The True Parent Phase of K1.9Fe4.2Se5: A Stripe-type Orthorhombic Phase Requiring a Superconducting Distortion
The origin of the four-fold Tc amplification in A_xFe_{2-y}Se_2 (>30 K) compared to FeSe (8 K) remains a central puzzle, complicated by a debate over the true superconducting (SC) parent phase--the I4/m (245) insulating matrix or a supposed I4/mmm metallic phase. Here, we resolve this ambiguity by identifying a novel "stripe-type orthorhombic phase" as the true parent phase of the high-Tc state, whose diffraction signature is the d2 peaks. Through decisive experiments, we demonstrate that this d2 parent phase is not intrinsically superconducting. Instead, superconductivity emerges only after this stripe phase undergoes a subsequent structural distortion, the definitive signature of which is the asymmetric broadening of the d1 main peaks. Our findings establish that the d2 peaks signal the formation of the parent phase, while the broadening of d1 peaks signals the transition into the superconducting state. This discovery of a two-step transition--formation of a stripe parent phase, followed by a superconducting distortion--provides a new mechanism for Tc amplification via controlled heterogeneity.