Phase Interlayer Shift and Stacking Fault in the Kagome Superconductor CsV3Sb5

The stacking degree of freedom is a crucial factor in tuning material properties and has been extensively investigated in layered materials. The kagome superconductor CsV3Sb5 was recently discovered to exhibit a three-dimensional CDW phase below TCDW similar to 94 K. Despite the thorough investigation of inplane modulation, the out -of -plane modulation has remained ambiguous. Here, our polarization- and temperature -dependent Raman measurements reveal the breaking of C6 rotational symmetry and the presence of three distinct domains oriented at approximately 120 degrees to each other. The observations demonstrate that the CDW phase can be naturally explained as a 2c staggered order phase with adjacent layers exhibiting a relative pi phase shift. Further, we discover a first -order structural phase transition at approximately 65 K and suggest that it is a stacking order -disorder phase transition due to stacking fault, supported by the thermal hysteresis behavior of a Cs -related phonon mode. Our findings highlight the significance of the stacking degree of freedom in CsV3Sb5 and offer structural insights to comprehend the entanglement between superconductivity and CDW.