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排序方式: 共有148条查询结果,搜索用时 22 毫秒
1.
P. Ramachandra T. Siva Rama Krishna Gautam R. Desiraju 《Journal of Chemical Sciences》1989,101(4):327-334
The compound 3-hydroxy-6-(4′-nitro)phenylazopyridine (1) was observed to incorporate several organic solvents of crystallisation, forming stable clathrates. These clathrates decompose
upon heating and the host material is chemically transformed. Single crystal X-ray diffraction studies on the ethanol clathrate
lead to a reasonable model for the host structure. But the guest molecules are severely disordered. Alternatively, the compound
forms twinned crystals. The chemically closely related compound2 does not form clathrates. 相似文献
2.
Dmitry A. Zyuzin Svetlana V. Cherepanova Elena B. Burgina Victor G. Kostrovskii 《Journal of solid state chemistry》2006,179(10):2965-2971
Genesis of the structure of zirconia particles prepared by precipitation of amorphous hydrated zirconia by ammonia from the ZrO(NO3)2 solution followed by a mild hydrothermal treatment (HTT) of precipitate, washing and calcination under air up to 1000 °C has been studied by X-ray diffraction (XRD), Raman and FTIRS. As revealed by FTIRS of lattice modes, the local structure of amorphous zirconia subjected to HTT is close to that in m-ZrO2. This helps to obtain nearly single-phase monoclinic nanozirconia (particle size 5-15 nm) already after a mild calcination at 500 °C. Stability of this phase with nanoparticles sizes below the critical value determined by thermodynamic constraints is due to its excessive hydroxylation demonstrated by FTIRS. Dehydroxilation and sintering of these nanoparticles at higher (600-650 °C) temperatures of calcination leads to reappearance of the (111) “cubic” reflection in XRD patterns. Modeling of XRD patterns revealed that this phenomenon could be explained by polysynthetic (001) twinning earlier observed by HRTEM. 相似文献
3.
Henning Henke 《无机化学与普通化学杂志》1993,619(2):311-317
Crystal Structure and Pseudosymmetry of a New Modification of Potassium Hexachloroniobate(V), KNbCl6. Comments on the Cubic Phase Long needles of KNbCl6 – invariably twinned around [100] – are obtained if the material is crystallized from SOCl2 solution. The structure has been determined from X-ray data collected with a single-crystal diffractometer at room temperature [space group P21/n, Z = 16, a = 6.894(1), b = 22.073(4), c = 23.337(3) Å, β = 91.00(1)°, R = 0.032 for 2 909 unique reflexions, 290 structural parameters]. Distorted NbCl6? octahedra and ?interstitial’? K+ ions are found to form similar arrangements, each of them corresponding to a closest packing of spheres with the layer sequence ACAB (stacking symbol hc). The resulting asymmetry in coordination by potassium is coupled with a strong off-centre displacement of the Nb atoms in any of the four independent chlorine polyhedra (0.14 Å on average). A pronounced pseudosymmetry accounts for the twinning. Since P21/m21/n21/b (no. 62) is already a good approximation of the real structure, only one formal step of symmetry reduction (index t2) is needed to create both, the observed twin law and the actual space group P1 21/n1. Above 180°C a reconstructive phase transition leads to the ‘face-centred cubic’ modification with ~ 10% lower density. 相似文献
4.
The effect of impurities on the growth of manganese carbonate crystals has been studied. Stereophotogrammetric analysis showed
that all crystal faces have the same Miller indices
and the peanut-shaped forms are twins with twin axis 0001. 相似文献
5.
Nanoparticles usually exhibit pronounced anisotropic properties, and a close insight into the atomic-scale deformation mechanisms is of great interest. In present study, atomic simulations are conducted to analyse the compression of bcc nanoparticles, and orientation-dependent features are addressed. It is revealed that surface morphology under indenter predominantly governs the initial elastic response. The loading curve follows the flat punch contact model in [1 1 0] compression, while it obeys the Hertzian contact model in [1 1 1] and [0 0 1] compressions. In plastic deformation regime, full dislocation gliding is dominated in [1 1 0] compression, while deformation twinning is prominent in [1 1 1] compression, and these two mechanisms coexist in [0 0 1] compression. Such deformation mechanisms are distinct from those in bulk crystals under nanoindentation and nanopillars under compression, and the major differences are also illuminated. Our results provide an atomic perspective on the mechanical behaviours of bcc nanoparticles and are helpful for the design of nanoparticle-based components and systems. 相似文献
6.
7.
A phase field theory for coupled twinning and fracture in single crystal domains is developed. Distinct order parameters denote twinned and fractured domains, finite strains are addressed and elastic nonlinearity is included via a neo-Hookean strain energy potential. The governing equations and boundary conditions are derived; an incremental energy minimization approach is advocated for prediction of equilibrium microstructural morphologies under quasi-static loading protocols. Aspects of the theory are analysed in detail for a material element undergoing simple shear deformation. Exact analytical and/or one-dimensional numerical solutions are obtained in dimensionless form for stress states, stability criteria and order parameter profiles at localized fractures or twinning zones. For sufficient applied strain, the relative likelihood of localized twinning vs. localized fracture is found to depend only on the ratio of twin boundary surface energy to fracture surface energy. Predicted criteria for shear stress-driven fracture or twinning are often found to be in closer agreement with test data for several types of real crystals than those based on the concept of theoretical strength. 相似文献
8.
9.
The microstructure of a Fe–Mn–Si–Al twinning-induced plasticity (TWIP) steel exhibiting remarkable work hardening rate under uniaxial tensile deformation was investigated using transmission electron microscopy to uncover the mechanism(s) controlling the nucleation and growth of the mechanically induced twins. The results show that the stair-rod cross-slip deviation mechanism is necessary for the formation of the twins, while large extrinsic stacking faults homogenously distributed within the grains could act as preferential sources for the activation of the deviation process. The influence of such features on the thickness and strength of the twins and the resulting mechanical behaviour is discussed and compared to similar works recently performed on Fe–Mn–C TWIP steels. 相似文献
10.
Phase‐pure ammonium catena‐polyphosphate IV [NH4PO3]x was synthesized by heating NH4H2PO4 in a tube furnace under an ammonia gas flow. The product contained single crystals of [NH4PO3]x IV appropriate for an X‐ray structure determination enabling structure refinement of this compound. The pseudo‐merohedrally twinned crystals of [NH4PO3]x crystallize in the monoclinic crystal system (P21/c, no. 14, a = 2270.3(5), b = 458.14(9), c = 1445.1(3) pm, β = 108.56(3)°, Z = 4, 2264 data, R1 = 0.076). In the unit cell the catena‐polyphosphate chain anions run parallel [010] with a chain‐periodicity P = 2 and a stretching factor fs = 0.94. The chain anions are interconnected through extensive hydrogen bonding towards the ammonium ions. Due to ‘chemical twinning’ a novel catena‐polyphosphate structure type is realized in [NH4PO3]x IV. The vibrational spectra of [NH4PO3]x IV are reported as well. 相似文献