Understanding the mechanisms governing the rapid formation of High Temperature Superconductors is essential for advancing scalable fabrication routes. Transient Liquid-Assisted Growth (TLAG) [1-5] offers a powerful approach for producing epitaxial REBa₂Cu₃O₇ (REBCO) films at exceptionally high growth rates (100 - 2000 nm s⁻¹), enabling low-cost and high-throughput production of superconducting coated conductors. However, the highly non-equilibrium and ultrafast nature of TLAG presents significant challenges for elucidating the phase formation pathways and kinetic parameters that control superconducting layer development.

To address this, we developed dedicated instrumentation for real-time monitoring of the TLAG process under controlled atmospheres. Time-resolved in-situ X-ray diffraction (XRD) experiments were performed at the ALBA Synchrotron with acquisition times of 100 ms per frame, enabling direct observation of precursor reactions, intermediate phase evolution, and the nucleation and growth of the REBCO. The setup allows precise control of temperature, heating rate, total pressure, and oxygen partial pressure, including sub-second variations in oxygen atmosphere, providing access to the relevant kinetic growth regime.

Complementary in-situ mass spectrometry was used to correlate structural transformations with gaseous reaction products, while simultaneous electrical resistance measurements tracked conductivity evolution throughout the process, offering direct insight into the emergence and growth rate of the superconducting layer.

Preliminary in-situ X-ray absorption spectroscopy (XAS) measurements further probed the electronic and chemical dynamics of the transient liquid. Tracking the Cu absorption edge revealed rapid changes in Cu oxidation state within specific temperature and oxygen partial pressure windows. A fast acquisition approach based on single-energy detection at the Cu¹⁺ feature enabled real-time monitoring of redox dynamics during ultrafast growth.

Together, time-resolved XRD and XAS provide a comprehensive picture of the structural, electronic, and chemical pathways leading to REBCO formation under non-equilibrium TLAG conditions. These insights define an optimized processing window and contribute to a mechanistic framework for controlling ultrafast superconducting film growth.

[1] L. Soler et al. 2024, Nat Commun 11, 344, https://doi.org/10.1038/s41467-019-13791-1

[2] L. Saltarelli et al. 2025, Advanced Materials, e10660, https://doi.org/10.1002/adma.202510660

[3] L. Saltarelli et al. 2022, ACS Applied Mater. Interf. 14, 43, 48582, http://dx.doi.org/10.1021/acsami.2c11414

[4] T. Puig et al. 2024, Nature Reviews Physics, 6, 132–148, https://doi.org/10.1038/s42254-023-00663-3

[5] E. Ghiara et al., Advanced Materials Technologies, e02441, https://doi.org/10.1002/admt.202502441