First principles study of lattice thermal conductivity and large isotope effect in materials

The isotope effect---the percent enhancement to a material's lattice thermal conductivity, $k$, with isotopic purification---depends on the interplay between phonon-isotope and phonon-phonon scattering. Diamond is known to have the largest measured room temperature (RT) isotope effect of any bulk crystal, achieving a $k$ enhancement of 50{\%}. Using an \textit{ab initio }Boltzmann transport equation approach, we have identified several other materials with far larger RT isotope effects [1]. In particular, we find that germanium carbide (GeC) and beryllium selenide (BeSe) have RT isotope effects of 450{\%}, almost an order of magnitude higher than that in diamond. Isotopic purification in these materials gives surprisingly high intrinsic RT $k$ values, over 1500 Wm$^{-1}$K$^{-1}$ for GeC and over 600 Wm$^{-1}$ K$^{-1}$ for BeSe, well above those of the best metals. These large values stem from fundamental material properties that give both enhanced phonon scattering by isotopes and weak anharmonic phonon-phonon scattering. The physical insights discussed in this work provide guidance for efficient manipulation of thermal transport properties of compound semiconductors through isotopic modification. \\[4pt] [1] L. Lindsay, D. A. Broido and T. L. Reinecke, Phys. Rev. B 88, 144306 (2013).