Since the crystalline front that is in contact with the glass phase changes during crystal growth, all possible PS 4 3alignments for each crystalline plane should be considered in the H total glass-ceramics calculation. Each crystal plane has four non-equivalent PS 4 3alignments perpendicular to the crystalline plane of interest. All non-equivalent faces of each crystalline plane were individually evaluated, and H total glass-ceramics was estimated as the mean value. Fig. illustrates four examples of unique PS 4 3alignments for the (100) and (010) planes. The planes (i)-(iv), where P atoms align linearly, were included in the H total glass-ceramics calculation.