Experimental and theoretical high pressure study of calcium hydroxyaluminate phases

Five calcium hydroxyaluminate phases have been investigated by synchrotron powder diffraction at high-pressure: two hydrogarnets, kuzelite, stratlingite and ettringite. The obtained bulk modulus, K₀, for kuzelite, stratlingite and ettringite were 23(1), 27(1) and 30(3) GPa, respectively. Kuzelite and stratlingite underwent transformations above 1 GPa likely releasing interlayer water. Kuzelite becoming markedly amorphous and stratlingite remained crystalline, K₀ = 58(6) GPa in the 1.5–5.5 GPa pressure range. The structural behavior for hydrogarnet samples is prone to the use of pressure transmitting media. K₀ for Ca₃Al₂(OH)₁₂ was 81(2) and 76(2) GPa for silicone oil and alcohol mixture, respectively. A similar study for Ca₃Al_1.7Fe_0.3(OH)₁₂ yielded 73(1) and 58(1) GPa for silicone oil and alcohol mixture, respectively. Atomistic calculations using periodic Density Functional Theory showed that the softening in iron-doped katoite, when compared to stoichiometric katoite, can be assigned primarily to greater compressibility of Ca−O dodecahedra, which overcompensates strengthening of hydrogen bonding between Al/Fe hydroxide groups.