Why are isotopically pure diamonds 50% more thermally conductive than other diamonds?
From THERMAL  CONDUCTIVITY  OF  NATURAL  AND  ISOTOPICALLY  ENRICHED  DIAMOND  - EFFECT OF NEUTRON IRRADIATION, D.P. White, Department of Physics, Merrimack College, N. Andover, MA 01845:  


  The thermal conductivity was calculated using the Callaway method.  This method takes into account  the  effects  of  intrinsic  three-phonon  normal  (N)  processes.    These  processes  do  not create thermal resistance directly but affect the thermal resistance by transferring phonons from frequencies where they are not scattered to frequencies where scattering is more efficient.  


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  In order to  explain  the  large  increase  (originally  much larger  than  expected)  in  terms  of  isotope scattering it was necessary to include the effects of N processes.  Subsequent analyses of the thermal  conductivity  of  enriched  diamond  have  included  N  processes.    Wei et.al.  used  the Callaway  expression  to  fit  experimental  thermal  conductivity  data  on  diamond  with  the  natural isotope  concentration  and  isotopically  enriched  (0.1% $\ce{^{13}C}$)  diamond  over  a  wide  temperature range (100-1000K).