Thermally induced effects on optical absorption in Ag-doped Bi12SiO20 single crystals

The spectral-temperature dependences of optical absorption of Bi12SiO20 crystals doped with Ag ions were measured in the visible range of photon energy hν. The temperature of the samples was varied in heating-cooling cycles in the interval T = 280 – 680 K. It was shown that the spectra fulfilled Urbach rule for hν > 2.1 eV at the forbidden zone width of 3.3 eV. The Urbach rule parameters, calculated within the frame of the electron-phonon interaction model, showed anomalous behavior in the interval T = 389 – 500 K. The observed peaks in the temperature dependences of optical absorption were attributed to the competition of thermally induced processes of color centers formation and destruction. The color centers were assumed in the form of Ag2+ – 2O- and Ag+ – O- complexes that are created due to thermal activation energy Ea1 = 0.1 eV and Ea2 = 0.145 eV, respectively. The spectrum of thermochromic effect characterizing non-reversible thermally induced weakening of optical absorption was detected and analyzed.