PHYSICAL REVIE%' 8 VOLUME 29, NUMBER 7 1 APRIL 1984 Structural and magnetic transitions in the Chevrel-phase compound FeMo6Ss J. M. Friedt Centre de Recherches Nucleaires, Strasbourg, France C. W. Kimball iYorthern Illinois University, DeEalb, Illinois 60175 A. T. Aldred, B. D. Dunlap, F. Y. Fradin, and G. K. Shenoy Argonne Rational Laboratory, Argonne, Illinois 60439 (Received 23 November 1983) Mossbauer-effect studies and susceptibility measurements have been carried out to characterize the magnetic and structural properties of the Chevrel-phase compound FeMo6S8. The Mossbauer spectra indicate that a structural phase transition occurs at To— - 100 K from an ordered form at low temperature to a disordered form of the material at high temperature, accompanied by a hardening of Fe vibrational modes. Susceptibility measurements give average magnetic moments of 1. 61p~/Fe atom and 1. 42@&/Fe atom in the regions below and above To, respectively. An antifer- romagnetic transition is observed at a temperature T~-— 41 K. Mossbauer spectra obtained below T~ show a distribution of magnetic hyperfine fields which are attributed to Fe atoms being dis- tributed among the crystallographically equivalent sites in the low-temperature structure. INTRODUCTION Interest in the Chevrel-phase materials MMoP'8 (M denotes a metal ion and X denotes a chalcogen) has been substantial for the past several years due to the occurrence of superconducting materials having high transition tem- peratures and extremely large critical fields. In addition, the observation of superconductivity in the presence of substantial concentrations of magnetic ions has stimulated a great deal of experimental and theoretical work. ' While materials formed from relatively large magnetic M ions, e.g. , trivalent rare earths, remain superconducting with a relatively low transition temperature, the addition of small magnetic ions, such as divalent d transition elements, de- stroys the superconductivity. This behavior is believed to be closely associated with those electronic and structural properties which determine the degree of interaction be- tween the M atoms and the Mo atoms, with the latter be- ing primarily responsible for the superconductivity of the materials. In particular, the crystallographic location of the large (rare-earth) and small (d-element) types of M atoms is not identical, because of a degree of delocaliza- tion of the M atoms from the 3-symmetry position which is small for the large atoms and large for the small atoms. Extensive crystallographic investigations have been car- ried out for a number of Chevrel-phase systems and the overall structure is now well known. Broadly speaking, this phase is formed by placing tightly bound molecular units of No+8 on an approximately cubic lattice, with a substantial hole remaining between these clusters which accommodates the M atoms. Because of a slight rotation of the Mo+s clusters around a body diagonal, the overall space group of the compound is rhombohedral with a tri- gonal axis connecting the body diagonals of those clusters. Single-crystal structural studies have shown that small M atoms can occupy two nonequivalent sets of sixfold sites distributed around the trigonal axis, with the occupation of each set being dependent on temperature and the con- centration of the M atom in the system. Figure 1 shows a portion of the Chevrel-phase structure consisting of the Mo+s clusters together with the two sixfold sites. Investigations of the Cu„Mo6S8 system have shown ' that both sixfold sites are occupied at high temperatures, and that this occupation leads to the rhombohedral struc- ture for these compounds. At lower temperatures, howev- M Site II M Site I FIG. 1. Portion of the Chevrel-phase structure showing the Mo6S8 clusters and the distribution of the two possible metal ion (M) sites around the trigonal axis. 29 3863 1984 The American Physical Society