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

Abstract

A tetragonal La 2ϩ center ͑symmetry C 4v ) was identified in single crystals of BaFCl and SrFCl doped with lanthanum with the aid of electron paramagnetic resonance ͑EPR͒/electron-nuclear double resonance ͑EN-DOR͒. This center forms a donor-acceptor couple with initially present F(F Ϫ ) centers. Switching takes place by illumination of appropriate wavelength. The kinetics of the process was monitored by EPR as La 2ϩ 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 2ϩ center allowed one to identify a d-d ͑the B 1 -E) transition, a charge-transfer band ͑for BaFCl at 10 940 cm Ϫ1 and at 17 890 cm Ϫ1 , respectively͒ and to obtain a value of 710 cm Ϫ1 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.

Instability of some divalent rare earth ions and photochromic effect

“…Direct and inverse optical transformations of divalent lanthanum (La 2+ ) ions into F centers have been studied in BaFCl-La and SrFCl-La crystals by optical means and EPR. Although the explanation of the authors of [39,40] differs from our interpretation, the similarities between the photo-induced electron transition from the divalent ion to the anion vacancy are worthy of attention.…”

Anomalous superhyperfine tensor observed in BaFCl doped with the5d1ionLa2+: Role ofGarcı́a-Lastra1, Bill2, Barriuso3 et al. 2007Phys. Rev. BSelf Cite

“…Additional problems appear in MFCl: La 2+ ͑M =Ba,Sr͒ when looking at experimental absorption bands of La 2+ impurity. 10 In fact, in addition to a crystal field band peaked at 10 900 cm −1 another more intense band peaked at 17 890 cm −1 has clearly been observed for BaFCl: La 2+ ͑see Fig. 3͒.…”

“…14,15 Electron paramagnetic resonance ͑EPR͒ spectra measured on MFCl: La 2+ ͑M =Ba,Sr͒ samples show a resolved superhyperfine ͑shf͒ structure due to the interaction of the unpaired electron with four equivalent F nuclei, but that from neighboring Cl nuclei is not resolved. 10 Another relevant experimental information comes from the measured gyromagnetic tensor ͑g͒ which is found to exhibit axial symmetry ͑g ʈ = 1.753, g Ќ = 1.889 for BaFCl: La 2+ ͒ the principal axis being the c axis of the host lattice 10 ͑Figs. 1 and 2͒.…”

“…Experimental data on La 2+ impurities in BaFCl and SrFCl lattices displaying the matlockite structure have recently been reported. 10 Although the La 2+ cation is rather unusual [10][11][12][13] it is, however, attractive from a fundamental standpoint. In this sense, its unpaired electron lies in a 5d orbital but the empty 4f orbital is only about 8000 cm −1 higher in energy in the free ion.…”

Photoreduction of Sm3+ in Nanocrystalline BaFCl

“…We note here that a range of possible defect centers were previously investigated in the context of light-induced valence state switching in single crystals of BaFCl:La, where it was concluded that the photoelectrons from La 2+ ions were most likely trapped at F centers. 22…”