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Currently i am doing some research on optical properties of a oxide material and for that purpose i have done some photoluminescence study on it. Because this is an ongoing research i cant divulge much information here. I have used "$Ex$" excitation wavelength for getting emission spectra for my oxide sample. I have prepared two types of oxide samples one is heat treated and the other not heat treated. After taking emission spectra data for both the samples I got peaks similar to this

Samples               Peak wavelength (nm)

Non Heat treated          a, b

Heat treated              b,c,d,e

*a,b,c,d & e are wavelength where emission peaks detected

Both samples (heat treated and non heat treated) has same chemical composition which i confirmed using powder XRD. As you can see from the data the heat treated sample has more emission peaks comparing to non heated samples. I can understand from this data that heat treatment to the sample causes increased defects in the sample. what i thought was that after heat treating a crystalline material; the defect in that material should decrease and not the other way round? Is my rationale wrong. Why heat treated sample has more defects?

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  • $\begingroup$ Can you be more specific on the XRD powder analysis? The same means really the same? If so, there should be no difference in other properties. The heating can reduce defects (disappear of a), but induce some different polymorphs, for example (c,d,e). Therefore I would first not call it a defect. If the defect is so common, that you can detect it in spectroscopy, it is not longer a defect, but something quite regular. $\endgroup$ – ssavec Nov 15 '13 at 8:59
  • $\begingroup$ @ssavec yes XRD has same chemical composition but more crystalline and the crystalline size also got increased after heating. I have gone through lots of the papers and the peaks a,b,c,d and e are reported in my oxide mateiral but not together. sometimes ab together or be,adb..etc $\endgroup$ – Eka Nov 15 '13 at 9:03
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It seems, that your oxide can have different crystal polymorphs and probably also other non-crystalline phases.

You synthesize your material as mixture of those, where the composition depends on the preparation process.

Upon heating and slow cooling you probably recrystallize the species to more thermodynamically stable polymorph (new peaks c,d,e) on the expense of less stable one (a). It can also depend on the speed of cooling, so you could obtain some metastable phases, etc...

Even then, treat your material always as an unkonwn mixture of uknown number of polymorphs and let the spectra tell you in which way the composition shifted.

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