The supplementary material for changes in the lattice constant of the r8 phase upon depressurization, changes in the lattice constant of the r8 phase at ambient pressure, the pair distribution functions of the r8, bc8, and hd-Si0.3Ge0.7 phases calculated from atomic structures relaxed at fixed experimental volumes using 80-atom SQS cells, and the first peak positions of the total and partial pair distribution functions for the r8, bc8, and hd-Si0.3Ge0.7 phases.