Fig. 4.

SOAP analysis of the coordination environments in the glass models. a Two-dimensional SOAP map of the Ge, Sb and Te local environments for each atomic species in the amorphous 225GST structures, generated in this work, with two (light blue), three (dark blue), four (black), five (dark green) and six (light green) other atoms in the nearest-neighbour positions. Environments away from the centre of each cluster correspond to under-/over-coordinated atoms inside the glass network. b Local environments of Ge atoms with 3, 4, 5 and 6 other atoms in the nearest-neighbour positions in the glass structure averaged over the thirty 225GST glass models. Despite the four-coordinated local environment being the most-favourable geometry that can be found in the glass, under- and over-coordinated Ge atoms are also possible geometrical arrangements within the amorphous network. c About 16% of the 5-coordinated Ge atoms were found to be 5-coordinated to only Te atoms inside the glass structure, whereas ~ 12% were coordinated to four Te atoms and one other Ge atom, corresponding to two structural motifs related to the localisation of the mid-gap electronic states. The distribution of coordination numbers around the atoms was calculated by using a geometrical bond-distance cutoff of 3.2 Å