The X-ray diffraction patterns show a glass-crystalline transition at some temperature in between 530 and 590 K. The crystalline phase persists up to 660 K. Then at some temperature in between 660 and 790 K there is a crystalline-glass transition. In order to determine more precisely these transition temperatures one must measure diffraction patterns in finer step. Also there may be some crystal-crystal transition in between 590 and 660 K. In amorphus or glassy phase the prominant broad hump is at the same position of the strong Bragg peaks of the crystalline phases suggesting a particular short range correlation persists in the glassy phase. If you have very good data with background correction you get a lot information about the structure of the phases. But before doing that you should first try to identify the crystalline phases. It is remarkable that there are so many transitions in the system. It can as well be composition changes rather than pure phase transitions.

The X-ray diffraction patterns show a glass-crystalline transition at some temperature in between 530 and 590 K. The crystalline phase persists up to 660 K. Then at some temperature in between 660 and 790 K there is a crystalline-glass transition. In order to determine more precisely these transition temperatures one must measure diffraction patterns in finer step. Also there may be some crystal-crystal transition in between 590 and 660 K. In amorphus or glassy phase the prominant broad hump is at the same position of the strong Bragg peaks of the crystalline phases suggesting a particular short range correlation persists in the glassy phase. If you have very good data with background correction you get a lot information about the structure of the phases. But before doing that you should first try to identify the crystalline phases. It is remarkable that there are so many transitions in the system. It can as well be composition changes rather than pure phase transitions.

Comparison with a polymer can be done: First the polymer is amorphous, and when heating it, the thermal energy is absorbed and used to make the polymer crystallise (i.e. it got enough energy to move its chains towards a more stable state. At a higher temperature, the thermal agitation becomes so high that the crystalline structure is progressively lost as the polymer undergoes its glass transition temperature. At even higher temperature, it melts and becomes totally amorphous again. If wonder how this system, which initially is a glass, can also behave like a polymer.

Moreover, in the present case, the XRD patterns at low and high temperature are rather identical.

Alain

It is also curious that a small Bragg peak around two-theta = 7 deg. persists in all amorphous diffraction diagram. I guess it is an impurity phase that stays crystalline at all temperature. In crystalline phases this peak is also perhaps there albeit looks stronger.

Impurity or part of the sample holder ?

Alain

In the amorphous phase there are two broad bumps corresponding to some atomic pair correlations and perhaps represent some Te-O, K-O or Te-K bond distances. but I guess such glassy phase must be discussed in literature and should be checked.

First of all, thank you so much for your responses and valuable comments!

Regarding the idea pointed out by Dr. Tapan Chatterji which suggests the presence of a crystalline impurity phase in the glass matrix, since the authors used a roller-quenching technique to prepare the samples, I think that this peak (at 7°) is more likely to depict the local structure of the glass rather than a crystalline impurity.

I noticed that the XRD patterns of glasses (having the same composition) show considerably different features. A priori, this difference (Please see the attached file) might be due to the glass preparation peculiarities (especially the quenching rate of glass melt).

Otherwise, how do you explain the presence of sharp peaks in the top two XRD patterns? And how do you consider their presence in a diffraction pattern of a glass? Is it seriously taken uncomfortable for an amorphous material?

The sharp peaks cannot belong to the glassy phase. The local structure of glass has no long range order and therefore generates only broad features in diffraction pattern.

I absolutely agree with you Dr Tapan Chatterji. On the other hand, I think that melt quenching rate has a significant role in the appearance of such sharp peaks in glass diffraction diagram.

As I mentioned previously, Akagi et al. used a twin-roller to quench melts with extremely fast rates. In my own case, I quenched melt using a mixture of ice, ethanol and NaCl (lower quenching rate compared with that achieved by Akagi et al.). In this sense, the difference in broadness of the peak at 13° might be explained by distinct quenching conditions.

The pattern at 760 K, can we attribute it to liquid phase (molten oxide) if it melts at all. The patterns at 590K and 660K are of crystalline material taken above glass transition temperature as mentioned by Dr Chatterji.

I see that the glassy phase has been produced by fast quenching; so it must be highly metastable. It is no wonder that it goes over to the crystalline phase by heating. But how does the substance go to the glassy state again at higher temperature? By the way what is the melting temperature of this substance?

At 590K and 660K there is a high level of crystallinity in the material, while at lower and higher temperatures the phase appears to be amorphous, which is evidenced by broad diffraction features. The more crystalline the material, the sharper the diffraction peaks are.

In general, other changes in the XRD patterns upon heating can be a result of lattice thermal expension or shrinkage (less common), both of which would result in peak shifts. There might also be a total change in the peaks, which would be a result of a phase transiton. One might say that what you see is the phase transition glass-crystalline-glass.

I agree with the answer by Chatterji and would add a hypothesis about the pattern variations in between 590K and 660K . In the absence of chemical modification in the sample, a variation in the space group + thermal expansion could change the pattern.

Thanks for your attention

Dear Mohammad,

I don't how much extent my answer helps you, but let me try to explain from my knowledge. After observing your PXRD pattern I draw the following conclusions.

Firstly you compound turned from semi-amorphous to crystalline in between 530-590K.

Secondly it changed its PXRD pattern in between 590-660K.

Afterwards it started to convert into amorphous(may be started melting).

Now the change occurred between 590-660K can be called as polymorphic transition. (enantiotropic system relationship).

To support my argument, I suggest you to follow the simple technique to make your result as strong.

Run DSC on this material, to know the exact crystal transition temperature (503-590K) and phase transition temperature(590-660K). I guess, for sure you will get two successive endotherms separated by a small exotherm for the phase conversion.

It suggests not eutectic but enantiotropic polymorphic relation ship within the same compound.

I will happy if it helps..

Good luck.

Dear Mohamed,

As you can see from the XRD patterns, there are some structural changes taking place upon heating. These changes reveal themselves as appearance of new peaks in the diffractogram. Also, one can see that the intensity of the peaks increase with temperature. Eventually, the composition comes to a melt and all peaks disappear.

Amorphization of the samples at high temperature.

If the diffraction patterns were taken in air there is possible that the crystal structure of the sample changes because it suffers lost weight . I suggest TGA measurements.

@Gallardo

In fact, the authors (Akagi et al.) measured the DSC curve of the glass sample which showed two crystallization peaks.

Those of you suggesting that at high temperature there can be a transition from crystalline to amorphous, this is not correct. An amorphous structure is metastable so it will form during heating of a crystalline sample. On heating your amorphous sample crystallizes and the crystalline phases change with temperature (as the first phases to crystallize from the amorphous matrix are usually not the equilibrium phases at that temperature. With increasing temperature you begin to form a melt with some crystalline phases remaining. This suggests you have crossed into a peritectic region. If you continue to increase the temperature you may cross into other peritectic regions with different crystalline phases in equilibrium with the melt before finally going into a fully liquid phase. I would look at the known phase diagrams in your system. Also if you perform DTA/DSC then crystallization from the amorphous structure will show up as a peak since it is an exothermic transition. The transition at higher temperature from crystalline to liquid + crystalline will show a dip since this is an endothermic transition.

at 590K it's a crystalline phase

at 660K it's another crystalline phase

above it's probably liquid (melting of the 2nd crystalline phase)

at 330 and 530K it's probably amorphous

a simple visual inspection of the sample may tell you if its liquid at 790K and solid at 330 and 530K

to know the exact temperature of the phase transitions, you have to perform a DSC measurement from room temperature up to 800K

each peak will give you the temperature each phase transition (i guess 3 here :

amorphous-crystal I

crystalI-crystal II

crystal II fusion)

a TGA experiment may tell you if there is a weight lost (perhaps there is a chemical transformation of the system? degradation with lost of vapor if you're lucky)

you may try to index the 2 different crystalline powder patterns (with fullprof for example)

you may even try to solve the crystalline structures on powder data (with dash for example)

here you have the crystal structure published on your compound

http://pubs.acs.org/doi/abs/10.1021/ic970497m?journalCode=inocaj

https://www.researchgate.net/publication/250533945_ChemInform_Abstract_Crystal_Structures_of_Potassium_Tetratellurite_K2Te4O9_and_Potassium_Ditellurite_K2Te2O5_and_Structural_Trends_in_Solid_Alkali_Tellurites

you can simply calculate the powder pattern with this structure

and compare with your data

it seems to exist as a hydrate also

http://www.springermaterials.com/docs/VSP/summary/lpf-sd/00073435.html

but at 500K, i guess it may have completely lost the water molecules...

Article ChemInform Abstract: Crystal Structures of Potassium Tetrate...

I agree that you need to run DSC to determine Tg and any crystalline melt temperatures over the range of your observations.  That will definitively answer the question regarding both Tg and any Tm's.  TGA will answer the question of either weight gain or loss over the range under study. 

The fact that the curves at 300K, 530K and 790K are virtually identical suggests either that something different is happening at 590K and 660K, OR that there is an instrument or measurement problem that is generating noise as those two intermediate temperatures.  As an experimentalist, I would always turn first to the question of error/noise and answer that question first, before investing further in explaining something that may not be real. 

Question:  what are the conditions of your samples at each of your temperatures?  How long does it take to ramp up the temperature between readings?  How long is the sample held at temperature before making measurements?

Assuming that your readings are reproducible, you appear to be seeing two different structures at 590K and 660K -- there are both shifts in peak positions and the appearance of different peaks between the two samples.  The system of titanium and oxygen as shown back in the 1960's by Gillis of the University of Kansas might shed some light on this. 

In addition to possible phase changes, one might suggest one phase falling out of solution in another as temperature rises.  That behavior is well known in organic systems.  But the return to the same behavior at an even higher temperature negates that thought in my opinion. 

As the interplanar distances increases, theta decreases. With increasing temperature generally the interplanar distances increases. Therefore, one may expect that with increasing temperature the 2theta value along the X axis in PXRD would decrease without changing the pattern. But if there is any phase transformation, then the pattern would change.

This answers really helped with my own XRD patterns was having a hard time describing the trend at the temperature was increased. But I have one question; when you have separation of peaks  like in the case of the first diagram; what could be the cause of that?

I have gone through all the given answer. I have one question, I have calcined ZSM-5 (molar ratio:23:1, SiO2:Al2O3) in muffle furnace. I have done XRD analysis of with and without calcined ZSM-5 and found the only intensity is changed. Now, i am not able to understand why peaks intensity changed. Please suggest me.

Dear Ranjeet,

if the positions of the peaks is not changed, after calcination, means (roughly) that the lattice geometry of you crystalline phase is not changed. Actually, a careful examination of the peak positions ( through the peak analysis by mathematical decomposition) should show that something is changed ( but only a little bit) in the lattice geometry.

Nevertheless, one cannot forget that your sample is not a single-crystal, but is a powder made by an infinity of microcrystals of different sizes. Calcination (and the consequent temperature decrease down to the room temperature) induces a variation in the size distribution of the microcrystals (grain breaking followed by the successive aggregation). This implies that one should observe an increase of the number of crystal planes having higher spacing ( lower two-theta diffraction angles), at the expence of the lower spacing, according to the X-ray powder (XRPD) diagram you obtained . This can be a reasonable way to explain the intensity changes)

All the best !

Dino

Thank you so much sir.