LATTICE POINT-DEFECTS AND FREE-CARRIER CONCENTRATION IN BI2+XTE3 AND BI2+XSE3 CRYSTALS

Reflectance in the infrared region and lattice parameters were measured on Bi2+xTe3 and Bi2+xSe3 single crystals. The plasma resonance frequency, N/m(perpendicular-to) and P/m(perpendicular-to), respectively, were determined. The process of incorporation of the superstoichiometric Bi is discussed from the changes of the optical and lattice parameters and by means of quantum-chemical calculation. Two possible ways of incorporation are considered: (i) forming anti-site defects Bi(Se(Te))' and (ii) forming seven-layer or nine-layer lamellae. It appears that at the lower concentrations of superstoichiometric bismuth there exist anti-site defects of the Bi(Te)' or Bi(Se)' type, whereas upon increasing the content of Bi atoms in the lattice of these chalcogenides could lead to the formation of seven-layer or nine-layer lamellae.