Optical F centre properties and phase transitions in KCN and NaCN crystals

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Abstract
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 4.2K1) and attributed to the 1s to 2p transition splits in to three components in the orthorhombic phase (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 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.