Entropy engineering in I–V–VI2 family: a paradigm to bestow enhanced average ZT in the entire operating temperature regime

Abstract

The design and development of n-type alloys in the mid-temperature regime (500–700) K with enhanced thermoelectric performance is of utmost necessity for the fabrication of thermoelectric devices. In this regard, the I–V–VI₂ family reveals superior thermoelectric performance, owing to the fact that group V elements have non-bonded electrons and high Z (atomic number), with a high Grüneisen parameter, which cause amplified anharmonicity and subsequently low intrinsic lattice thermal conductivity. However, the irony is that the well-studied alloy of this family, AgBiSe₂, undergoes phase transition in the operating temperature range. Thus, of paramount importance is restricting the phase transition and bringing it down below room temperature (RT), along with stabilizing a highly symmetrical crystal structure in the extended operating temperature range. Efforts were made to synthesize a cubic n-type AgBiSeS alloy belonging to the I–V–VI₂ compounds (unlike AgBiSe₂) that is stabilized throughout the temperature range, as the S element aids in strengthening of the chemical bonds. In addition, the alloy was further stabilized by forming a solid solution with PbSe, which aids in increasing the configurational entropy and thereby increases the chemical space of the system. The resultant alloys possess intrinsically low lattice thermal conductivity ranging from 0.38–0.74 W m⁻¹ K⁻¹ in the entire operating range. Consequently, the peak ZT was reported as ∼0.6 at 780 K, with an average ZT value of 0.3 for the alloy (AgBiSeS)_0.5(PbSe)_0.5 within 300–823 K. Although the reported ZT is low, the methodology of entropy-driven structural stabilization in the operating temperature regime was adapted to attain a highly symmetrical, stable structure for practical applications.