrödinger and Poisson equations and the dispersion property of each type of phonon modes is considered in the derivation of Fermi's golden rule to evaluate the transition rates. It is indicated that the interface and half-space phonon scattering play an important role in the process of 1-2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. The present work can be helpful for the structural design and simulation of new semiconductor lasers.     

Li mobility in Li0.5 − xNaxLa0.5TiO3 perovskites (0 ≤ x ≤ 0.5): Influence of structural and compositional parameters

The dependence of Li mobility on structure and composition of Li_0.5 − xNa_xLa_0.5TiO₃ perovskites (0 ≤ x ≤  0.5) has been investigated by means of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. At 300 K, all samples display a rhombohedral superstructure (R-3c S.G.), where octahedra are out of phase tilted along [111] direction of the ideal cubic cell. The elimination of the octahedral tilting is responsible for the rhombohedral–cubic transformation, detected near 1000 K. In these perovskites, La and Na cations are randomly distributed in A sites, but Li ions are fourfold coordinated at unit cell faces of the cubic perovskite. Lithium conductivity, σ_300 K, decreases with the sodium content, decreasing from values typical of fast ionic conductors, 10^− 3 S/cm, to those of good insulators, 10^− 10 S/cm, when the interconnectivity between vacant A sites is lost (x > 0.3). In samples with x < 0.3, dc conductivity displays a non-Arrhenius behaviour, decreasing activation energy from ~ 0.37 to 0.25 eV when the sample is heated between 77 and 500 K. The temperature dependence of B_Li factors shows the existence of two regimes for Li motion. Below 373 K, Li ions remain partially located near square oxygen windows that connect contiguous A sites, but above 400 K, extended Li motions become dominant. The additional decrease of activation energy from 0.25 to 0.16 eV (low-temperature ⁷Li NMR value), should require the full elimination of octahedral tilting which is only produced above 1000 °C.