The Ph3E−E‘Me3 (E =
Si, Ge, Sn; E‘ = Si, Ge, Sn) compounds exhibit two morphotropic steps
in the crystalline state: a [E = E‘ = Si; E = Si, E‘ = Ge; E
= Ge, E‘ = Si, E = E‘ = Ge (space group P3̄)]
→ b [E = Ge, E‘ = Sn; E = Sn, E‘ = Ge (space group
Pna21)] → c [E = E‘ = Sn (space group
P3̄)]. Ph3Sn−SnMe3
reverts to group a. Due to the long Sn−Sn bond, it is not
isometric, and therefore not isostructural, with the group a compounds;
the relaxed relationship is termed as
homeostructural.