使用EXCEL进行数据分析.ppt

衍射花样指数化
The first step in analysing unknown powder pattern is often an attempt to find a unit cell that explains all observed lines in the spectrum. You do not need additional crystallographic data, although if it exists it makes for faster and more reliable results. The material to be analysed must be single phased and the experimental material must be very accurate.
Indexing programs use only the positional information of the pattern and try to find a set of lattice constants (a,b,c,a,b,g) and individual Miller indices (hkl) for each line. The form of equations to solve plicated for the general case (triclinic) in direct space but is straightforward in reciprocal space. In the latter the set of equations is:
Q = h2A + k2B + l2C+ hkD + hlE + klF
where the Q-values are easily derived from the diffraction angle Q. This set has to be solved for the unknowns, A, B, C, D, E, F, which are in a simple way related to the lattice constants. Finding the proper values for the lattice parameters so that every observed d-spacing satifies a bination of Miller indices is the goal of indexing. It is not easy even for the cubic system, but it is very difficult for the triclinic system.
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
Reciprocal Space
Direct space posed of unit cells and its contents, wh