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Optical absorption of Bi12SiO20 crystals containing nonstoichiometry defects in the cation sublattices was studied in the temperature range 80 ÷ 600 K. It is shown that, at absorption coefficients the Urbach rule is fulfilled; while at indirect transitions with the participation of optical phonons take place. The value αt depends on temperature. The parameters of the Urbach rule change regularly when the Bi:Si stoichiometry is violated. The features of the isoabsorption dependences and the parameter σ characterizing the temperature change of the edge absorption are revealed. [ABSTRACT FROM AUTHOR]

The correlation between photochromic effect and the effect of inducing the photoconductivity in undoped and Al-, Ga-, Sn-, Cr-, Mn-, V-, Fe-, Cu-, Ag-, Mo-doped Bi 12 SiO 20 single crystals have been investigated. It is shown that the photochromic effect is conditioned by the intra-center optical transitions in deep centers, while the effect of inducing the doped photoconductivity is conditioned by filling of the shallow levels in forbidden zone. Characteristic peculiarities of doped ions influence on the photochromic properties of Bi 12 SiO 20 crystals are studied. [ABSTRACT FROM AUTHOR]