Local 3D real space atomic structure of the simple icosahedral Ho11Mg15Zn74 quasicrystal from PDF data

We present a new complementary strategy to quasicrystalline structure determination: The local atomic structure of simple icosahedral (si) Ho₁₁Mg₁₅Zn₇₄ [a(6D) = 5.144(3)Å in a sphere of up to r = 17Å was refined using the atomic pair distribution function (PDF) from in‐house X‐ray powder diffraction data (MoKα₁, Q_max = 13.5Å⁻¹; R = 20.4%). The basic building block is a 105‐atom Bergman‐Cluster {Ho₈Mg₁₂Zn₈₅}. Its center is occupied by a Zn atom – in contrast to a void in face centred icosahedral (fci) Ho₉Mg₂₆Zn₆₅. The center is then surrounded by another 12 Zn atoms, forming an icosahedron (1st shell). The 2nd shell is made up of 8 Ho atoms arranged on the vertices of a cube which in turn is completed to a pentagon dodecahedron by 12 Mg atoms, the dodecahedron then being capped by 12 Zn atoms. The 3rd shell is a distorted soccer ball of 60 Zn atoms, reflecting the higher Zn content of the si phase compared to the fci phase. In our model, 7% of all atoms are situated in between the clusters. The model corresponds to a hypothetical 1/1‐approximant of the icosahedral (i) phase. The local coordinations of the single atoms are of a much distorted Frank‐Kasper type and call to mind those present in 0/1‐Mg₂Zn₁₁. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Molecular symmetry and ferroelectricity in pure organic molecular crystals

The molecular symmetry of all the existing organic ferroelectrics was analyzed with an intention to develop a criterion to search for new low molecular weight organic ferroelectric. We present here a summery of the analysis; it is conclude that in order to exhibit ferroelectricity in crystalline state the molecules should necessarily poses symmetry higher than C₁, since this appears to be an important requirement to ensure reorientability of the electrical polarization of the crystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultraviolet Photoelectron Spectra of Mono Metal Atom Encapsulated Fullerenes

Abstract

Ultraviolet photoelectron spectra of mono metal atom encapsulated fullerenes, M@C₈₂ (M = La, Gd and Sc), are measured with a synchrotron radiation light source. When the excitation energy is tuned, spectral intensity oscillation is observed in these mono metal atom encapsulated fullerenes as was in empty fullerene, C₈₂. Their incident photon energy dependence is essentially the same. Their spectra deeper than 5 eV are almost identical and are similar to those of empty fullerene. The noticeable difference among the spectra is in the energy region between the Fermi level and 4 eV, which reveals the degree of electron transfer from the metal atom to the fullerene cage.     

Chiral symmetry breaking in unstirred crystallization

Statistics of nucleation of chiral forms of sodium bromate from unstirred aqueous solutions was studied. It was established that bimodal, trimodal and unimodal distributions of enantiomers are obtained in unstirred crystallization. It was also found out that probabilities of the creation of L or D crystals and racemates R, as well as the presence of D, L, and R peaks in distributions depend on crystallizer size, supersaturation and temperature. Nucleation at low supersaturations in small, closed crystallizers leads to the formation of pure enantiomers, and to bimodal distributions with D and L peaks at any temperature. At high supersaturations in large, open crystallizers the formation of racemates and unimodal distributions with racemate R peaks results. In open crystallizers at the lowest temperatures and at the highest temperatures used in crystallization from aqueous solution racemates of sodium bromate are preferentially formed, but in a wide range of intermediate‐temperatures apart from racemates, pure enantiomers are efficiently formed which leads to trimodal distributions. The spontaneous formation of pure enantiomers in crystallization from unstirred, unseeded solutions is caused by the chiral symmetry breaking phenomenon, the same as that discovered in stirred crystallization. The conservation of chiral symmetry is, in unstirred crystallization, one of the two possibilities, and the other one is the breakage of symmetry. Both of them occur in nature. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure, characterization, and EPR of a novel Cu(II) compound with 4-aminopyrimidine. A compound with a very high lattice symmetry

The new copper(II) coordination compound [Cu(4apm)₄(CF₃SO₃)₂](H₂O)^8/3, (in which 4apm = 4-amino-pyrimidine) is studied. This compound is investigated in detail by spectroscopy, EPR, and X-ray crystal structure analysis. The compound crystallizes in the rare cubic spacegroup Im-3m with a,b, and c = 16.823(2) Å, Z = 6. The copper(II) atom has an elongated octahedral geometry, of which the basal plane is formed by four nitrogen atoms of four monodentate ligands with a Cu–N distance of 2.010(4) Å, while the apical positions are occupied by oxygen atoms of the triflate anion with a Cu–O distance of 2.606(5) Å.     

Topology and symmetry analysis of rare earth borocarbides structural family,analogy to hexaferrites and relation to properties

The topology and symmetry analysis was applied to a series of rare earth borocarbide compounds, which have been gaining increasing interest due to their magnetic and thermoelectric properties. Using principles of OD theory, the crystal structures were deconvoluted into L(1) (B₁₂ icosahedra and C‐B‐C chain) layers and L(2) (rare earth and B₆ octahedral) layers. The arrangement of B₁₂ icosahedra in the L(1) layer is equal to close packed spheres, however, symmetry of the B₁₂ block lowers symmetry of the resulting layer from P 6/mmm to P 3m1. Both layers, L(1) and L(2) possess symmetry P 3m1 and the conjugation of L(1) with L(2) layers occurs in accordance with the symmetry elements. No disorder may appear here because of equal symmetry of single layers and layer pairs and it is not a classical OD family. Only the increasing of the amount of one type of layers, namely L(1), provides the structural variations. Close analogy to the hexagonal ferrites family has been found. Topology and symmetry analysis reveals principles in the building up of the structural family, gives an insight into the particular order‐disorder formation mechanism/criteria of these homologous borocarbide compounds and as the result relation to the properties (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three‐dimensional thermoelastic stresses in off‐axis oriented single crystals with hexagonal symmetry

A three‐dimensional (3D) thermoelastic stress analysis is carried out on a single crystal with axisymmetric geometry but with a hexagonal crystallographic symmetry. The crystallographic orientation is off‐axis with respect to the cylindrical coordinate system. By applying a Fourier series expansion with respect to the rotational angle ϕ of the cylindrical coordinates, the 3D boundary value problem is reduced to a sequence of 2D ones on the meridian plane, which are solved by the finite‐element method. In our example, the off‐axis orientation is towards a direction of high symmetry, and therefore only four of the six stress tensor components are non‐zero. In the end, the stress tensor is projected onto the slip system of the crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

掺杂Rh离子SrTiO3晶体缺陷形成能和电子结构的研究

使用CRYSTAL-09软件包模拟计算了完整SrTiO3晶体的电子结构以及Rh离子掺杂SrTiO3晶体的缺陷形成能和电子结构.结果表明Rh离子在晶体中将优先占据Sr格位;Rh掺杂SrTiO3晶体在禁带中出现了一个4d态杂质能级,导致禁带宽度减小,可见光区的光催化活性提高;解释了Rh掺杂SrTiO3晶体具有较高H2产量的原因机理.     

Er掺杂金刚石缺陷的第一性原理研究

本文以第一性原理计算为基础,研究了Er掺杂后金刚石的电子结构、能级跃迁及N、B原子共掺杂对金刚石Er相关缺陷的影响。首先对Er掺杂后金刚石的形成能与结合能进行计算,结果表明掺杂后的稳定结构为Er原子周围存在1个空位的结构,稳定价态为+3价。然后预测零点跃迁能(ZPL)是0.807 eV,对应激发的光子波长为1 536.289 nm。最后对N、B原子共掺杂计算,结果表明N、B原子的掺入可以使形成能降低,增加结构稳定性。Er掺杂金刚石使其在近红外光谱发光,为Er金刚石色心的应用提供了理论依据。     

理论设计ZnTiS3化合物光电性能的第一性原理计算

根据实验上合成LiNbO₃(LN)构型的ZnTiO₃铁电化合物,基于第一性原理的方法设计研究了化合物LN-ZnTiS₃(LN构型)的特性。计算结果表明LN-ZnTiS₃化合物满足力学稳定条件。根据化学势平衡相图分析,LN-ZnTiS₃在常压下不会形成稳定结构,但施加外部压力可以形成稳定结构。电子态密度和带隙的计算结果表明,LN-ZnTiS₃的价带顶(VBM)主要由S-p轨道组成,导带底(CBM)则由Ti-d轨道组成,硫原子的替代可以促进体系费米能级以上的电子状态大幅度下降到较低的能级,从而减小LN-ZnTiS₃的带隙。LN-ZnTiS₃的带隙计算值为1.04 eV,可以促进可见光的吸收,可以看出LN-ZnTiS₃是一种潜在的高效率光伏材料。     

Bandgap characters in GaAs‐based ternary alloys

The existence and origins of the bowing character in the bandgap variation of GaAs‐based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp³s * tight‐binding (TB) method with, and without, the inclusion of the spin‐orbit coupling effects, and the first‐principle full‐potential linear augmented plane wave (FP‐LAPW) technique are applied on both the common‐cation GaSb_xAs_1‐x and the common‐anion Ga_1‐xIn_xAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common‐cation alloys (GaSb_xAs_1‐x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made‐available‐cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common‐anion alloys (Ga_1‐xIn_xAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

六钛酸钾(K2Ti6O13)晶须几何构型、能量及电子结构的第一性原理计算

本文采用基于密度泛函理论的第一性原理计算方法,对具有单斜结构六钛酸钾( K2Ti6O13)晶体的几何构型、能量及电子结构进行系统研究.计算所得K2Ti6 O13晶体的晶格常数、原子位置等晶胞参数均与实验值吻合,且形成热与结合能的计算结果显示该晶体具有较高的相结构稳定性;进一步的电子结构分析表明,K2Ti6O13晶体呈现具有间接带隙的半导体性质,在K2Ti6O13内部,Ti-O间成键作用明显强于K-O,而K-Ti间却未明显成键,且Ti(d)与O(p)轨道电子间较强的共价键相互作用直接决定了K2Ti6O13晶体的相结构稳定性.     

In原子替位位置对新型正交GaN影响的第一性原理研究

铟(In)原子替位位置对开发新型正交GaN的储氢材料具有重要意义。当前关于In原子替位位置对正交GaN材料的影响研究相对薄弱。本文基于第一性原理研究了不同In原子替位位置下InGaN材料的形成能、电子结构、弹性特性和力学稳定性。结果表明,通常情况下间隔三个原子的In原子替位位置的形成能最小且该体系最易形成。在相同的掺杂情况下,该结构的InGaN材料也具有较大的带隙宽度以及较小的弹性模量、体积模量、剪切模量与弹性模量,这意味着其抗压能力、抗剪切应力的能力较弱,韧性以及硬度较低。此外,声子谱计算结果表明,间隔三个原子的InGaN材料在环境压力下也具有良好的力学稳定性。本研究为正交GaN的新型储氢超材料的研究提供了理论依据。     

Crystal structure and spectroscopy of a hydrogen-bridged one-dimensional Cu(II) complex containing both octahedral and square pyramidal geometries in the same unit cell

Single crystals of the helical hydrogen-bridged one-dimensional Cu(II) complex, [Cu(stpy)₂(CH₃COO)₂(H₂O)₂] (1) [Cu(stpy)₂(CH₃COO)₂(H₂O)] (2), are prepared and characterized by elemental and thermal analyses, IR, electronic and X-ray crystal structure determination. The crystals are monoclinic, of space group C2/c, with unit cell parameters a = 31.842(7) Å, b = 5.9829(10) Å, c = 30.970(14) Å, = 111.78(3)°, Z = 4. The asymmetric unit contains two different types of Cu(II) polyhedra, namely, octahedron and square pyramid within the same unit cell. 1 has elongated octahedral geometry with two nitrogen atoms from stpy and two oxygen atoms from synmonodentate acetate ligands, transcoordinated to Cu(II) in the basal plane. The oxygen atoms of the two water molecules occupy the axial positions. 2 has Cu(II) coordination polyhedra similar to 1, except that only one of the apical positions is occupied by a water molecule. The structure consists of two independent linear chains, one involving octahedral (1) and the other involving square-pyramidal (2) polyhedra, held by hydrogen bridges. The Cu–Cu intra- and interchain separations in both 1 and 2 are 5.983 and 8.214 Å. The unit cell packing shows weak -stacking between adjacent coordinated stpy ligands in the chain, resulting in ladder-type structure. Further, the extended packing reveals helical arrangement of Cu(II) polyhedra in the lattice.     

PbWO4晶体中铅氧空位对对晶体光学性质的影响

本文利用完全势缀加平面波局域密度泛函近似,按照能量最低原理采用共轭梯度方法,对含铅氧空位对的PbWO4晶体进行结构优化处理.在此基础上,计算了含铅氧空位对的PbWO4晶体的电子结构、复数折射率、介电函数及吸收光谱,并与完整的PbWO4晶体进行了比较.结果表明:PWO晶体中铅氧空位对的存在对PbWO4晶体的光学性质没有显著的影响.