6533b870fe1ef96bd12cef69

RESEARCH PRODUCT

On nature of the transient IR-absorption STE-like bands at 0.15–0.36eV in alkali halides

subject

description

New evidence is given that two classes (A and B) of the transient IR-absorption bands: (A) with max. at 0.15-0.36 eV (in NaCl : I, NaBr, NaI, KCl : I, KBr : I, RbCl : I, RbBr : I), due to on-centre self-trapped exciton and (B) with max. at 0.27-0.36 eV (in NaCI, KCl, KBr, RbCl), due to shallow trapped electrons or bound polarons, are caused by the same defect-shallow trapped electron (e-) at the substitutional (cation: c-site) alkali impurity cation (M + ): [M - ] 0 c e - . The A- and B-class IR bands have the same location, similar shape, half-width (exactly coincide for KCl : I and KCl at 80 or 10 K with the same vibration structure). It is established that the same Mollwo-Ivey plot curves E 0 = a/d n (d--nn anion-cation distance, n-exponent, a-constant) are fulfilled for both IR band classes, if we plot instead the IR band peak energy values the more definite values of the IR band zero-phonon line energy E 0 (for NaCI, KCl, KBr, RbCl and KCl : I) and/or the IR band low-energy edge energy E 0 (±0.03 eV) values (for NaBr, NaI, NaCl : I, KBr : I, RbCl : I and RbBr : I). These data are significant additional evidence that the A-class IR bands are caused by the same type defects as the B-class IR bands-by the shallow electron traps, i.e., centres [M + ] 0 c e - Two types of the [M + ] 0 c e - centres are predominant: (i) [Li + ] 0 c e - in NaX host crystals with E 0 29.4/d 4.72 and (ii) [Na + ] 0 c e - in KX and RbX host crystals with the relation E 0 6.15/d 2.74 .