REVISITED VIBRONIC MODEL FOR LI+ ION AND FA(LI) CENTER IN ALKALI-HALIDES

The controversial far-infrared optical absorption in Li-doped alkali halides is now attributed to transitions between the low-lying energy levels of an anharmonic oscillator composed of three halogen-ion pairs centred at the cation site which vibrate in the T1u ungerade mode. This oscillator is electronically coupled to the impurity dragging it into rotation over <111> sites. Several experimental features derive simply from our model such as the absence of any clear-cut lithium-isotope effect and the low-frequency range. The capital effect of an F centre at a <001> nearest-neighbour (nn) site is to reduce the dimensionality of vibration forcing the impurity to rotate in the perpendicular (001)-plane. This upgrades the optical frequency in proportion to (3/2)1/2 and shrinks the off-centre displacement. We finally show that recent computer-simulation data on Li-doped alkali halides provide a solid ground for regarding the Li off-centredness as a pseudo-Jahn-Teller phenomenon.