Resolving the metastable Si-XIII structure through convergent theory and experiment

Silicon allotropes have emerged as a pivotal frontier for engineering materials with tailored optical, electronic, and thermal functionalities, opening routes toward integrated photonics, energy harvesting, and quantum technologies, while avoiding rare or toxic elements. High-pressure experiments have revealed several metastable silicon allotropes, among which Si-XIII stands out. First observed more than 20 years ago, this phase has remained structurally unidentified, representing a significant gap in our understanding of elemental silicon allotropy. In this work, a convergent methodology is employed combining advanced theoretical modeling with experimental characterization to finally resolve the long-standing structural assignment of Si-XIII. We construct a structural model guided by experimental observations, validate it through first-principles optimization, and systematically test it against multiple experimental signatures. All key fingerprints of this phase — interplanar spacings, Raman frequencies, thermodynamic stability, and kinetic pathways — are consistently rationalized by our proposed crystal structure. These findings provide a crucial missing piece in the high-pressure phase diagram of silicon, demonstrate the power of integrating computational predictions with experimental validation to resolve complex structural problems in materials science, and open new avenues for the controlled synthesis and functional exploitation of metastable Group-IV allotropes.