Given that ${\mathrm{Al}}_2{\mathrm{O}}_3$ (alumina) is a primary component of silicate magmas in deep planetary interiors, elucidating the structural transformations of amorphous alumina ($a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$) under extreme pressures is crucial for understanding the evolution and dynamics of deep silicate magmas. However, the extremely low glass-forming ability of alumina has made it technically difficult to synthesize sufficient quantities of pure $a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ for measurement. Structural information has thus been largely confined to amorphous nanofilms at ambient pressure, limiting our understanding of structural transformations under ultrahigh-pressure conditions. Here, we have employed recently established pulsed-laser deposition techniques to synthesize high-quality, millimeter-sized $a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, which enabled us to conduct in situ Brillouin scattering spectroscopic measurements in a diamond anvil cell, measuring acoustic velocities under pressures up to 174 GPa. The results reveal two distinct pressure regimes in transverse acoustic velocity: (i) a monotonic increase up to $\sim 120$ GPa, and (ii) a steep increase above $\sim 120$ GPa. These behaviors mirror those observed in $\mathrm{MgSi}{\mathrm{O}}_3$ glass and can be interpreted as (i) a transition from $4^{++}$ to 6-fold Al-O coordination, and (ii) the emergence of coordination states beyond 6-fold. The experimental observation of $6^{+}$ coordination under extreme pressure provides novel insights into the densification mechanisms of $a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$ and silicate melts within terrestrial planet interiors, offering essential constraints on the dynamics and evolution of deep magma oceans during early planetary formation.

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• Acoustic phonons
• Earth's interior
• High-pressure studies
• Glasses
• Brillouin scattering & spectroscopy