Expanding my XRD applications – Q&A

Because the geometrical reason for X-ray diffraction (XRD) during the measurement, the diffracted beam must be considered from the incident X-ray beam. Please refer to the theory of XRD and Bragg’s law.

The slides were prepared to give you more understanding about XRD theories. So, the boxes in slide 12,13 were unit cell, and the others were grain/crystallite of the sample in slide 13-18.

We could plan one if there are many requests.

Firstly, we need to do a Search/Match to judge what phases (minerals) are present. Then we could start a Rietveld refinement to quantify them, and as a result, divalent iron can be calculated based on chemical composition and wt.%.

Yes, it is a spiking method. If there are no standard/reference samples, we can do the math according to the ICH guideline. That uses signal and noise.

You can refer to “Vegard’s law” which describes that peak position shifts as a function of how much dopants were substituted by seeing a peak shifting from the non-doped sample. There is some linear correlation between doping ratio and shifting.

A peak broadening is observed when particle size is close to nano range. And because X-ray penetration depth of the sample can be limited against large metallic particles having a greater X-ray absorption coefficient.

I do not think so. Theoretically, the micro-strain starts from zero. But, if you measure your sample using residual stress method, you can derive + and – value of stress by measuring one (hkl) plane with a various tilt in omega or chi to monitor peak shifting.

CHC and MHC+ chambers are designed for controlling humidity and temperature for organics. But, TTK600 does not have any humidity sensor and humidifier in the system, but you can still insert humid gases by connecting hoses and flask, then flow inert gas into the water of flask. It is kind of indirect way.

It sounds like your sample is consisted of two similar structures. Even though those phases are chemically different from each other, they could be identical in crystallographic properties including crystal system, space group and unit cell parameters. This is called isomorphs which is the most difficult job for XRD. Example: Steel304 vs Steel403, LiNiMnO2 vs LiMnCoO2, Fe vs Fe: Co…

In Bragg-Brentano geometry, you cannot because the X-ray will penetrate whole layers of the sample if the thickness is below several tenth of micrometres.

So, you need to use a Parallel beam geometry instead. You can fix the incident beam angle (omega) during the measurement around 1~2 degrees, then you can only move detector in 2 theta. The appropriate incident angle to capture a very thin skin layer of your sample is depending on a) incident angle, b) density of sample, c) mass absorption coefficient and d) layer thickness in accordance with Lamber-Beer equation. You can vary the omega angle (fixed value) to get a depth resolved XRD patterns. It is called GI-XRD (Glazing Incidence X-Ray Diffraction).

By finding peaks and fitting a profile. HighScore software will make your job easier. Feel free to contact us.

It could be covered in a separate session in the future. It indeed takes half a day.

It is hard to derive structural information from the amorphous material through XRD instrument. By the way, the peak position of the hump is related to the original crystalline phase of that material. So, organic compounds show their peak position at around 5 to 20 degrees while inorganic compounds have 20 to 50. Metallic is far away from them, 35 to >90. If you really want to extract more structural information of your amorphous sample, you can refer to PDF (Pair-Distribution Function) Analysis that can also be done by our XRD equipped with Ag-tube and some specially required parts. Once you measure amorphous or disordered material by PDF technique, you finally get the boding lengths between atoms from the short-range orderings even they are not crystalline.

If two amorphous phases have a similar chemical composition, then we have no chance to distinguish them due to the humps that appear at the same 2θ position. However, if they are quite different in chemistry, we may see two separated humps at different positions.

There are several ways to do that in HighScore software. If you can add a certain amount of known crystalline powder into the sample, we could use an “internal standard method” by using Rietveld algorithm. If adding is not allowed, then we could use an “external standard methods” by measuring 100% crystalline materials with very similar chemical composition as a standard. By doing so, we will get some constant to calibrate it and calculate amorphous contents of the unknown material.

They are all rhombohedral structure in crystal system, and the space group also same. So, you can distinguish them by seeing peak intensity ratio of (003) and (104) for example. The best way is measuring XRF. Please get more info from us.

HighScore Plus from Malvern Panalytical

You can use any commercial XRD software like our HighScore software that helps you to extract FWHMs easily.

Yes, right. But those factors can be handled and corrected by software afterwards. More serious issue would be inhomogeneity of the powder sample. So, keeping a homogeneity is key by mixing them well or measuring many samples to get an average. 

In case of mixture sample, you need to be careful about the refinement of any atomic coordinates when the peaks are overlapped and/or other factors are influencing the intensity as well. So, do the sample prep and measurement as perfect as possible to get a randomly oriented PXRD pattern. The best way is to use K-alpha1 system and fill your powder into the glass capillary for the Debye-Scherrer geometry in order to ignore preferred orientation effect. 

If the dopant is smaller than the host ion, the peak will be shifted to the higher angle side, vice versa. Moreover, if the dopant deteriorates the host lattice, the peak widths will be broadened with respect to the influences of the dopant.

Yes or No. It depends on the case. Can you tell us a bit more information?

We need to separate your request into two: firstly, dislocation density in the metals/alloys could be handled by powder guys, but residual stress could be covered by metallurgy team who are specialized in stress/texture analysis.

No. You can also use SC-XRD (Single Crystal XRD) or TEM. But it is no doubt that the easiest way is use of PXRD. The advantages of PXRD is giving you short measurement time, minimum sample preparation and representative result in statistics compared to the others.

Those are the same.

It strongly depends on the sample quality and measurement conditions. But, in mathematics, the peak intensity must be higher than 10 times of standard deviation of the background. Then it can be recognized as a peak and quantifiable.

We need to say in weight fraction. So LoD (limit of detection) would be 1 wt.% as a best.

Depends. But the smoothing is always not recommended for analysing a PXRD pattern due to the mistreatment.

If you are talking about a single phase, it would be relatively similar. But every single reflection has its own directions. So, same size of crystallites will not be obtained. That is why we use an average value to ignore that effect.

Yes. Because the peak intensity is a function of number of electrons at the certain lattice plane. 

Because a peak at higher angle represents a small distance between atomic planes. If some defects existed in your lattice, overall structure would still be okay. But local part is not. 

At very least, 3 peaks are needed. But, recommend getting more than 5. 

Usually, we need to say that it depends on a weight fraction since scattering power of X-ray is related to the number of electrons. 

In general, below 0.02 deg would be considerable. But make sure that what is the angular accuracy or reproducibility of your instrument. We have 0.01 deg in error.

Due to the presence of air, and some reflections from the sample and holder surface.

At higher angle, you can see more small changes in the lattice, for example, local distortion/inhomogeneity in atomic position/defects (dislocation, point or screw defects, stacking faults, anti-domain, and so on)

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