Hello chemists,
I see in the literature that most crystals that are transparent at room temperature have the elements oxygen, carbon, or fluorine in them.
Please comment on any general rule for transparent compounds.
Thank you.

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On Friday, April 10, 2020 at 5:42:50 PM UTC-6, omni...@gmail.com wrote:

Hello chemists,
I see in the literature that most crystals that are transparent at room temperature have the elements oxygen, carbon, or fluorine in them.
Please comment on any general rule for transparent compounds.
Thank you.

Your conclusions may reflect your sampling procedures. KBr, for example, is transparent and is used to make windows and other optical elements in spectrometers. Lots of transition-metal crystals, while coloured, are transparent across at least some of
the optical range. And there are clear counterexamples, such as graphite, which is just an allotrope of carbon. The truly opaque materials that come to mind are metals and network solids (graphite, boron, silicon, ceramics, etc.), but some network solids
(e.g. quartz) are clear.

You will note that many opaque materials are conductive to a greater or lesser extent, which isn't a coincidence. A material with metallic conductivity should be reflective, and therefore opaque, according to the theory of metals. Semiconductors, which
have medium-sized bandgaps, will be opaque if the bandage is smaller than the energy of optical photons because they will absorb these photons.

Insulators, on the other hand, have bandgaps larger than the frequency of an optical photon, and so will tend to be transparent. The situation with transition-metal compounds is best thought of slightly differently by thinking about d to d transitions of
the metals in a given environment, but again, they typically don't absorb right across the spectrum and so are transparent at some optical frequencies.

Ceramics are a slightly different story. They are usually insulators, but common ceramics are opaque because of their irregular microscopic structure, which scatters light.

So as a rule, if you want a material to be transparent, look for insulating materials. If you additionally want it to be clear, stay away from transition-metal compounds, other than those with d^0 or d^10 electronic configurations.

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On Saturday, April 11, 2020 at 7:02:35 AM UTC-10, Marc Roussel wrote:

On Friday, April 10, 2020 at 5:42:50 PM UTC-6, omni...@gmail.com wrote:

Hello chemists,
I see in the literature that most crystals that are transparent at room temperature have the elements oxygen, carbon, or fluorine in them.
Please comment on any general rule for transparent compounds.
Thank you.

Your conclusions may reflect your sampling procedures. KBr, for example, is transparent and is used to make windows and other optical elements in spectrometers. Lots of transition-metal crystals, while coloured, are transparent across at least some of

the optical range. And there are clear counterexamples, such as graphite, which is just an allotrope of carbon. The truly opaque materials that come to mind are metals and network solids (graphite, boron, silicon, ceramics, etc.), but some network solids
(e.g. quartz) are clear.

You will note that many opaque materials are conductive to a greater or lesser extent, which isn't a coincidence. A material with metallic conductivity should be reflective, and therefore opaque, according to the theory of metals. Semiconductors, which

have medium-sized bandgaps, will be opaque if the bandage is smaller than the energy of optical photons because they will absorb these photons.

Insulators, on the other hand, have bandgaps larger than the frequency of an optical photon, and so will tend to be transparent. The situation with transition-metal compounds is best thought of slightly differently by thinking about d to d transitions

of the metals in a given environment, but again, they typically don't absorb right across the spectrum and so are transparent at some optical frequencies.

Ceramics are a slightly different story. They are usually insulators, but common ceramics are opaque because of their irregular microscopic structure, which scatters light.

So as a rule, if you want a material to be transparent, look for insulating materials. If you additionally want it to be clear, stay away from transition-metal compounds, other than those with d^0 or d^10 electronic configurations.

Thank you Marc,
The " transition-metal crystals" that are transparent will be studied,
or you could give an example of a chromium crystal that is transparent.

The conductive indium tin oxide is transparent, like glass, across
many colors. My list is growing, thanks to your comment. Here is a link to
a page with my paper on chromium antiferromagnetism… https://pyramidalcube.blogspot.com/p/chromium.html

Alan Folmsbee MSEE

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On Saturday, April 11, 2020 at 11:33:20 AM UTC-6, omni...@gmail.com wrote:

The " transition-metal crystals" that are transparent will be studied,
or you could give an example of a chromium crystal that is transparent.

In order to get a chromium compound with a d0 configuration, you would have to have chromium in the Cr(V) oxidation state, but there aren't many compounds like that.

Crystalline scandium(III) compounds should be transparent in the visible range.

The conductive indium tin oxide is transparent, like glass, across
many colors.

Interesting. I guess these transparent conductive oxides are semiconductors with band gaps just big enough not to absorb in the visible.

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On Sunday, April 12, 2020 at 5:55:29 AM UTC-10, Marc Roussel wrote:

On Saturday, April 11, 2020 at 11:33:20 AM UTC-6, omni...@gmail.com wrote:

The " transition-metal crystals" that are transparent will be studied,
or you could give an example of a chromium crystal that is transparent.

In order to get a chromium compound with a d0 configuration, you would have to have chromium in the Cr(V) oxidation state, but there aren't many compounds like that.

Crystalline scandium(III) compounds should be transparent in the visible range.

The conductive indium tin oxide is transparent, like glass, across
many colors.

Interesting. I guess these transparent conductive oxides are semiconductors with band gaps just big enough not to absorb in the visible.

Chromium (V), with five bonds, is existing because the nucleus has two rings of protons, with each ring made from ten protons. See photo:

https://pyramidalcube.blogspot.com/p/chromium.html

The white protons and brown neutrons are in a structure called
the tetrahexahedron (pyramidal cube). This produces self-passivation.

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