High‐Pressure Behavior of Mullite: An X‐Ray Diffraction Investigation

Using synchrotron X‐ray diffraction and diamond anvil cells we performed in situ high‐pressure studies of mullite‐type phases of general formula Al_4+2xSi_2?2xO_10?x and differing in the amount of oxygen vacancies: 2:1‐mullite (x = 0.4), 3:2‐mullite (x = 0.25), and sillimanite (x = 0). The structural stability of 2:1‐mullite, 3:2‐mullite, and sillimanite was investigated up to 40.8, 27.3, and 44.6 GPa, respectively, in quasi‐hydrostatic conditions, at ambient temperature. This is the first report of a static high‐pressure investigation of Al₂O₃–SiO₂ mullites. It was found that oxygen vacancies play a significant role in the compression mechanisms of the mullites by decreasing the mechanical stability of the phases with the number of vacancies. Elevated pressure leads to an irreversible amorphization above ~20 GPa for 2:1‐mullite and above 22 GPa for 3:2‐mullite. In sillimanite, only a partial amorphization is observed above 30 GPa. Based on Rietveld structural refinements of high‐pressure X‐ray diffraction patterns, the pressure‐driven evolution of unit cell parameters is presented. The experimental bulk moduli obtained are as follows: K₀ = 162(7) GPa with K₀′ = 2.2(6) for 2:1‐mullite, K₀ = 173(7) GPa with K₀′ = 2.3(2) for 3:2‐mullite, K₀ = 167(7) GPa with K₀′ = 2.1(4) for sillimanite.