  Given the consensus that pressure improves cation order in most of known
materials, a discovery of pressure-induced disorder could require
reconsideration of order-disorder transition in solid state physics/chemistry
and geophysics. Double perovskites Y2CoIrO6 and Y2CoRuO6 synthesized at ambient
pressure show B-site order, while the polymorphs synthesized at 6 and 15 GPa
are partially-ordered and disordered respectively. With the decrease of
ordering degrees, the lattices are shrunken and the crystal structures alter
from monoclinic to orthorhombic symmetry. Correspondingly, long-range
ferrimagnetic order in the B-site ordered phases are gradually overwhelmed by
B-site disorder. Theoretical calculations suggest that unusual unit cell
compressions under external pressures unexpectedly stabilize the disordered
phases of Y2CoIrO6 and Y2CoRuO6.
