Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in AlTe 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 AlTe and AlTe 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 AlTe, a square planar chain (SPC) known as polytelluride [Te] is neutralized by the covalently bonded [AlTe] 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 AlTe monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the AlTe monolayer. The calculated values of 1.8 and 0.5 W m K along the - and -axes at 600 K are one order of magnitude lower than those of AlTe, and even lower than monolayers that contain heavy cations like BiTe. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of AlTe, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped of, respectively, 1.5 and 0.5 at 600 K is achieved for AlTe. The pioneering of AlTe compared to that of AlTe 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.