Pressure dependence of sound velocity, shear modulus, and yield strength of the amorphous 4:1 methanol-ethanol mixture to 78 GPa

High-pressure elastic and plastic behavior of the amorphous solid, formed after solidification of the mixture of methanol and ethanol in the volume ratio 4:1 (4:1-ME), was investigated using the technique of time-domain Brillouin scattering (TDBS). Isotropic nature of this solid, whose bulk modulus exceeds that of diamond at pressures above 60--70 GPa (provided the earlier-measured dependence of its density on pressure, \ensuremath{\rho}($P$), holds at $P$>60 GPa), was confirmed thanks to the high 3D resolution of the TDBS technique. This permitted establishing of the relationship between elastic and plastic properties at high pressures because elastic response of the amorphous solid was not affected by anisotropy and texture, as is the case for crystalline solids. Using the TDBS technique, we measured pressure dependence of the Brillouin-oscillation frequencies, ${f}_{\mathrm{B}}$($P$), of 4:1-ME and, consequently, of its longitudinal sound velocity, ${V}_{\mathrm{L}}$($P$), and shear modulus, $G$($P$), to $P=78$ GPa. To access pressure dependence of its yield strength, ${\ensuremath{\sigma}}_{\mathrm{y}}$($P$), we used the here-established secondary pressure scale, based on our experimental ${f}_{\mathrm{B}}$($P$), and measured the maximal pressure gradients in the compressed samples which are proportional to ${\ensuremath{\sigma}}_{\mathrm{y}}$. We found that the rapid increase of bulk modulus of 4:1-ME with pressure, deduced from the \ensuremath{\rho}($P$) measured earlier to 60 GPa and extrapolated here to 78 GPa, is not accompanied by a similarly strong increase of its shear modulus and yield strength. The two parameters remain significantly below those of diamond at atmospheric and high pressures. Comparison of the here-measured ${\ensuremath{\sigma}}_{\mathrm{y}}$($P$) with the dependences $B$($P$) and $G$($P$) indicated that ${\ensuremath{\sigma}}_{\mathrm{y}}$ can be similarly well approximated by linear functions where $B$ or $G$ serves as the variable.