Experimental section 
In Fig. 13, the first step is depicted by the horizontal arrow with a grey arrowhead. Hexagonal ice (big turquoise hexagon) is compressed from atmospheric pressure to 1.6 GPa. Following in essence the protocol by Mishima et al. , subsequently, decompression to 1.1 GPa is performed (T ~ 77 K; compression/decompression rate: 0.1 GPa min−1).
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