Ag-109 NMR investigation of atomic motion in the incommensurate and paraelectric phases of proustite (Ag3AsS3)

Ag-109 NMR line shape, T-1, Hahn spin echo, and Carr-Purcell-Meiboom-Gill ( CPMG) spin-echo measurements were performed in proustite (Ag3AsS3) between 420 and 47 K in order to understand the role of atomic motions in phase tra nsitions in a system containing an incommensurate phase. These measurements indicate a definite change in the motional characteristics of Ag ions near 240 K. Above 240 K, an Ag+ ion executes fast jump motions between its six inequivalent chemically shifted sites. However, below 240 K, these motions are too slow to affect the line shapes. The NMR line shape results are cons istent with the existence of a 3q incommensurate phase between 60 and 50 K. Between 180 and 55 K, Hahn and CPMG spin-echo decay measurements suggest t hat slow motions exist. These motions are attributed (1) in the paraelectri c phase (above 60 K) to slow jumping of an Ag+ ion between its inequivalent sites with a jump time of the order of the Hahn spin-echo time and (2) in the incommensurate phase (50-60 K) to ultraslow motion of the incommensurat e modulation wave. This paper constitutes a report of the effect on the Hah n spin-echo decay due to atoms jumping between sites characterized by diffe rent chemical shifts.