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 Post subject: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Tue Apr 23, 2013 1:54 pm 
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Came across a really neat sample when I was playing around in the lab recently. I was experimenting with different color laser pointers and happen to notice that a purple laser pointer caused this particular crystal to fluoresce bright red. This is pretty unusual for the crystals that I work with. I should note that a green laser pen shows the similar effect but it isn't as photogenic :)

Wondering what different knee jerks out there might be for the cause? The crystal is about 20mm thick, and 160mm across. Both sides are polished slightly.

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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Tue Apr 23, 2013 2:31 pm 
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You are seeing photoluminescence reaction of chromium in corundum. All sapphires contain enough chromium for triggering this reaction if excitation source (laser) is powerful enough. Even most pure colorless synthetic corundum does have this reaction, not always strong enough for naked eye but always detected with spectrometers.

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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Tue Apr 23, 2013 3:07 pm 
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mikko wrote:
You are seeing photoluminescence reaction of chromium in corundum. All sapphires contain enough chromium for triggering this reaction if excitation source (laser) is powerful enough. Even most pure colorless synthetic corundum does have this reaction, not always strong enough for naked eye but always detected with spectrometers.


Cool thanks! I have a faint ruby crystal!

If I wanted to reduce the red fluorescence, can I anneal it out? Maybe 1800 C in an oxidizing atmosphere for 24 hrs (if I cut a small piece out?) I'm new to all this fluorescence and color center stuff. It is incredibly fascinating to me. I'd love to set up a little experiment and play a little. Any suggestions?


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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 8:58 am 
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Hi

JSquare wrote:
If I wanted to reduce the red fluorescence, can I anneal it out? Maybe 1800 C in an oxidizing atmosphere for 24 hrs ?


i don't know how you could reduce chromium amount by annealing it.......... :roll:

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alberto

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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 9:05 am 
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Why the eye roll? I'm new to this... but i've been reading a little on this forum. I came across these two links from other threads. Seems plausible... no?

http://www.gemstonebuzz.com/heating

http://lgdl.gia.edu/pdfs/su03a1.pdf


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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 10:27 am 
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Neither of the articles you posted refer to "annealing" a stone to reduce fluorescence.
That is not possible.

The articles you link to are completely unrelated to your topic.


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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 10:46 am 
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Barbra Voltaire wrote:
Neither of the articles you posted refer to "annealing" a stone to reduce fluorescence.
That is not possible.

The articles you link to are completely unrelated to your topic.


OK-- I am seeing now where I'm not aligned with you guys, but I'm reading... and I'm stuck... I guess I'll lay out my logic and you guys can shoot down what doesn't seem right.

1- The fluorescence in my sample is likely low concentration Cr impurity.
2- Cr takes the place of Al in the crystal lattice
3- There are 32 other elements ranging from Ti to C that could also take the place of the Al, each of these can change the color.
4- Annealing at higher temperatures can change the hue of a stone, where oxidizing will generally decrease the color intensity and reducing will increase the color intensity
5- Annealing at higher temperatures can cause impurities to enter or leave the crystal depending on the surrounding environment. Packing the stone in a concentration of one of those 32 elements will create color, where packing it in pure Al203 will reduce color.

I feel like there is a riff between what I've been reading and what you guys work with every day... Hopefully you guys don't mind sharing some insight and guiding me towards the right understanding. I don't get to work with colored stones very often, so i'm just trying to absorb what I can here.


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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 1:12 pm 
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Annealing at higher temperatures can cause impurities to enter or leave the crystal depending on the surrounding environment. Packing the stone in a concentration of one of those 32 elements will create color, where packing it in pure Al203 will reduce color.


The key for understanding is the knowledge about diffusion process. Appendix "What is diffusion?" in the G&G article you posted is excellent resource... read it carefully and you will see remarkable differences in different ions diffusion rates.

Only smallest ions are able to diffuse in corundum crystal significantly enough for having practical use. Chromium is way too large. Additionally, heat treatment does not change the surrounding corundum crystal structure enough for distorting Chromium/Aluminum coordination (and luminescence properties).

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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 1:25 pm 
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mikko wrote:
Quote:
Annealing at higher temperatures can cause impurities to enter or leave the crystal depending on the surrounding environment. Packing the stone in a concentration of one of those 32 elements will create color, where packing it in pure Al203 will reduce color.


The key for understanding is the knowledge about diffusion process. Appendix "What is diffusion?" in the G&G article you posted is excellent resource... read it carefully and you will see remarkable differences in different ions diffusion rates.

Only smallest ions are able to diffuse in corundum crystal significantly enough for having practical use. Chromium is way too large. Additionally, heat treatment does not change the surrounding corundum crystal structure enough for distorting Chromium/Aluminum coordination (and luminescence properties).


Really appreciate the response. I'll keep doing my homework and post up if I find anything that's outside of the ordinary.


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 Post subject: Re: Bright Red Fluorescence from Purple Laser in sapphire
PostPosted: Wed Apr 24, 2013 1:26 pm 
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mikko wrote:
Only smallest ions are able to diffuse in corundum crystal significantly enough for having practical use. Chromium is way too large.


i'd only add....this is the reason why surface diffusion is partically only performed with Ti and Fe oxides.........Surface diffused rubies are extremely rare...........pratically few more than a gemmo curiosity............

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alberto

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