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Difficulties in indexing monoclinic crystalline materials by wide angle X-ray diffraction

Xian Chen's picture

For monoclinic polycrystalline materials, how can we uniquely index the X-ray diffraction spectrum so that we can calculate the accurate lattice parameters a, b, c and beta? Due to the fact that there are four unknown variables coupled to determine d-spacings, we can not easily read out the lattice parameters until enough diffraction peaks positions are obtained correctly. This leads to two questions

1. How do we get the full diffraction peaks from a monoclinic polycrystalline sample if it is partially textured?

2. Even if we got more than four peaks, how do we correctly index the pattern so as to calculate four lattice parameters a, b, c and beta? Are there any tricks to index monoclinic sample, especially when the monoclinic structure is modulated along c axis?

 PS: we use TOPAS software to refine the peaks positions which will give a guess of indexing. But too many suggested results come from the software, how do we identify which solution is the correct one? 

Thanks 

i am not very clear about the monoclinic material. I mainly study diamond cubic, fcc and hcp, which are more specific than monoclinic.

you said there are many results from software. i don't know about topas, i use a opensource software lauetool to get the indexation. in lauetool, to select the best indexation, on just generate a simulated pattern, then use a norm to quantify the discrepencies between the simulated one and experimental one. the smaller the better 

as for textured sample, i don't know the size of the beam you use. i use very small beam(the scale of 1micrometer), so texture do not cause much trouble

Xian Chen's picture


Thank you very much! I will download lauetool and try to do some simulation. but there still exists difficulties to choose simulated
pattern.


TOPAS can generate possible indexing based on the given peak positions and assumed space group.


For monoclinic system there are lots of (hkl) planes satisfying reflection conditions, therefore resulting in many many peaks in simulated pattern. And most of them don't have enough intensity but will appear in the output peak list.


It is hard to say which solution is the best because not every peaks can be detected during 2theta scan. Although many simulated indexing can fit the observed peaks quite well, there are quite a few peak positions left out and not matching anything.


Are there any method specific to analysis monoclinic system? Do we need to prepare single crystal measurement?

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