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Smart phonon control boosts efficiency in eco-friendly thermoelectric material

A research team has discovered how to make a promising energy-harvesting material much more efficient—without relying on rare or expensive elements. The material, called β-Zn₄Sb₃, is a tellurium-free thermoelectric compound that can convert waste heat into electricity.

In their study in Advanced Science, scientists used advanced neutron scattering techniques to peek inside the crystal and found something surprising: tiny heat vibrations (called phonons) were being disrupted by "rattling" atoms inside the structure. This phenomenon, known as phonon avoided crossing, dramatically slowed down how heat travels through the material.

Thanks to this effect, the material's thermal conductivity dropped to extremely low levels—great news for thermoelectric performance. Even better, the researchers found that the single-crystal version of this material also conducts electricity better than its polycrystalline counterpart, reaching a high power conversion efficiency of 1.4%.

These results show that smart phonon control can lead to high-performance, eco-friendly materials for converting heat into power.

In thermoelectric materials, avoided crossing refers to the interaction between propagating phonons and localized vibrational modes, where their energy dispersions repel each other rather than intersect. This phenomenon occurs under specific conditions, such as crystal symmetries or vibrational mode couplings.

However, when researchers developed the single-crystal β-Zn₄Sb₃, they observed an unexpected, avoided crossing, revealing unique phonon behavior that deviated from conventional thermoelectric materials.

The article explores the thermoelectric performance of single-crystalline β-Zn₄Sb₃, a tellurium-free material, by uncovering the microscopic mechanisms that lead to its ultralow lattice thermal conductivity (κ_L).

Using inelastic neutron scattering (INS), the researchers provide the first experimental observation of avoided crossing between longitudinal acoustic phonons and low-energy rattling modes. This interaction causes a significant reduction in phonon group velocity—from over 4000 m/s to about 591 m/s—and shortens phonon lifetimes to under 1 picosecond, both of which contribute to strongly suppressed heat transport.

The β-Zn₄Sb₃ single crystal achieves a κ_L of approximately 0.36 W/m·K in the 300–600 K range and a peak thermoelectric figure of merit (zT) of 1.0 at 623 K. Additionally, device-level testing shows a conversion efficiency (η) of 1.4% in a single-leg thermoelectric module—one of the highest reported for undoped Zn₄Sb₃.

Structural characterizations via TEM reveal a grain-boundary-free lattice with uniformly distributed moiré fringes, attributed to Zn concentration variations.

These nanoscale features further enhance phonon scattering without degrading electronic performance. Compared to polycrystalline samples, the single crystal exhibits significantly better electrical conductivity due to fewer defects and optimized carrier mobility.

"This discovery shows how heat flow can be engineered to design more efficient and sustainable energy technologies—without depending on scarce resources," says Prof. Hsin-Jay Wu.