The continuous temperature-induced change of the ligand field strain in a pseudo-Jahn-Teller thermochromic crystal bis(NN-diethylethylenediamine)copper(II) tetrafluoroborate was studied. In situ powder photodiffraction was employed to demonstrate that at the low-temperature limit of the gradual lattice stability another structural phase transition can be triggered by photoinduction of the intramolecular ligand-to-metal charge transfer. The minute differences in the cationic structure imposed by the isomorphic ionic substitution result in increase of the temperature of the photoinduced phase transition of about 30 K relative to the perchlorate salt. (c) 2006 Elsevier B.V. All rights reserved.