Phase engineering on high-entropy transition metal dichalcogenides and the entropy-enhanced thermoelectric performance

High-entropy structures in layered compounds, especially transitional metal dichalcogenides (TMDCs), have powered the field with disordered and versatile chemical compositions, showing great potential in various functional applications, including energy storage and catalysis. However, the reported high-entropy phases are mainly 1T phases, 2H phases are rare, and approximately 3R phases are still lacking. Here, phase engineering of high-entropy TMDCs is achieved by tuning the chemical composition of (Mo_0.5W_0.5)_1−<i>x</i>(Nb_0.5Ta_0.5)_<i>x</i>Se_2+<i>δ</i>, 0 ≤ <i>x</i> &lt; 1, and −0.1 ≤ <i>δ</i> ≤ 0.3. A phase diagram is constructed to guide the synthesis of pure 2H/3R phases over a wide composition/entropy range. The increase in VB-group element content and Se overdose facilitated the formation of 3R phases, whereas the opposite occurred for 2H phases. Thermodynamic first-principles calculations evaluate the stability of phases in different polytypes and compositions, matching well with the composition-dependent crystalline habits. Moreover, the optimized thermoelectric performance, with a figure of merit (<i>zT</i> = 0.36@723 K) in 2H phase of <i>x</i> = 0.2, is attributed to the low thermal conductivity (<i>κ</i>) caused by the high-entropy effect, which is one of the highest among (Mo/W)Se₂-based materials. Our work enriches high-entropy TMDCs with versatile polytypes, expanding their potential uses for various fields.