Orthorhombic Inverse Perovskitic Ba3TtO (Tt = Ge,Si) as Zintl Phases

The isotypic title compounds are obtained in high yield from the reactions of Ba, BaO, and Ge (Si) in welded Ta containers slowly cooled from 1100 °C. The structure of Ba₃GeO was determined by single-crystal X-ray diffraction (orthorhombic symmetry; Pnma (No. 62); a = 7.591(1), b = 10.728(1), c = 7.551(1) Å; Z = 4; R = 0.058, R_w = 0.065 for 780 reflections (I > 3σ(I)) with 2θ_max = 60°)). The structure consists of slightly deformed OBa₆ octahedra that are tilted by £ 14° with respect to their positions in the ideal inverse perovskite structure. These distortions optimize eight of the original twelve equal Ba–Ge distances. The ideal cubic Ca₃SiO (a = 4.699(1) Å) has also been synthesized.     

YBa2Cu3Ox超导体的光谱研究

能带是决定固体特性的头等重要的因素。我们用光谱方法对单相YBa₂Cu₂O_x高T。超导体进行了研究,测量了样品的反射-吸收谱、Raman光谱和荧光光谱.其350和500nm吸收带以及390nm,560nm荧光峰来自晶格中Cu⁺发光中心,是跃迁过程¹A_1g(3d¹⁰)-¹E_g和~3E_g(3d⁹4s¹)。其720nm和860nm荧光峰来自Cu²⁺发光中心,是由于自由离子光谱项²D在八面体晶场、正交晶系晶场中分裂为5个能级之间的跃迁。此外,还有一些光谱可能来自晶格中Cu³⁺发光中心,其481cm^-1和551cm^-1声子对说明了312cm^-1处声子对与电子系统是强耦合。     

Na30.5AgxGa60—x: A Three‐Dimensional Network Composed of Ga3, Ga12, and Ga18 Clusters

Single crystal X‐ray structural determinations for Na_30.5Ag_2.6(2)Ga_57.4(2) and Na_30.5Ag_6.4(2)Ga_53.6(2) (P6/mmm, Z = 1) reveal Ga₃ triangles, Ga₁₂ icosahedra, and Ga₁₈ hexacapped hexa gonal prisms that are interconnected into a three‐dimensional network. Silver substitutes on the outer gallium sites of the Ga₁₈ unit and presumably stabilizes the structure by reduction of the overall electron count. The two compositions are close to the limits of a nonstoichiometry region in Ga—Ag content, whereas the sodium content does not vary significantly. Stacking of the (Ga, Ag)₁₈ “drums” along creates a channel in which some typically less well localized sodium atoms reside; other sodium cations lie between the clusters and bond to gallium atoms in typical roles. Apparently isotypic analogues of this structure that are more or less well ordered are also known for other combinations of elements. Extended Hückel MO and band calculations were performed in order to gain a better understanding of the silver compound. The variable electron count observed falls in a region of relatively nonbonding states with only a small dependence on energy.     

相对论性核与乳胶(Ag)碰撞中灰粒子的系统研究 *

用描写相对论性核 核碰撞的LUCIAE模型和相应的事件产生器系统地研究了 1 4.6 ,6 0和 2 0 0AGeV的O核、2 0 0AGeV的Si和S核以及 1 1 .6AGeV的Au核与乳胶 (Ag)碰撞中灰粒子产生的平均多重数、多重数分布以及角分布 3个物理量同入射能量、射弹质量及碰撞中心度间的关系 ,还研究了再散射在灰粒子产生机制中的作用 .LUCIAE的这些研究结果与相应的EMU0 1乳胶实验结果都相一致 .     

算术级数中的陈景润定理 *

设N为一充分大的偶数且q≥ 1 ,(l_i,q) =1 (i=1 ,2 ) ,l₁+l₂ ≡N(modq) .证明了方程N =p+P₂ ,p≡l1(modq) ,P₂ ≡l₂ (modq)对区间 1 ,N^1/37中除了ON^1/37log^-5 N个例外的整数q ,都有无穷多解 ,其中p是素数 ,P₂ 为一至多有2个素因子的殆素数 .     

Limit cycles in a generalized Gause-type predator–prey model

We study the problem of the existence of limit cycles for a generalized Gause-type predator–prey model with functional and numerical responses that satisfy some general assumptions. These assumptions describe the effect of prey density on the consumption and reproduction rates of predator. The model is analyzed for the situation in which the conversion efficiency of prey into new predators increases as prey abundance increases. A necessary and sufficient condition for the existence of limit cycles is given. It is shown that the existence of a limit cycle is equivalent to the instability of the unique positive critical point of the model. The results can be applied to the analysis of many models appearing in the ecological literature for predator–prey systems. Some ecological models are given to illustrate the results.     

涡旋成对分布和对二元速度关联函数的影响

本文考虑了涡旋之间相互作用的影响,假定均匀各向同性湍流场是由涡旋对在整个空间等几率分布而成,这样计算出的二元速度关联函数f(γ/λ)在远处略有提高,这个结果是与Bennett和Corrsin的实验结果和其他实验结果相合的。     

Synthesis, structure, and bonding of BaTl(3): an unusual competition between cluster and classical bonding in the thallium layers.

The new BaTl(3) compound has been synthesized and characterized by physical property measurements and electronic structure calculations. Its structure (Cmcm) is a new intermediate in the Ni(3)Sn family (P6(3)/mmc), and consists of thallium layers formed from two-center bond formation between the parallel chains of face-sharing octahedral clusters. The valence electron concentration (VEC) of the thallium layers is consistent with their two-dimensional nature, in comparison with those in other AX(3)-type compounds with one- or three-dimensional anionic networks with the same building blocks and different VECs. The unique geometric features of the anionic thallium layers bring on an unusual competition between inter- and intracluster bonds. Detailed studies of the energetics of BaTl(3) reveal for the first time the important role of cation-anion interactions in the bonding competition in such an anionic substructure.     

Diastereoselective amplification of an induced-fit receptor from a dynamic combinatorial library

A high-affinity, induced-fit receptor for NMe4I was discovered using dynamic combinatorial chemistry. The addition of the guest to a dynamic combinatorial library made using a racemic mixture of chiral building blocks caused the strong and highly diastereoselective amplification of the receptor at the expense of other library components. The receptor and its mode of binding were characterized by NMR, ITC, and re-equilibration experiments, from which it was deduced that the receptor probably forms a folded four-stave barrel shape on binding of the guest.     

Phase stabilization through electronic tuning: electron-poorer alkali-metal-indium compounds with unprecedented In/Li clusters

Three alkali-metal-indium compounds K34In(92.30)Li(12.70) (I), K14Na20In(91.82)Li(13.18) (II), and K14Na20In(96.30) (III) (all Rm) have been synthesized and characterized by structural and physical property measurements and electronic structure calculations. Novel mixed In/Li anionic icosahedra and fused icosahedra form in I and II. All three contain In28 as the first triply fused In icosahedra, which are further linked into (In28)In(In28) sandwich adducts in compounds I and II and (In28)In2(In28) in III. Stabilization of these electron-poorer phases through electronic tuning occurs via two different structural (redox) perturbations, either by substitution of certain indium atoms in the clusters by electron-poorer lithium atoms or by the introduction of defects and disorder in the fused cluster (III). The preferential occurrence of either substitutions or defect formation in the clusters is consistent with extended Huckel band calculation results for both the ideal pure indium phase and the Li-substituted equivalent. Model (ideal) and experimental EF values (based on stoichiometries) fall around a pseudogap in DOS. All three compounds are metallic according to both EHTB band calculations and measured resistivities. The cations (A = K, Na) in all the three structures generate A136 clathrate-IotaIotatype networks with remarkably specific and transferable cation dispositions around the two types of anionic cluster units.