R1+εFe4B4化合物中游标卡尺结构对称性表示的两种途径

本文从R_1+εFe₄B₄(ε=0.1)的两个亚结构各自的对称性出发,由两种途径对该化合物的游标卡尺结构对称性进行了描述。结果表明:通过建立公度的长周期超结构模型,可以恢复R_p(Fe₄B₄)_q三维空间对称性,并和p,q的奇偶性组合有关。在p,q同为奇数,p为偶数而q为奇数和p为奇数而q为偶数的情况下,其对称性分别由P4₂/n,Pccn 关键词：     

超冷铯分子0u+(6P3/2)长程态的高灵敏光缔合光谱研究

利用发展的调制俘获损耗荧光光谱技术实验测量了超冷铯分子0_u⁺(6P_3/2)长程态的高灵敏光缔合光谱. 光谱探测范围较国际已有报道扩大了60 cm^-1, 观察到25个长程区域新的振动能级. 通过LeRoy-Bernstein公式对振动束缚能数据进行拟合, 获得了超冷铯分子0_u⁺(6P_3/2)态的长程系数C₃ 为16.103±0.010. 构建了超冷铯分子0_u⁺(6P_3/2)态长程区域的势能曲线.     

BiVO4的晶体结构和光学观察

对于提拉法合成的BiVO₄单晶进行了X射线衍射分析。室温下该晶体属于单斜晶系,空间群为B2/b,z=4,晶胞参数为:a=7.247?,b=5.096?,c=11.702?,γ=134.18°。进行了二维、三维帕特逊分析和三维电子密度计算。全矩阵最小二乘法修正给出335个独立反射的R=0.067,R_w=0.058。经吸收和次级消光校正后,R=0.058。Bi和V原子各占据一套4e特殊点系,O原子占据两套一般点系。V³⁺是四配位,形成规则的VO₄四面体。Bi³⁺是八配位,形成畸变Bi-O十二面体。BiVO₄在T_c温度上下空间群各为I4₁/a和I2/b(或B2/b)。由四方到单斜相变没有正交中间变体。相变显然是由Bi-O多面体孤对畸变所驱动。Bi³⁺和V⁵⁺沿c轴上下交替位移,这就显示单斜BiVO₄具有层状结构的特征。这一特征由光学观察得到证实。该晶体在垂直干c轴的方向极易开裂而产生平坦的解理面。选取具有胞状结构圆盘形晶体、自然解理晶片和粉末晶粒三种试样进行了光学观察,并研究了晶体透过率与波长关系。 关键词：     

IrB和IrB$lt;sub$gt;2$lt;/sub$gt;力学性质的第一性原理计算

采用平面波赝势密度泛函理论对0–100 GPa静水压下P₁ -IrB（空间群Pnma）和P₅ -IrB₂（空间群Pmmn）结构的平衡态晶格常数、弹性常数等性质进行了研究. 研究结果表明,P₁ -IrB不可压缩性随着压强的增加而增强;P₅ -IrB₂ 结构在0–100 GPa范围内弹性常数、体弹模量、剪切模量均呈现出有规律的变化,当所加压强为50 GPa时,杨氏模量和在b方向的晶格常数发生异常变化. 对零压下P₁ -IrB和P₅ -IrB₂ 的电子结构的研究发现,二者均没有一个明显的带隙,主要原因为Ir原子和B原子间的共价作用. 从P₁ -IrB和P₅ -IrB₂的能带结构和态密度图可以发现这两种结构均有金属性. 关键词： 1 -IrB')" href="#">P₁ -IrB 5-IrB₂')" href="#">P₅-IrB₂ 第一性原理 力学性质     

K离子掺杂增强NaErF4体系上转换发光

制备了一系列Na_1-xK_xErF₄@NaLuF₄的核壳纳米结构,核中K⁺掺杂摩尔分数变化范围为0%~8%。XRD分析结果揭示这些具有不同K掺杂浓度的纳米粒子均为β-相纳米结构。研究结果表明：随着K⁺浓度的增加,纳米结构中Er³⁺~650 nm处的红带发光强度呈现先增强后减弱的规律,当K⁺摩尔分数为4%时,Na_0.96K_0.04ErF₄@NaLuF₄纳米晶的发光强度达到最大,为未掺杂K⁺的NaErF₄@NaLuF₄纳米晶发光强度的3.7倍。其发光增强的原因在于K⁺的掺杂降低了Er³⁺微环境晶场宇称对称性,提高了Er³⁺离子⁴F_9/2→⁴I_5/2能级辐射跃迁几率,进而增强了Er³⁺的650 nm红带的上转换发光强度。     

Dy,Co共掺杂对BiFeO3陶瓷磁特性和磁相变温度Tc的影响

采用快速液相烧结法制备BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0, 0.05, 0.1, 0.15)陶瓷样品. 实验结果表明: 所有样品的主衍射峰与纯相BiFeO₃相符合且具有良好的晶体结构, 随着Co³⁺掺杂量的增大, Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品的主 衍射峰由双峰(104)与(110)逐渐重叠为单峰(110), 当掺杂量x>0.05时, 样品呈现正方晶系结构; SEM形貌分析可知: Dy³⁺, Co³⁺共掺杂使BiFeO₃晶粒尺度由原来的3—5 μ减小到约1 μ. 室温下, BiFeO₃样品表现出较弱的铁磁性, 随着Dy³⁺和Co³⁺掺杂, BiFeO₃样品的铁磁性显著提高. 在外加磁场为30 kOe的作用下, Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0.05, 0.1, 0.15)的M_r分别为0.43, 0.489, 0.973 emu/g; M_S分别为0.77, 1.65, 3.08 emu/g. BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品磁矩M随着温度T的升高而逐渐减小, Dy掺杂使BiFeO₃样品的T_N由644 K升高到648 K, 而T_C基本没有变化. Dy和Co共掺杂导致BiFeO₃样品磁相变温度T_C由870 K降低到780 K, 其T_C变化主要取决于Fe-O-Fe反铁磁超交换作用的强弱和磁结构的相对稳定性. 关键词： 铁磁电材料 磁滞回线 磁相变温度     

Eu3+掺杂钛酸钇晶态薄膜的制备与高效红色荧光发射

采用溶胶-凝胶法制备了Eu³⁺掺杂的钛酸钇晶态发光薄膜。通过X射线衍射(XRD)对薄膜的结构和结晶过程进行了分析,利用荧光分光光度计对薄膜的发光性质开展了测试和研究。XRD结果表明,薄膜包含立方相Y_xTi_1-xO_1-0.5x晶粒,该晶粒属立方晶系,Fm3m(225)空间群,晶胞参数a=0.530nm,晶粒尺寸约为17nm。荧光光谱表明,Eu³⁺掺杂的Y_xTi_1-xO_1-0.5x薄膜显示了强的红光发射,其中Eu³⁺的⁵D₀→⁷F₂超灵敏跃迁为最强一组。紫外氙灯、准分子激光器、汞灯等是这种发光薄膜的有效激发源。     

1,2-环己二酮基态光解离反应的理论研究

通过密度泛函理论计算研究了1, 2-环己二酮(α-CHD)基态光解离反应的势能面. 通过IRC方法确定了5个主要的反应通道, 相应的产物分别为P₁(c-C₅H₈O+CO), P₂(2C₂H₄+2CO), P₃(CH₂CHCH₂CH₂CHO+CO), P₄(2CH₂CO+C₂H₄)和P₅(CH₃CHCO+CH₂CHCHO). 获得了反应过程中反应物、产物、中间体和过渡态的结构参数. 详细阐述了这些通道的反应过程, 分析了其反应机理, 总结出最优的反应路径为α-CHD→c-C₅H₈O+CO. 理论分析与实验结果相符. 获得的结果为进一步进行与1, 2-环己二酮相关的研究提供有价值的信息.     

P2O5-CuI-CuCl系统非晶态铜离子导体的EXAFS,IR,Raman光谱研究

本文用透射法EXAFS和IR,Raman光谱等手段研究了以P₂O₅,为基体的非晶态铜离子导体的微观结构,得出了非晶结构网络中存在[PO₄]^3-,[CuO₄]^6-,[CuO₃I]^5-,[PO₃I]^2-等四元骨架结构单元,Cu⁺在结构网络空隙中有两种不同的分布状况。并在此基础上对材料在50℃左右存在的二级相变及Cu⁺的其它传导性能进行了定性的描述。 关键词：     

用夸克模型探索π-N相互作用

本文中用夸克模型,Warke近似,并作为补偿,在夸克唯象势中加入中程势(-1)^T+JKre^(-r/a₂),计算了π-N相互作用势,从此势计算了L_2T,2J=S₁₁,S₃₁,P₁₁,P₃₁,P₁₃和P₃₃六种道的相移。在入射π介子动能E_π^k(lab)=0—700MeV范围内,与由实验方法决定的相移进行了比较。 关键词：     

La–Zn Substituted Hexaferrites Prepared by Chemical Method

La–Zn substituted M-type Ba hexaferrite powders were prepared by sol-gel (Mx) and organometallic precursor (Sk) methods with Fe/Ba ratio of 11.6 and 10.8, respectively. The compositions (LaZn) _x Ba_{1 ? x} Fe_{12 ? x} O₁₉ with 0.0 ≤ x ≤ 0.6 were annealed at 975°C/2 h. The cationic site preferences of nonmagnetic La³⁺ instead of Ba²⁺ ions and Zn²⁺ instead of Fe³⁺ ions were determined by Mössbauer spectroscopy. The La³⁺ ions substitute the large Ba²⁺ ions at 2a site and for x ≥ 0.4 also at 4f₂ site. The nearly all Zn²⁺ ions are placed at the 4f₁ sites. The thermomagnetic analysis of χ(?) confirms that only the small substitutions for x ≤ 0.4 can be taken as a single-phase hexaferrites. The coercivity H _c almost does not change at x = 0.2 for (Mx) samples and further decrease up to x = 0.6. For (Sk) samples at substitution x = 0.2 the values of H _c are decreasing and at higher x the values nearly do not change. The Curie points, T _c, slowly decrease with x for both (Mx) and (Sk) samples.     

Complex permittivity and complex permeability of Sr ions substituted Ba ferrite at X-band

M-type hexagonal ferrite composition, Ba_(1−x)Sr_xFe₁₂O₁₉ (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), was prepared by a two route ceramic method. Complex permittivity (ε′−jε″) and complex permeability (μ′−jμ″) have been measured using a network analyzer from 8.2 to 12.4 GHz X-ray diffraction confirmed the M-type hexagonal structure and a scanned electron micrograph was used to analyze the grain size distribution of ferrite. Substitution of Sr²⁺ ions causes an increase in porosity that deteriorates the electromagnetic and microstructural properties in the doped samples. Both dielectric constant and dielectric loss are enhanced in comparison to the permeability and magnetic loss over the entire frequency region. This is due to a resistivity variation and the formation of Fe²⁺ ions, which increases the hopping mechanism between Fe²⁺ and Fe³⁺ ions.     

Effect of iron doping at Mn-site on complex impedance spectroscopy properties of Nd0.67Ba0.33MnO3 perovskite

The effect of Fe-doping at Mn-site on the structural and electrical properties of Nd_0.67Ba_0.33Mn_1?xFe_xO₃ (0 ≤ x ≤ 0.05) perovskites has been investigated. X-ray diffraction patterns show that the structural parameters change slightly due to the fact that the Fe³⁺ ions replacing the Mn³⁺ have similar ionic radius. The electrical properties of these samples have been investigated using complex impedance spectroscopy technique. a function of the frequency at different temperatures. When increasing the Fe-content, a decrease of dc conductivity was observed throughout the whole explored temperature range and the deduced activation energy values are found to increase from 128 meV for x = 0 to 166 meV for x = 0.05. The curves of the imaginary part of impedance (Z″) show the presence of relaxation phenomenon in our samples. The complex impedance spectra show semicircle arcs at different temperatures and an equivalent circuit of the type of R_g + (R_gb//C_gb) has been proposed to explain the impedance results.     

Tetragonal rare earth (R) Iron borides,R1+εFe4B4 (ε≅0.1), with incommensurate rare earth and iron substructures

Crystallographic and magnetic properties of a new structural series of ternary borides with composition R_1+εFe₄B₄ (R = Ce, Pr, Nd, Sm, Gd, Tb, 0.11(Pr) ≤ε≤ 0.15(Tb) are reported. The compounds are built of incommensurate substructures of rare earth atoms (linear strings ?? c?), iron atoms (chains of edge sharing tetrahedra ?? c?),and boron atom pairs. A single crystal X-ray diffraction study of one representative (Sm_1.13Fe₄B₄) based on a commensurate structure model (composition : Sm₁₇(Fe₄B₄)₁₅, a = 7.07 Å, c ≡ 17c_Sm ≡ 15c_Fe= 58.69 Å, space group P4₂/n) revealed a periodic twist modulation of the Fe tetrahedra chains around c?. Magnetic susceptibility measurements on single crystals of another representative (Nd_1.11Fe₄B₄) revealed ferromagnetic ordering at T_c = 13 K. Above this temperature the magnetic properties are dominated by ferromagnetic inclusions (Fe₂B, Nd₂Fe₁₄B).     

Non affinity of Fe2+ for B sites in iron thiochromites and compared band structures of oxides and sulfides spinels

Departure from stoichiometry in vapor grown FeCr₂S₄ was studied using Mössbauer spectroscopy. The paramagnetic Mössbauer spectra give evidence of two singlets and two doublets which correspond respectively to A site Fe²⁺ ? Fe³⁺, Fe^II in T_d symmetry and in symmetry lower than T_d. The following ionic distribution has been deduced:

$\text{(Fe}{}^{\mathrm{2+}}{}_{\mathrm{1?y}}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{y}}\text{)|Cr}{}^{\mathrm{3+}}{}_{\mathrm{2?x}}\text{□}{}_{\mathrm{x}}\text{|S}{}_{\mathrm{4?z}}\text{□}{}_{\mathrm{z}}$

Compounds in the system Fe_1+xCr_2?xS₄ have been studied for 0 ? x ? 0.1. The spectra are solved assuming Fe^II in A site with T_d symmetry, A site Fe^II with lower symmetry and B site Fe³⁺. No Fe²⁺ appears in B site. These features are discussed in terms of schematic band structures implying single electron narrow bands. The non-affinity of Fe²⁺ for B sites of iron thiochromites is discussed in relation with B site Cr²⁺ level.     

Debye temperature and magnetic ordering in K x Ba1−x Fe2S3

⁵⁷Fe Mössbauer measurements are reported for the series K _x Ba_1?x Fe₂S₃, x ≤ 0.3, at temperatures between 4.2 K ≤ T ≤ 294 K. A decrease of the Debye temperature from 435 to 405 K with x, indicates a weakening of the stiffness of the Fe sublattice. The ordering temperatures, taken from the appearance of magnetic hyperfine splitting in the spectra, are approximately 40 K lower for x ≥ 0.1. The values of the centre shift and the small temperature dependence of the quadrupole splitting strongly supports that similar to the border compound BaFe₂S₃ also the K containing samples should be characterised as mixed valence compounds.     

Etude De La Conductivite Electrique Des Ferrites Oxyfluores A Structure Spinelle

An investigation of the electrical conductivity of some oxyfluoride spinels of formula Zn_x²⁺Fe_1?x³⁺[M²⁺ Fe³⁺]O_4?xF_x (M = Fe, Co, Ni) and Fe³⁺[N_x²⁺Fe²⁺Fe_1?x³⁺]O_4?xF_x (N = Fe, Ni) shows that the conduction depends on the composition of the B sites: the activation energy increases, the conductivity and the Fe₃O₄ transition temperature decrease as the substitution rate of Fe³⁺ by N²⁺ in the B sites increases. The authors conclude to a hopping mechanism between the B cations; the anionic sublattice and the cationic A sublattice do not participate in the conduction.     

Phase stratification in the Nd1 − x BaxCoO3 − δ (0.3 ≤ x ≤ 0.54) system

Slowly cooled Nd_{1 ? x} Ba_xCoO_{3 ? δ} samples were two-phase in the concentration interval 0.3 ≤ x ≤ 0.46. One of the phases had O-orthorhombic lattice distortions (Pbnm) characteristic of ferromagnetic samples with x ≤ 0.3, and the other phase had tetragonal distortions (P4/mmm) characteristic of samples with x ≥ 0.46. Tetragonal distortions were caused by ordering of Nd³⁺ and Ba²⁺ ions. Samples with ordered neodymium and barium ions (Nd_{1 ? y} Ba_{1 + y} Co₂O_{6 ? γ} at ?0.08 ≤ y ≤ 0.08) experienced metal-dielectric and orientation magnetic phase transitions.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.