OPTICAL F-CENTER PROPERTIES AND PHASE-TRANSITIONS IN KCN AND NACN CRYSTALS

KCN and NaCN crystals present a structural cubic to orthorhombic phase transition respectively at T1=168K and 288K with a ferroelastic ordering of the CN- ions and an antiferroelectric phase transition respectively at T2=83K and 172K. The optical properties of F centres are studied between 4.2K<T<300K. In absorption the single Gaussian band observed in both crystals in the cubic phase (T>T1) and attributed to the 1s to 2p transition splits in to three components in the orthorhombic phase (T<T1). Below T2 one observes a marked blue shift of the low energy band and, in KCN, the appearance of a small new band at 1.5 eV. These phenomena are explained theoretically by the symmetry change of the crystalline structure at T1 and by the presence of a local electric field for T<T2 due to the alignment of the CN- electric dipoles which produces a internal Stark effect. The small absorption band in the near IR is assigned to the partially allowed (field mixing) 1s to 2s transition.