Spin and Optical Fingerprints of Pressure-Driven Double Topological Phase Transition in Monoclinic BaIn₂As₂

Distinguishing topological from trivial phases is central to condensed matter physics, yet resolving distinct nontrivial regimes that share the same Z ₂ classification remains challenging. Here, using first-principles calculations, we show that monoclinic BaIn₂As₂ undergoes two successive pressure-driven topological phase transitions within the same P2/m structural phase, evolving from a topological insulator at ambient pressure to a trivial insulator at 16 GPa and then to a high-pressure topological semimetal at 26 GPa. The two nontrivial phases share the same Z ₂ = 1 classification but exhibit distinct surface spectra and opposite low-energy optical anisotropy. We establish a two-level identification scheme in which spin Hall conductivity provides a fingerprint for separating nontrivial phases from the trivial phase, with possible inverse spin Hall electrical readout, whereas monoclinic symmetry-enabled in-plane reflectance anisotropy resolves the two nontrivial phases. We further propose a diamond-anvil-cell platform combining yttrium-iron-garnet-based spin pumping with polarization-resolved infrared reflectance.