Wavelike tunneling of phonons dominates glassy thermal conductivity in crystalline <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cs</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>Bi</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">I</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mi>Cl</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>
Jingyu Li, Liuming Wei, Zhuoyang Ti, Le Ma, Yuli Yan, Guangbiao Zhang, Pengfei Liu
Abstract
Intrinsically low lattice thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{L}}$ in halide perovskites is of great interest for energy conversion applications. Here, based on first-principles calculations, we systematically study the lattice thermal conductivity of the recently synthesized layered perovskite ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{6}{\mathrm{Cl}}_{3}$. By using renormalized force constants extracted from lattice dynamics, our calculated ${\ensuremath{\kappa}}_{\mathrm{L}}$ is 0.227 and 0.130 ${\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ along the in-plane and cross-plane directions at 300 K, respectively, which agrees well with the experimental values (0.223 and 0.209 ${\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ parallel and perpendicular to the Bridgman growth direction). Meanwhile, ${\ensuremath{\kappa}}_{\mathrm{L}}$ follows a nonstandard ${\ensuremath{\kappa}}_{\mathrm{L}}\ensuremath{\propto}{T}^{\ensuremath{-}0.237}$ dependence on heating, originating from the dual particle-wave behavior of heat-carrying phonons where wavelike tunneling dominates $>72$% of the contribution to the total ${\ensuremath{\kappa}}_{\mathrm{L}}$ when $T>$ 300 K. Further analyses imply that ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{6}{\mathrm{Cl}}_{3}$ manifests the coexistence of metavalent bonding, loosely bonded rattling atoms with thermally induced large-amplitude vibrations, and stereochemical lone pair activity, which induces strong anharmonicity with the soft low-lying modes, causes a mixed crystalline-liquid state, and, finally, produces unexpectedly glassy thermal conductivity. Our work pinpoints the microscopic origin of ultralow ${\ensuremath{\kappa}}_{\mathrm{L}}$ in ${\mathrm{Cs}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{6}{\mathrm{Cl}}_{3}$, which is important for designing efficient materials in halide perovskites for energy conversion.