DIMENSIONAL EFFECTS OF DEBYE-WALLER FACTORS IN LAYERED CRYSTALS

It has been found by He atom scattering and LEED, that the mean square vibrational amplitude LT AN BR u//c**2 RT AN BR normal to the basal planes of graphite does not saturate at low temperatures to the zero point energy value predicted by Debye theory, but decreases substantially below that value as T approaches zero. This result has been interpreted as being due to the strong anistropy and reduced dimensionality of graphite, and has been observed, so far, only for surface atoms of graphite and 1T-TaS//2. The (00 18) reflection for graphite, and the (00 16) for TaS//2, have been monitored as functions of T between 300 and 5K. While the Debye model works well in both cases above 100 K, LT AN BR u//c**2 RT AN BR saturates to a value 10. 2% lower than that predicted by the model in the case of graphite, and 23. 8% in the case of TaS//2.