check the phase diagram! If the phase diagram tells you have full miscibility then Vegard's law usually works. The miscibility depends largely on the cation sizes and their preferred coordination. If you have access to the ICDD PDF4+ database/software you can also extract those data for quite a number of mixed series.

In your case however, you are dealing with a film: the shift can also be caused by the presence of residual stress. Even if you have an amorphous substrate, you can easily have a columnar film (like yours probably). So my suggestion is to check your stress and texture: if you have a 4 circle machine it is easy, otherwise try at least a few measurements at different omega tilts. You should see the peak(s) shifting and changing in intensity

Dear Sir,

depending on the way of "alloying" of both materials, one may think of

Vegards law (but I´m not sure wether it holds for oxides to

the same extent as for metals) .

Regards, FM

check the phase diagram! If the phase diagram tells you have full miscibility then Vegard's law usually works. The miscibility depends largely on the cation sizes and their preferred coordination. If you have access to the ICDD PDF4+ database/software you can also extract those data for quite a number of mixed series.

In your case however, you are dealing with a film: the shift can also be caused by the presence of residual stress. Even if you have an amorphous substrate, you can easily have a columnar film (like yours probably). So my suggestion is to check your stress and texture: if you have a 4 circle machine it is easy, otherwise try at least a few measurements at different omega tilts. You should see the peak(s) shifting and changing in intensity

the shift may be caused due to the 20% of oxide adding to your virgin 80% oxide.. the foreign paricles(20% oxide) may be interacting and affecting on the Anamorphicity nature. shifting to higher degree may be due 20% oxide is tending the 80% oxide. in other words the added oxide may be having the peak toward higher degree but due to bonding between host oxide and added oxide the expected peak could not emerge in your XRD peak

Hi all, thank you for your suggestions...

Hi Frank, I shall look into this...

Hi Matteo, where can I obtain phase diagrams of the transition metal oxides? (I have searched but find all kind of mixed results - it is like looking for a needle in a haystack)

I do not have a 4-circle machine, I have however investigated the films by SEM. This was hard and the images are not great (because the films are only 50nm), but indeed they seem to be columnar.

Does this mean the likely hood is stress? I will attempt to investigate further.

Additional Information: The mixed-metal films were DC sputtered from a 80:20 alloyed metallic target. (If this helps?)

Also: Is the substrate the likely cause of both sputtered and evaporated films possessing the same crystalline orientation? Rather than mixed peaks? I find it strange that despite different deposition techniques I get similar results, especially when the other study (employing very similar conditions, and from a very reputable source), obtains rater different results!

(I have performed about 15 repeatable depositions so I believe my results also)

Micro strain

I'm not sure that your 2theta shift could be due to the stresses because you are talking about a difference value of 1.3°. In my opinion It's too much. Moreover if you think that this shift is caused by the internal stresses you need to check all the other XRD peaks to confirm if all the peaks are affected by this change in 2theta position. It's likely that the extra peak is related to the presence of mixed phase or non-stoichiometric compounds made of the two metal oxides.

For what concerns the high level of orientation I think that It could be due to the substrate because despite you having used two different techniques, you have found the same result. I don't have a lot of experience in alloys but I don't think that the orientation depends on the mixed target.

Hi Valeria,

Thank you for your input. Indeed the shift is large, which is in-part why I asked on here. I cannot check all of my other peaks (as there appears to be only one VERY faint peak at the 200 position - and it is only detectable on some of my films). Although vague, this peak does seem to be shifted by about 1.2 degrees so this is indicative of stress I believe?

I don't think the main peak is a new peak or a mixed peak as the other material's closest peak is at just under 40 degrees.

Thank you also for confirming it may be my substrate (I don't know how to explain this though), it is strange I can achieve very orientated films, not that it is a bad thing.

Hi Matthew... I think there are two possibilities. 1) May be you are getting new crystal growth orientation that is due to mixture. 2) Generally, the small shift about 1 degree or little more could be due to the stress developed in the film.

If the shifted peak is not broadened and if there are no humps

in it, then lattice parameter(s) must have slightly shrunk. The cause may depend

on one or sevral factors like, interaction radii of the new metal,

new packing strength, addition or subtraction of defects etc....

Hello Matthew,

In thin films, residual stresses of various origin can be generated:

- thermal stress (if the thermal expansion coefficients of the substrate and the film are different and if the growth is performed at a temperature different from RT)

- epitaxial stress (if epitaxial relationship between the film and the substrate exists)

- stress due to grain boundaries and impurities [Hoffman, Thin Solid Films 34, 185 (1976)]

- in thin films grown by sputtering, energetic particles can also create a lot of defects ("atomic peening effect"), that can also induce large stress. For example, in Y2O3 (bixbyite) thin films grown by ion beam sputtering, it has been shown that atomic peening can lead to the formation of anti-frenkel pairs (point defects) and to oxygen vacancy aggregates (loops) that have strong effects on the residual stresses [Lacroix et al., Phys. Rev. B 84, 014104 (2011)]. The formation of these oxygen vacancy loops is seen as a process to accommodate locally the non stoichiometry of the film.

Did you measure the composition in your films? Maybe a similar process could happen in your layers, and could be an explanation of your XRD observations?

Hy. If there are peak shiftings this means displacement of the bunch of dispersors at [h1 k1 l1] positions to the displaced [h2 k2 l1] positions, and as has been observed from some colleagues this can be done by stress but also due to oriented films and by tilting the surface you can even enhance the diffraction peak intensities in order to analyze in which Miller planes is 2D growing your film, even kinetic growth parameters can be obtained through this analysis. Hoe this helps. Best regards.

Hi, Babu - Yes there is no broadening of the peak.

Thank you Bertrand, the references are very useful (will have a look over the weekend). I obtained an idea of the composition of the films via EDX, which indicates the stoichiometry has changed very little from the target composition, other than the addition of oxygen (obviously). Unfortunately I cannot get very accurate EDX (though it is pretty good) due to the thickness of my films(50nm) which makes the beam go through.

Daniel, microstrain (i.e. root mean strain) cannot give a peak shift.. by definition it is the variance of strain (with respect to the average that causes the shift). It gives broadening (that is not observed) or asymmetry (if anisotropic).

If there is stress then a simple Rietveld refinement should not work (as the shift of the peaks would not me monotonic!).

If you can decouple the omega (also called theta) and 2theta arms, then do a 2theta scan on a couple peaks at increasing omegas just to probe planes that are differently oriented with respect to the surface.. it's not like a 4-circle but you can get some good ideas. Remember that for a given 2theta you are limited in the range of thetas you can use (above theta = 2theta/2 you see no signal.. so you better choose a good single high angle peak).

I would not say that 1 degree is unreasonable for combined substitution + stress. I don't know the elastic constants and the cell parameters by heart, and I cannot therefore say anything about the expected shift without doing any calculation! If the peaks shift to high angle then the film is likely in tension.

Yes vacancies can also play a role (I have seen some reasonable shift in nanoparticles) but there must be a good amount!

Thanks again Matteo, I shall ask if I can do this (as I have not decoupled the theta and 2theta arms before). This is the first time I have mixed oxides and the first time I have personally observed such a shift.

The causes may be due to structural and/or microstructural changes in these materials. The investigation of the problem by X-ray crystallographic methods can help understand the possible origin of this interesting problem.

In general, a small addition of an atom in a solid solution causes changes in the unit-cell parameters and in the unit-cell volume obtained by X-ray diffraction methods and complementary techniques (for example, electron microscopy).

Hi Matthew Navees,

have compared the intensity of the peaks at 37.1 (2 theta) and 38.4.

If you take a grazing angle diffractogram, you

may get most of the peaks. Take the unit cell parameters

of the parent oxide and refine the cell parameters

with new peak positions. The refined cell will indicate

the lattice parameter shift. If there are unindexed peaks,

it will indicate the existence of new phase(or unreacted

added oxide)

Dear all, I think that more or less all of the answers given so far contain meaningful information, but in any case it is not sufficient just looking at one single peak. Stress and strain may lead to shifts in both directions, i.e. smaller AND larger Bragg angles may be observed for the same sample for different Bragg peaks. Thus an analysis on several reflections has to be done and solved CONSISTENTLY.

Good luck, Dirk

Thanks again to all,

Yes Dirk, it is the problem I have that my films are so oriented I only have one peak to work with which makes things more of a mystery.

This is the first time I have had to go into depth with XRD since it is the first time I have had unexpected results. I will try decoupling the arms as Matteo has suggested but I need to wait for someone whom has done this on my apparatus is available, which is taking longer than I would like.

Zahida - The intensities are more or less equal.

The shift in peaks and change in intensity of the peaks may be due to the change in bond length due to bond stretching of the mixed metal oxide films.

In mixed metal oxides one must see the phase diagram as suggested by Matteo Leoni. Then see if individual oxides have peaks in between 37 and 39. If they have then the mixing or formation can be justified. We can apply Vegards law and estimate composition and compare with Composition by other methods. Please try to index the peak with JCPDS of all associted and expected Ternery systems. Thgis may give some clue.