AG-109 NMR LINESHAPE STUDY OF THE 3Q INCOMMENSURATE INSULATOR PROUSTITE (AG3ASS3)

We have observed the NMR lineshape for Ag-109 in the paraelectric, inc ommensurate and commensurate phases of the 3q incommensurate insulator proustite (Ag3AsS3). In the paraelectric phase we observed discrete n arrow lines representative of the six Ag sites in the unit cell. In th e incommunsurate region the narrow lines are replaced by two broad, fe atureless peaks, typical of a 3q incommensurate modulation. This is th e first observation of such incommensurate line broadening for a dipol ar nucleus in an insulator. In the commensurate region a larger number of discrete lines are observed, indicative of an increase in the size of the unit cell.