Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al 2 Te 5 monolayers.

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al 2 Te 3 and Al 2 Te 5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al 2 Te 5 , a square planar chain (SPC) known as polytelluride [Te 3 ] 2- is neutralized by the covalently bonded [Al 2 Te 2 ] 2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al 2 Te 5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al 2 Te 5 monolayer. The calculated κ L values of 1.8 and 0.5 W m -1 K -1 along the a - and b -axes at 600 K are one order of magnitude lower than those of Al 2 Te 3 , and even lower than monolayers that contain heavy cations like Bi 2 Te 3 . Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al 2 Te 5 , leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit ( zT ). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al 2 Te 5 . The pioneering zT of Al 2 Te 5 compared to that of Al 2 Te 3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.