K alpha and K beta intensity is completely depends on metal target that is Copper where as c/a ratio is completely depends on the sample what u keep for charecterization. 

I believe K alpha 1 to alpha 2 is always 2:1

please do not judge it by the peak height - the intensity = the integrated area of the peak.

Dear Wei,

I am not judging the exact intensity by the peak height (but this might be OK for just rough estimation, if the FWHM are similar, and usually it is so for Kalpha1 and Kalpha2 peaks...).

But anyway - when I refine the profile (just LeBail fit) with a fixed Kalpha1:Kalpha2 ratio of 2:1, then the fit is poor, and Kalpha1 peaks are underestimated, while Kalpha2 peaks are overestimated. When the Kalpha1:Kalpha2 relative intensity is allowed to be refined, then the fit improves (but still I am not fully satisfied), and such refined ratio is 3.045:1.

Could it be a problem with the measurement?

You may need to work carefully with the peak shape function, but keep Kalpha1:Kalpha2 ratio of 2:1.

Hi Tomasz,

Sure, there is something wrong. The ratio between Kalpha1 and Kalpha2 must be 2:1. I think this is a bad monochromator adjustement. You have to check where the monochromator is located: if you've got Kalapha1 and Kalpha2 on your diffractograms, it means that's you've got a secondary monochromator (or eventually a Kbeta filter but in this is case you won't have any ratio trouble) placed on the diffracted beam, between your sample and the detector.

The first thing to do is to do a new measurement with a standart like Si or LaB6 powder and check the ratio between Kalpha1 and Kalpha2. If it's the same as your experiment, it's really a monochromator adjustement problem.

Thanks a lot Eric!

I have just asked for the measurement of a standard.

The integral intensity of CuKa2 is about 51.3% of the a1-Signal, not exactly 2:1. Also the line width of a2 is a little bit higher than the width of a1. This is caused by the different lifetime of the different L-states after loosing an electron in the K-shell. But this is not sufficient different from 2.1 to explain your ratio of 3:1.

As others already explained this is most probably caused by a misaligned monochromator. Do you use a focusing primary beam monochromator (SiO2, Ge, ...)? Is the behaviour the same at low and high Bragg angles? If this is not the case also sample preparation may cause spurious effects.

None of these explanations clarify the huge difference in relative intensity. I'd like one of you to post a diffractogram demonstrating 2:1 ratio when convenient.

Here is my experience using only a Ni filter on a Cu tube and a military grade Quartz Transverse Field Oscillator mono-crystal. I used the simple Bragg spectrometer arrangement to evaluate the instrumental resolution. Meaning, beam exiting the tube with only a Ni foil to go through and then impinging on the sample surface directly. Also included is an example of high quality GaP monocrystal with a Brinell indentation and various grid abrasive test. Similar ratios were observed in the case of GaAs and other Monocrystals as well. The ratio seem to be similar (certainly not 2:1) for all the monocrystalline samples we considered. I've always wondered about this discrepancy myself :-) What do you think?

We actually used the "fixed" separation between KAlpha 1 & 2 in these diffractograms to precisely calibrate the Omega angular range in each case. Both these were optimized Omega (decoupled) scans. In these cases even the integrated intensity computation may not yield the 2:1 ratio. I'd like to see a diffractogram demonstrating this 2:1 ratio if available. Thanks!

The fact that most of you are discussing polycrystalline samples should NOT make that much of a difference. Should it?

https://www.flickr.com/photos/85210325@N04/7852409924/in/album-72157632724063041/

https://www.flickr.com/photos/85210325@N04/9561740689/in/album-72157632724063041/

Dear Tomasz,

Can you tell for which material did you obtain this XRD pattern? Are you sure that you have a linear Y-axis in the presented powder XRD pattern (numerical Y-data are not seen in the figure)?

Regards, Biserka

by eye I think everbody would say: misaligned mono and, probably, also old tube. The shape of the profiles is rather odd with the wrong ratio and also with some peculiar tails (like a broad component would be under each peak). There is actually no Ka1 and Ka2 with a ratio of 2:1. Already Robert Haberkorn pointed out that the ratio should not be perfectly 2:1. Actually, the published studies show that the pure emission profile from the target element is rather complex and it can be described by using multiple peaks (5 usually suffice for CuK). When you use a monochromator you simply introduce a band-pass filter (a smooth window, not a sharp one) to trim the emission of the tube. If you do not center that filter properly, you simply cut too much on one of the sides of the emission multiplet. You can choose how to trim, so in principle you could eliminate a larger fraction of the Ka2 component or have ratios that are quite far from 2:1 going towards a 1.1. Working with the 2:1 guarantees that the shape of the emission profile remains sufficiently close to the original one (easier to model!) as the band-pass window is centered roughly in the middle of the emission doublet. If you trim one of the sides you slightly change the shape and as a consequence the apparent relative spacing of the components differs from the expected one. The diffractometer is a complex object and you can experience slight variations in the relative ratio, distance and shape of the two components by varying the angle. Remember you have a specimen (flat and with its angle-dependent absorption), an instrument (with angle-dependent aberrations) plus the secondary crystal (the mono) with its own mosaicity!

Are you using an energy discriminator on the detector? If so then that could also be something to check in case the energy window is set too small.

Dear Tomasz,

Did you get XRD pattern of the standard? Please, let us see it.

Best regards,

Biserka

"misaligned mono and, probably, also old tube"

By "using an energy discriminator on the detector", how?

Why aren't (appear) the Kα1: Kα2 ratios in the case of mono-crystals like Quartz, GaAs, GaP, Si, and most others not the veritable 2:1 (51.3%) in all of our observations? Even for InP, AlGaAs, CdTe, you name it! I'm yet to see experimentally this ratio anywhere close to 2:1. This must be some sort of a "powder thing" :-) Most of our observations were for the fully "optimized" condition where the (hkl) plane observed was parallel (if not coincident) with the "diffractometer axis" (rocking axis). My sample in this case was the "monochromator" after the Ni filter. The use of a "monochromator" in Tomasz's case would have potentially eliminated the Kα2, wouldn't it?

In reality, I never paid attention to it a whole lot. Now I will. In the past I never checked the "Integrated Intensity". Visually it doesn't seem to be anywhere near the 50%. I'll check both peak intensity and integrated intensity values from henceforth. However, most of the XRD instrumentation these days appear to have pretty good beam conditioners and not much trouble attenuating the Kα2 component. I never see the Kβ these days. I suppose I ought to delve into powder diffraction with 2D imaging concepts :-) Any of you gentle folks engaged in such activities? Please share your experiences :-)

I would like to mention that,

• The vertical scale is not displayed in the diffractogram. Linear or Log? Please clarify. Based on the discussion I'd assume "linear".
• What about displaying the "known standard" diffractogram in the same reciprocal space range (2θ range) with the same instrument and optics? Couldn't be that hard, could it? A quick scan to estimate the Kα1: Kα2 ratio for any NIST SRM would reveal a whole lot immediately, wouldn't it? This is a practice blatantly missing from a significant amount of XRD studies in literature. Certainly on RG.  What is your excuse for not having it before sending us all in to an interesting synergistic pontificatory expedition? I'm not complaining, just curious :-)

Cheers!

Joachim the idea is good but the difference in the wavelengths is too little to give any appreciable effect. Moreover there is no element that has an absorption edge in between Ka1 and Ka2 maxima. I know.that patterns at Ka and Kb have been used in the past to profit of a difference in absorption.. and this is the way the Kb filter works

Ravi too often XRD is seen as a service and not as a proper analytical technique and not always the technician that runs the machine is an expert in it.

Hi Tomasz,

Have you specifically stated that you do have an primary beam monochromator?  I hope so beccause it sure looks like it is simply set to incompletely eliminate kalpha2.

Something no one else has mentioned, yet is that at high angles the kalpha1 and 2 can be separated by enough to have different Lorentz polarization Factors, but nowhere nearly as big as the effect you see, and not at the angles you are looking at.  But it does change it from the expected ratio output by the tube.

Mostly, pay attention to Matteo Leoni :)

Dear Tomasz,

Please do show us the XRD pattern of any other powder sample collected by your XRD equipment. What kind of detector are you using? Do you have a pulse-height discriminator? Looking forward for further details from you,                 

With regards, Biserka

Thank you all for the answers in this topic. I do not know what caused the problem physically, as I work distantly just on the data refinement. Anyway, measure the same sample using another diffractometer gave refineable results. :-)

What was the difference?

Dear Ravi,

Now the ratio of Kalpha and Kalpha2 peaks is visually around 2:1, and Rietveld refinement is possible.