High‐purity La3‐xTe₄ Material with Superior Thermoelectric Performance Synthesized via Te‐vapor Transport and Solid‐State Diffusion

La 3‐x Te 4 is a very promising high‐temperature candidate applied in next‐generation Radioisotope Thermoelectric Generators (RTGs). Conventional synthesis of such materials is based on the mechanochemical method, which makes the sample difficult to purify due to the high‐energy ball milling. In this report, a novel synthetic method is developed, which utilizes Te‐vapor transport and solid‐phase diffusion to efficiently produce the RE 3‐x Te 4 phases (RE = La, Ce, Pr, Nd). Notably, this method obviates the requirement for high‐energy ball‐milling instruments, conventionally indispensable in the mechanochemical syntheses. For as‐synthesized La 2.74 Te 4 material, a high figure of merit of 1.5 is achieved at 1073 K, owning to the reduced electronic thermal conductivity with metal impurities well eliminated.