Abstract:
On the basis of a great body of experimental data it was concluded [1] that neither s conduction electrons nor s valence holes undergo self-trapping in wide -gap (Eg > 5 eV) binary ionic crystals. On the other hand, the self-trapping of p-holes takes place in all alkali halide crystals as well as in many binary and complex metal oxides. In addition, there are only few cases of the self-trapping of d-electrons in non-cubic dielectrics (e.g., for Sc2O3 see [2] and references therein). The main goal of the present study is to elucidate the peculiarities of electronic excitations and electron-hole processes in pure CaSO4 as well as in CaSO4 doped with two-valent Mn2+ ions оr three-valent ions (Tb3+, Gd3+, Dy3+, Eu3+), the effective charge of which is compensated by Na+ ions in cation sites. It has been shown recently [3] that RE3+-Na+ dipole pairs serve as efficient hole traps, which manifest themselves as the peaks of thermally stimulated luminescence (TSL) at 220-260 K [3]. Alternatively, there is no TSL peaks at 210- 260 K in CaSO4:Mn2+ and CaSO4:Eu2+ phosphors. The ~50 K TSL peak was detected in all the above-mentioned doped CaSO4 being even a dominant one in the especially pure CaSO4 sample. This TSL peak (measured for RE3+-impurity emission) can be tentatively ascribed to the recombination of the electrons, that turn mobile at such temperatures (i.e. their self-trapping is over), with the holes still localized near different RE3+ ions. The emission and excitation spectra were measured at 8-10 K using synchrotron radiation of 4-35 eV at the SUPERLUMI station of HASYLAB. The excitation spectra, measured with an instrumental resolution of ~0.3 nm, were normalized to equal quantum intensities of synchrotron radiation falling onto a crystal. The reference signal for normalization was recorded from a sodium salicylate. The emission spectra of Tb3+ centres were measured using a 0.3 m Czerny-Turner monochromator-spectrograph SpectraPro-308i equipped with a liquid nitrogen cooled CCD detector (spectral resolution of ~0.2 nm).
