Dear Panda,

You have raised a good question. To be very frank no one knows the answer for it as yet. Answers provided by Subhajit Saha based on the effect of point defects on luminescence efficiency is well known but is the not the appropriate answer to your question. The answer may lie in determining in which crystal structure such defects (activator, vacancies, interstitials, dislocations, site symmetrty etc) are accomodated to the appropriate levels so@@ as to get optimal luminescence efficiency

Dr Areunachalam Lakshmanan

Saveetha Engineering college, Chennai

I worked on cellulose. My understanding is that crystallinity is related to the organization of polymeres. Hydrogen bonds make cellulose very crystalline. It means the more crystallinity, less is the space between polymeres. In my case; data showed that the chemical reaction with acid was slow when the crystallinity is high. It could be the same for luminescence.

Thank you, Sir, for your suggestions.

However, I work on luminescence studies of polycrystalline materials. I observed low luminescence from poor crystalline material. I do not know how to account that.

Regards

Sitakanta Panda.

Dear Sitakanta,

In general, luminescence efficiency is directly proportional to crystallinity in case of phosphor materials. Generally, when some luminescence activators are doped into a host to develop a phosphor, various kind of defects are introduced in the lattice. In most of the cases these defects are: vacancy due to charge imbalance, volume defect due to different ionic radii of host and dopant atoms, different surface states etc. In order to achieve strong luminescence, it is necessary to reduce these defects as minimum as possible. Charge compensation by adding extra co-activator, is a strategy that can solve the charge imbalance problem. You can have a look into the following papers to get a clear idea.

1. https://pubs.rsc.org/en/content/articlehtml/2013/dt/c3dt51411k

2. https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.5b03500

3. https://pubs.acs.org/doi/abs/10.1021/acsomega.7b01525

Best regards,

SS

Dear Subhajit,

Thank you very much. It is useful.

Dear Panda,

You have raised a good question. To be very frank no one knows the answer for it as yet. Answers provided by Subhajit Saha based on the effect of point defects on luminescence efficiency is well known but is the not the appropriate answer to your question. The answer may lie in determining in which crystal structure such defects (activator, vacancies, interstitials, dislocations, site symmetrty etc) are accomodated to the appropriate levels so@@ as to get optimal luminescence efficiency

Dr Areunachalam Lakshmanan

Saveetha Engineering college, Chennai

Dear Sir,

Thank you very much for your valuable clarifications.

With warm regards

Sitakanta

- Nice to see reply from stalwarts @ Arunachalam Lakshmanan of the Luminescence and Dosimetry fields.

Already Lakshmanan Sir has explained. I only want to add that crystallinity is desirable for luminescence as of now there is no mathematical relation for the same. There are other confounding factors which drastically affect the luminescence efficiency; as mentioned by Lakshmanan Sir. We always noted that in explaining concentration quenching, crystal lattice parameter, a - enters the relation i.e luminescence centres are too-too close. First principle simulations are not perfect ( solving N*10^23 equations which are coupled for defects, activators, centres etc. etc.and stochastic) for luminescence as large number of atoms and defects etc. need to be considered. So crystallinity may be essential but not sufficient criteria exceptions are there. Glasses, polythene, leaves etc. exhibit feeble luminescence.

Even in same host lattice, different dopants give TL glow curves of varying efficiency and so on-difficult to bind in single rule.

In some phosphors mere inclusion of an impurity has been reported to change the crystal structure as well as luminescence efficiency. Similarly small differences in preparation temperature has also been reported to change the crystal structure as well as luminescence efficiency. So there may not be a simple answer to this question. Preparation method, firing/preparation temperature/ type of crystals/impurity ions all of them can change the crystal structure as well as luminescence efficiency. One must look into the crystal parameters, unit cell dimensions, ionic radii of impurity ion etc and then only one can propose a crystal specific model as an answer to this question

Dr Lakshmanan