Generally, optical band gap of glasses decreases with an increase of modifier content due to the formation of NBOs. But in my oxide glasses, increase of optical band gap with increase of modifier has been observed. Kindly, explain the reason behind it.
Can you provide little more details about your glasses? For example, silicate, borate, borosilicate or any other? and which modifier you are adding?
This is because modifier added to borate and silicate glasses can have different effects on the structure and other properties. Modifier added to silicate glasses is likely to increase NBO's, but same modifier added to borate glasses will FIRST converts BO3 groups to BO4 group i.e. network formation, and after certain concentration it will start creating NBO's. You can refer to a very good book "Introduction to glass science and technology" by JE Shelby.
I agree with the answer above. If the glass contains boron, the conversion of the NBO to bridging oxygens in the BO4 units will shift the edge toward the UV. There are papers showing this effect in the sodium borate glasses. Don't remember the references, but they are pretty old, maybe the 60's or 70's. I've measured the same effect myself, so know it happens.
@ James Shelby
Sir,
Happy to see u on Researchgate. Ur Book by name "Introduction to glass science and technology" has helped me a lot in my work. I used to refer it regularly.
As per kind suggestion of Mr. Satwinder Singh, I have gone through this book to find the answer of this particular question but I didnt find any clear concept based on this. Kindly, help me out of this. I found such behavior in my tellurite glasses.
Thanks,
@Naziya Parveen
Kindly refer to Chapter 5, Structure of glasses, Section 5.2 Alkali Borate Glasses, Page 94-98 (2nd Eition book). You will get a detailed discussion on the structure and formation of NBO's in Borate glasses on the addition of modifiers as suggested by Respected Dr. James Shelby. However, you will not get discussion on particularly band gap in this section, but you can easily co-relate the NBO generation discussed here with your results.
Regards.
i also face a same problem, where i found that addition of Ag2O create NBO in tellurite glass system. however after do an analysis on optical absorption , i found that Eopt increase with the addition of Ag2O content. is it any suggestion/ paper that can i refer to explain this observation? i have look at HOMO-LUMO but still not fully understand. thank you
Since I have never worked with tellurite glasses and have not followed the literature on those glasses, I'm afraid I can't help you there. If similar behavior to that of the borates occurs, then similar explanations may work. But since I know nothing about the structure of those glasses, I have no idea.
I want to tell my idea but Im not sure its correct or not!!
When you add modifier, NBOs increase. Therefore ionic bond changes to covalent bond.Generally, covalent bond is stronger than ionic bond so electrons move from valence bond to conduct bond difficulty;this means increasing of band gap.I Hope I could help!!
Thank you, for the suggestion. it is possible to relate HOMO and LUMO for the increase of Eopt? but im still not clear about HOMO and LUMO.
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