Positron lifetime measurements and electronic structure of CeNiSn

Positron lifetime measurements have been carried out in CeNiSn, in the temp erature range of 5 to 300 K. In the 300 and 150-K temperature interval the positron lifetime is seen to decrease from 178+/-1 to 176+/-1 ps. Below 150 K the lifetime shows a gradual increase and attains a value of 181+/-1 ps at 35 K, below which the lifetime shows a small decrease. This behavior of the positron lifetime is seen to correlate with earlier measurements [Y. Uw amoto et at, J. Magn. Magn. Mater. 104, 643 (1992)] on the thermal expansio n coefficient in CeNiSn, which has been understood in terms of electronic s tructure changes associated with the system transforming from an independen t Kondo regime to a Kondo insulating state, via an intermediate mixed valen t regime. Strong support for these electronic structure changes are provide d by nb initio positron lifetime calculations which are seen to be in good agreement with the positron experiments.