  We present temperature-dependent single-crystal diffraction results on seven
antifluorite-type $A_2MeX_6$ compounds with $Me$=Os or Ir: K$_2$OsCl$_6$,
$A_2$OsBr$_6$ with $A$=K, Rb, Cs and NH$_4$, and K$_2$Ir$X_6$ with $X$=Cl and
Br. The structural transitions in this family arise from $MeX_6$ octahedron
rotations that generate a rich variety of symmetries depending on the rotation
axis and stacking schemes. In order to search for local distortions in the
high-symmetry phase we perform refinements of anharmonic atomic displacement
parameters with comprehensive data sets. Even at temperatures close to the
onset of structural distortions, these refinements only yield a small
improvement indicating only small anharmonic effects. The phase transitions in
these antifluorites are essentially of displacive character. However, some
harmonic displacement parameters are very large reflecting soft phonon modes
with the softening covering large parts of the Brillouin zone. The occurrence
of the rotational transitions in the antifluorite-type family can be remarkably
well analyzed in terms of a tolerance factor of ionic radii.
