[SiO4F] -trigonal bipyramids differs markedly: In the former, energetically unfavorable case, there are only two such linkages between adjacent [SiO4F] -units, compared to four for the latter case. It seems plausible to conclude that the local distortions that occur as a consequence of the formation of [SiO4F] -trigonal bipyramids can be accommodated more easily when these units are distributed rather evenly in the structure, leading to a lower energy for the F@Si1_2 and F@Si1_4 structures.