Photoinduced phase transition of $\mathrm{Cu}(\mathrm{dieten}{)}_2{X}_2$ $(\mathrm{dieten}=N,N$-diethylethlenediamine, $X={\mathrm{BF}}_4$ and ${\mathrm{ClO}}_4)$ has been investigated by means of x-ray-absorption fine structure spectroscopy. Upon UV-light $(>\mathrm{}250\mathrm{}\mathrm{nm})$ irradiation at 30 K, the materials undergo a photoinduced structural transition. Cu K-edge x-ray-absorption near-edge structure spectra have clarified that the initially planar ${\mathrm{CuN}}_4$ unit is distorted tetrahedrally. Although the structural change within the first nearest neighbors around Cu is similar to that in the thermally driven transition (the high-temperature phase), the configuration of the surrounding ethyl and ethylene groups in the photoinduced phase was found to resemble that of the low-temperature phase rather than that of the high-temperature one. Namely, no noticeable reconstruction of ethyl groups takes place, which occurs in thermally activated transition. The photoinduced state is conclusively regarded as a new metastable state that is structurally not equivalent either to the high- or low-temperature phases.

• Received 25 December 2002

DOI:https://doi.org/10.1103/PhysRevB.67.172104