Ultrahigh-pressure structural evolution of amorphous-Al₂O₃ from acoustic velocity measurements

Abstract

Given that Al2O3 (alumina) is a primary component of silicate magmas in deep planetary interiors, elucidating the structural transformations of amorphous alumina (a-Al2O3) 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-Al2O3 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-Al2O3, 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 ~120 GPa, and (ii) a steep increase above ~120 GPa. These behaviors mirror those observed in MgSiO3 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-Al2O3 and silicate melts within terrestrial planet interiors, offering essential constraints on the dynamics and evolution of deep magma oceans during early planetary formation.