Achieving Superior Thermoelectric Performance in Ge₄Se₃Te via Symmetry Manipulation with I-V-VI₂ Alloying

Although orthorhombic GeSe is predicted to have an ultrahigh figure of merit, ZT approximate to 2.5, up to now, the highest experimental value is approximate to 0.2 due to the low carrier concentration (n(H) approximate to 10(18) cm(-3)). Improving symmetry is an effective approach for enhancing the ZT of GeSe-based materials. With Te-alloying, Ge4Se3Te displays the two-dimensional hexagonal structure and high n(H) approximate to 1.23 x 10(21) cm(-3). Interestingly, Ge4Se3Te transformed from the hexagonal into the rhombohedral phase with only approximate to 2% I-V-VI2-alloying (I = Li, Na, K, Cu, Ag; V = Sb, Bi; VI = Se, Te). According to the calculated results of Ge0.82Ag0.09Bi0.09Se0.614Te0.386 single-crystal grown via AgBiTe2-alloying, it exhibits a higher valley degeneracy than the hexagonal Ge4Se3Te. For instance, AgBiTe2-alloying induces a strong band convergence and band inversion effect, resulting in a significantly enhanced Seebeck coefficient and power factor with a similar n(H) from 17 mu V K-1 and 0.63 mu W cm(-1) K-2 for pristine Ge4Se3Te to 124 mu V K-1 and 5.97 mu W cm(-1) K-2 for 12%AgBiTe2-alloyed sample, respectively. Moreover, the sharply reduced phonon velocity, nano-domain wall structure, and strong anharmonicity lead to low lattice thermal conductivity. As a result, a record-high average ZT approximate to 0.95 over 323-773 K with an excellent ZT approximate to 1.30 is achieved at 723 K.