Stereochemically Active Lone‐Pair Electrons Driven High‐Performance and Self‐Powered X‐Ray Detection in Cs ₂ TeMo ₃ O ₁₂ Crystal

X‐ray detection is crucial for medical imaging and industrial non‐destructive testing. However, conventional materials face limitations such as low absorption efficiency, high dark current, and the necessity for external power bias. This study presents a novel inorganic polar oxide crystal, Cs 2 TeMo 3 O 12 , as a high‐performance direct‐conversion X‐ray detector material, particularly for self‐powered applications. The stereochemically active lone‐pair electrons of Te 4+ induce a non‐centrosymmetric polar structure, which generates a built‐in electric field that enables efficient charge carrier separation under zero bias. Along the polar Z‐direction, the crystal exhibits a high resistivity of 5.08 × 10 14 Ω cm, a considerable mobility‐lifetime product of 1.08 × 10 ‒3 cm 2 V ‒1 , and an excellent sensitivity of 436 µC Gy air ‒1 cm ‒2 under a 2000 V cm −1 electric field. Most notably, the device operates effectively in a self‐powered mode, achieving a sensitivity of 178 µC Gy air ‒1 cm ‒2 and an ultralow detection limit of 10.5 nGy air s ‒1 . Furthermore, it demonstrates exceptional operational stability with negligible dark current drift. This work not only introduces Cs 2 TeMo 3 O 12 as an exceptional candidate for low‐dose and self‐powered X‐ray detection but also provides a novel design strategy leveraging lone‐pair electrons for developing advanced photoelectric materials.