From: mroussel@shaw.ca   
      
   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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