Light-induced valence-state switching in BaFCl:La and SrFCl:La

A tetragonal La[2+] center (symmetry C[4v]) was identified in single crystals of BaFCl and SrFCl doped with lanthanum with the aid of electron paramagnetic resonance (EPR)/electron-nuclear double resonance (ENDOR). This center forms a donor-acceptor couple with initially present F(F-) centers. Switching takes place by illumination of appropriate wavelength. The kinetics of the process was monitored by EPR as La[2+] and the unswitched F center are paramagnetic. The results of our experimental investigation of this kinetics are presented. A foregoing spectroscopic characterization of the La[2+] center allowed one to identify a d-d (the B[1]-E) transition, a charge-transfer band (for BaFCl at 10 940 cm[-1] and at 17 890 cm[-1], respectively) and to obtain a value of 710 cm[-1] for the spin-orbit coupling constant in the ground state. In order to narrow the choice of possible acceptor-donor partners a detailed EPR/optical search was further done to identify a number of lattice defects and oxygen centers—in addition to a La-oxygen molecular structure