it is possible in practice as well, with a very well colimated X-ray beam on the source and detector side  (like the one from a synchrotron, from example - in this case the intensity in the beam is still high enough for performing the experiment). another advantage of the synchrotron beam would be its energy (wavelength) tunability. So by increasing lambda (decrease E), the 2th angles will also increase, so one might not need to go so low in 2theta

It might be that also some lab setups with careful beam conditionning and small detector slits will also going lower than 1 deg if needed ...

Regards

     cristian

We use a Bruker D8 Discover X-Ray Diffractometer in transmission configuration to make routine analyses starting from 0.15-0.2 degrees. So it is possible to make such powder diffraction analyses without access to a synchrotron source..

By using a double crystal analyzer between your sample and detector, you can reach extremely low angles. Down to a Q of 10-4!

Some very good answers here already.

If you are using a laboratory diffractometer you might also consider changing to a longer wavelength anode such as Cr K-alpha, which will shift the diffraction peaks to higher 2theta compared to with Cu K-alpha radiation, and avoid overlap with the direct beam. But perhaps you are doing so already? Could you provide some detail as to what equipment you are using?

Following what has written Edward Andrew Payzant I would like to call the attention on Bragg equation λ = 2dsinθ. What is fixed is the interplanar spacing d; for a given value of d, larger is the wavelength λ larger is θ. One should always refer to d values and not to  θ values as unfortunately is frequent to find in papers even in top class journals. Therefore, even with a "normal" diffractometer it is possible to see at accessible angles a diffraction peak by using a longer wavelength.

True, Giovanni, however as the wavelength increases a number of things become more problematic:

Small scattering volume, large absorption

Dynamical effects become more pronounced

windowless setup is needed (i.e. no Kapton windows).

d is definitely an improvement over θ, but I'm a Q man myself.

One man gathers what another man spills.

I think the peak that Raul - Augustin Mitran got using Bruker D8 Discover X-Ray Diffractometer, was from  primary beam. 

Wow. That's a bold statement to tell someone they don't know if they are measuring the direct beam or the scattered beam. Ouch!

Sorry. However, in the case of 106 cm Sample to detector distance (using Bruker D8 Discover X-Ray Diffractometer) the primary beam produces a strong peak about 2θ of 0.1 degrees

Mohamad Amin, this is my experience too with the direct beam producing a peak up to ~ 0.18 degrees with a Cu Kalpha1 source. So I would say that it is possible to measure most ordered mesoporous silicas with such a setup. Some of the SBA-15 materials with the largest reported pores have d100

Thank you Raul-Augustin Mitran for your reply. Yes, it is possible to characterise 2θ peak of about 0.3 degrees when you use 106 cm Sample to detector distance. Theoretically, you can characterise 2θ peak 0.08 to 3.2 degrees using this high distance, but in practice the primary beam produces a strong peak about 2θ of 0.1 degrees.

Raul,

don't you need some extra attachments for you instrument, like knife-edge collimators? I don't know if they were manufactured for Discover series. Also, a primary Göbel-mirror and a grazing incidence Soller (for 0D detectors) could improve your set-up... Maybe you work with capillary stage? As far as I know, on powder specimens below 1°(2Th) with laboratory XRD you don't get information on your crystalline compounds, and for small angle scattering (for nano- and mesoporous structure investigation) the "routine" diffractometer set-up is not applicable. Correct me if I'm wrong. Regards,

Feri

Dear Dr. N. Venkatathri,

It's a nice problem. It is possible with certain XRD system(which is depend on detector size and Goniometer space for movement  of the detector etc). I have used XPert MPD, Philips system (NIT Rourkela). If  I am not wrong some of your colleagues also taking data from the same XRD. Using the existing system I have started 0.2-0.5 deg with special care like using lowest divergence slit (0.5 deg or less) and receiving slit (programmable receiving slit PRS=0.02 deg.). I think presently, it is bit difficult to set that PRS value. I am not sure, it better to discuss with Prof. S. C. Mishra, HOD of Metallurgical Engg and Materials Centre, NIT Rkl., He is also an expert in X-ray physics.

With best regards

B. Mallick

Dear Feri,

I do use a focusing Göbel-mirror and 0.5 mm collimator, with the possibility of using a circular divergence slit . The LYNXEYE detector from Bruker can also work in 0D mode, although I did not find this necessary so far.  I don't know of any crystalline material that has diffraction peaks below 1 degree 2theta. For ordered mesoporous materials (defined as having pores between 2-50 nm), such as SBA-15 or MCM-41, you get diffraction peaks from the ordering of the pores. These can be readily measured, provided they do not overlap with the primary beam. On occasion, I have been able to also measure scattering peaks, although the background was very high (for example as the SAXS data in Schmidt-Winkel, P.; Lukens, W. W.; Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D., Mesocellular Siliceous Foams with Uniformly Sized Cells and Windows. Journal of the American Chemical Society 1998, 121 (1), 254-255).

Of course, we are open for any collaborations.

Best wishes,

Raul

I underline once more that quoting θ values without giving the value of the used wavelength is meaningless.

Yes I agree with Ferraris. Higher wavelength (such as Co, Fe, or Cr K-lines) would shift the angular positions of the diffraction lines upward and give better resolution in principle. Smaller number of diffraction lines are however observed in the same angular range. Mo K-alpha for example gives greater number of peaks in a given scan, but leads to porer resolution, especially at low angles.

I agree very much with Giovanni. In precious opal, colloidal SiO2 particles (spheres of about 0.5 um) are packed to form the well known ccp or hcp structures. These structures have been studied using visible light which is about the same length scale, and the colour of opal in white light is due to diffraction. Just google on opal diffraction and you will eventually find that Bragg's law has to be modified introducing the refractive index for visible light (not 1 as for X-rays) and Snellius. Maybe some of this is of use for your problem, you should at least be able to check the unit cell hypothesis.