Evidence of Highly Anharmonic Soft Lattice Vibrations in a Zintl Rattler

Abstract

Here, we present lattice dynamics associated with the local chemical bonding hierarchy in Zintl compound TlInTe₂ , which cause intriguing phonon excitations and strongly suppress the lattice thermal conductivity to an ultralow value (0.46-0.31 W m^-1 K^-1 ) in the 300-673 K. We established an intrinsic rattling nature in TlInTe₂ by studying the local structure and phonon vibrations using synchrotron X-ray pair distribution function (PDF) (100-503 K) and inelastic neutron scattering (INS) (5-450 K), respectively. We showed that while 1D chain of covalently bonded <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mfenced><mml:mi>I</mml:mi> <mml:mi>n</mml:mi> <mml:msub><mml:mrow><mml:mi>T</mml:mi> <mml:mi>e</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mfenced> <mml:mrow><mml:mi>n</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mi>n</mml:mi></mml:mrow> </mml:msubsup> </mml:math> transport heat with Debye type phonon excitation, ionically bonded Tl rattles with a frequency ca. 30 cm^-1 inside distorted Thompson cage formed by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup><mml:mfenced><mml:mi>I</mml:mi> <mml:mi>n</mml:mi> <mml:msub><mml:mrow><mml:mi>T</mml:mi> <mml:mi>e</mml:mi></mml:mrow> <mml:mn>2</mml:mn></mml:msub> </mml:mfenced> <mml:mrow><mml:mi>n</mml:mi></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mi>n</mml:mi></mml:mrow> </mml:msubsup> </mml:math> . This highly anharmonic Tl rattling causes strong phonon scattering and consequently phonon lifetime reduces to ultralow value of ca. 0.66(6) ps, resulting in ultralow thermal conductivity in TlInTe₂ .