Mechanoluminescence from piezoelectric crystals of an europium complex

Mechanoluminescence (ML) from crystals of an europium complex was investigated by microphotography. Crystals prepared by recrystallization from acetone solution are noncentrosymmetric, piezoelectric ones. We have succeeded in catching the images of ML occurring at the moment of cleaving these crystals along their axis. From these ML images, we understand that the formation of piezoelectric charges is one of the most important processes for ML excitation.