dilational (dilatational) transition

A transition in which the crystal structure is dilated (or compressed) along one (or more) crystallographic direction(s) while the symmetry about that direction is retained. Examples:

The transition of a CsCl-type structure to a rock salt structure in which dilation occurs along the three-fold axis.
The transition at $T D$ in quenched NiS in which volume expansion occurs without change of symmetry on going from a metallic state ( $T > T D$ ) to a semiconducting state ( $T < T D$ ).

Source:

PAC, 1994, 66, 577 (Definitions of terms relating to phase transitions of the solid state (IUPAC Recommendations 1994)) on page 581