Dissertation

Investigations of some novel ternary transition metal chalcogenides with low-dimensional properties

Greaney, Mark Alan

1992

Overview

Magnetic studies were conducted on the solid solutions: TlCo$\\sb2$S$\\sb{\\rm x}$Se$\\sb{\\rm 2-x}$ (0.0 $\\le$ x $\\le$ 2.0). These materials crystallize in the ThCr$\\sb2$Si$\\sb2$-type structure with the cobalt atoms forming sheets in the ab plane sandwiched between chalcogen layers. Resistivity measurements on sintered pellets demonstrate that all of the compositions are highly metallic. The magnetic phase diagram was determined by magnetic susceptibility measurements. The Se-rich phases (0.0 $\\le$ x $<$ 0.75) order antiferromagnetically, the S-rich phases (0.75 $<$ x $\\le$ 2.0) order ferromagnetically, and the intermediate composition (x = 0.75) shows competition between antiferromagnetic and ferromagnetic behavior. The change in magnetic ordering type as a function of x is discussed in terms of the RKKY model. The magnetic structures of TlCo$\\sb2$X$\\sb2$ (X=S,Se) were determined at room temperature and at 4 K by powder neutron diffraction methods. The magnetic structure of TlCo$\\sb2$S$\\sb2$, which orders ferromagnetically at 97 K, has been shown to consist of ferromagnetically-ordered cobalt atom layers which lie parallel to the ab plane with their magnetic moments aligned in this plane. Adjacent planes of cobalt atoms order ferromagnetically. The powder neutron diffraction data of TlCo$\\sb2$Se$\\sb2$ is consistent with a magnetic structure composed of ferromagnetically-ordered cobalt atom planes with their magnetic moments aligned in the ab plane as for TlCo$\\sb2$S$\\sb2$; but in this case, the adjacent cobalt atom planes order antiferromagnetically. These magnetic structures are consistent with earlier oriented-crystal magnetic susceptibility measurements. Magnetic studies were performed upon the linear chain antiferromagnets: Ba$\\sb2$MnX$\\sb3$ (X=S,Se,Te) and the solid solutions: Ba$\\sb2$MnS$\\sb{\\rm x}$Se$\\sb{\\rm 3-x}$ (0.0 $\\le$ x $\\le$ 3.0) and Ba$\\sb2$MnSe$\\sb{\\rm x}$Te$\\sb{\\rm 3-x}$ (0.0 $\\le$ x $\\le$ 3.0). The magnetic susceptibility data were analyzed in terms of the Bonner-Fisher model for a spin 5/2 Heisenberg ion. The magnitude of the coupling constants were found to be inversely proportional to the Mn-Mn separation in the phases. This is consistent with the superexchange mechanism being operative in these insulating phases. The effects of diamagnetic \"impurities\" upon the magnetic behavior was determined by studying the solid solutions: Ba$\\sb2$Mn$\\sb{\\rm 1-x}$M$\\sb{\\rm x}$S$\\sb3$ (M=Zn,Cd; x = 0.01, 0.10). Dramatic reductions in one dimensional magnetic ordering are observed even at these low levels of impurity-doping. The effects of paramagnetic ion substitutions was determined by study of the solid solutions: Ba$\\sb2$Mn$\\sb{\\rm 1-x}$M$\\sb{\\rm x}$Se$\\sb3$ (M=Fe; 0 $\\le$ x $\\le$ 1.0). The magnetic susceptibility data were analyzed in terms of the reduced-spin model of Emori. Study of the magnetic susceptibility behavior of the series Ba$\\sb2$MS$\\sb3$ (M=Mn, Fe, Co) demonstrate that the magnitude of the magnetic coupling constant in this series is inversely proportional to the metal-to-metal separation in these linear chain phases.

• Publisher: ProQuest Dissertations & Theses
• Subject: Chemistry / Inorganic chemistry / Physical chemistry
• ISBN: 9798208123737