第6章 衍射花样指数化与应用概述解析.pptVIP

衍射花样指数化 There are two general approaches to indexing, the exhaustive and the analytical approach. Both of these approaches require very accurate d-spacing data. The smaller the errors, the easier it is to test solutions because there are often missing data points due to intensity extinctions related to the symmetry or the structural arrangement or due to lack of resolution of the d-spacing themselves. The earliest approaches were of the exhaustive type and were done by graphical fitting or numerical table fitting. Indexing Programs The methods currently implemented are shown bold. They are selected through the item Indexing in the main menu. Program Author Type ITO Visser analytical TREOR Werner exhaustive POWDER DICVOL CUBIC The programs use a set of common parameters, e.g. the wavelength and a method specific set. After you have clicked on a program with the left mouse button indexing is immediately started with the active parameter set. Depending on the problem and computer type the program run can take from seconds to many minutes. All solutions are computed and stored internally. The best one is displayed at the top of the screen, the next to best at the bottom. For an overview of all solutions select Solutions in the main menu. Miller Indices Triplet of integer numbers uniquely assigned to a Bragg reflection. The notation is usually in the form of (hkl). Formally spoken, Miller Indices represent coordinates of lattice points in reciprocal space. Reciprocal Space Direct space is composed of unit cells and its contents, whereas reciprocal space is a lattice whose lattice points are Bragg reflections. Direct and reciprocal space are tightly coupled. Lattice constants A set of maximally six floating numbers representing the unit cell. As crystal symmetry grows the number of lattice constants needed to describe the metrics of a unit cell reduce. In the cubic system there is only one constant. 衍射分析应用 物质对射线的衍射产生了衍射花样或衍射谱，对于给定的单晶试样，