Thermoelectric materials capable of converting waste heat into electricity have garnered significant attention for energy harvesting applications. Among these, nanostructured tellurium-based compounds exhibit exceptional thermoelectric performance due to their unique electronic and thermal transport properties. Key materials such as GeTe and AgSbTe2 demonstrate high thermoelectric efficiency through strategic engineering of phase transitions, vacancy manipulation, and metastable nanostructures.

Phase transition engineering plays a crucial role in optimizing the thermoelectric properties of tellurium-based materials. GeTe, for instance, undergoes a structural phase transition from rhombohedral to cubic at approximately 700 K. This transition significantly impacts carrier concentration and phonon scattering. By alloying with elements like Sb or Bi, the phase transition temperature can be tuned, stabilizing the cubic phase at lower temperatures. The cubic phase exhibits degenerate semiconductor behavior with high carrier mobility, while the rhombohedral phase introduces anisotropic transport properties. Precise control over this transition allows for enhanced power factors and reduced thermal conductivity, leading to improved thermoelectric performance. Similarly, AgSbTe2 displays a complex phase transition behavior influenced by stoichiometry and temperature. Engineering these transitions through compositional adjustments enables optimization of both electronic and thermal transport properties.

Vacancy manipulation is another critical strategy for enhancing the thermoelectric performance of tellurium-based nanostructures. GeTe is known to exhibit intrinsic Ge vacancies, which act as p-type dopants, increasing hole concentration. However, excessive vacancies can degrade carrier mobility. By carefully controlling vacancy concentration through annealing or doping, optimal carrier densities can be achieved. For example, introducing small amounts of Pb or Mn can compensate for Ge vacancies, fine-tuning the electrical conductivity. In AgSbTe2, Ag vacancies dominate the electronic transport properties. The formation energy of these vacancies is relatively low, making them highly sensitive to synthesis conditions. Post-synthesis treatments such as quenching or annealing can be employed to adjust vacancy concentrations, thereby optimizing the power factor. Additionally, vacancy clusters and their interactions with dopants can further reduce lattice thermal conductivity by enhancing phonon scattering.

Metastable nanostructures offer a promising avenue for decoupling electronic and thermal transport in tellurium-based thermoelectrics. GeTe can form metastable phases with nanoscale domain structures, such as twinning and stacking faults, which strongly scatter phonons without significantly affecting electron transport. These nanostructures are often stabilized through rapid quenching or templated growth techniques. Similarly, AgSbTe2 can form nanocomposites with secondary phases like Sb2Te3 or Ag2Te, creating interfaces that scatter mid- and long-wavelength phonons. The size and distribution of these nanostructures are critical; domains in the range of 5 to 20 nm are particularly effective for phonon scattering while maintaining good electrical conductivity. Metastable phases can also be induced through high-energy mechanical milling or spark plasma sintering, which introduce strain and defects that further suppress thermal conductivity.

The interplay between these strategies—phase transition engineering, vacancy manipulation, and metastable nanostructures—enables unprecedented control over the thermoelectric properties of tellurium-based materials. For instance, optimized GeTe-based systems have achieved ZT values exceeding 2.0 in the mid-temperature range, while AgSbTe2-based materials demonstrate ZT values above 1.5 at similar temperatures. These performance metrics are a direct result of synergistic effects between electronic structure modifications and phonon scattering mechanisms.

One of the challenges in working with nanostructured tellurium-based thermoelectrics is maintaining stability under operational conditions. Phase transitions and vacancy dynamics can lead to property degradation over time, particularly at elevated temperatures. Advanced characterization techniques such as in-situ transmission electron microscopy and high-resolution X-ray diffraction are essential for understanding these phenomena and designing more stable materials. Additionally, interfacial engineering between nanostructured domains can improve mechanical robustness while preserving thermoelectric performance.

Future developments in this field will likely focus on further refining nanostructural control through advanced synthesis techniques. Atomic layer deposition and molecular beam epitaxy offer precise control over composition and structure at the nanoscale, enabling the creation of tailored interfaces and defect configurations. Computational modeling, particularly density functional theory and molecular dynamics simulations, will play an increasingly important role in predicting optimal compositions and nanostructures for enhanced thermoelectric performance.

In summary, nanostructured tellurium-based thermoelectrics represent a highly tunable class of materials with exceptional energy conversion potential. Through deliberate engineering of phase transitions, vacancy concentrations, and metastable nanostructures, their thermoelectric properties can be optimized for practical applications. Continued advancements in synthesis and characterization will further unlock their potential, paving the way for more efficient waste heat recovery systems.