CHARGE-DENSITY-WAVE CAUSED BY REDUCING THSE3 BY ONE-ELECTRON - SUPERSTRUCTURE AND SHORT-RANGE ORDER IN ATH(2)SE(6) (A = K, RB) STUDIED BY X-RAY-DIFFRACTION, ELECTRON-DIFFRACTION, AND DIFFUSE-SCATTERING

The two isostructural compounds, ATh(2)Se(6) (A = K, Rb), adopt the or thorhombic space group Immm. ATh(2)Se(6) has a two-dimensional structu re which is related to the ZrSe3-type structure with K+/Rb+ cations st abilized between the layers. These compounds represent the intercalate d form of ThSe3 with 0.5 equiv of alkali metal ion. The stacking arran gement of the layers is slightly modified from that of ZrSe3 in order to stabilize the newly introduced alkali metal ions between the layers . Electron diffraction studies reveal a static charge density wave (CD W), due to electron localization, resulting in 4a x 4b superstructure. An atomic pair distribution function analysis and spectroscopy confir med the presence of diselenide groups in the ZrSe3-type layer (invisib le by the single-crystal structure analysis) and support the notion th at these Se atoms in the [Th2Se6] layers accept the extra electron fro m the alkali metal, and this results in breaking one out of four disel enide bonds. The superstructure is due to ordering of the three Se-2(2 -) and two Se2- species along both directions. Optical absorption, Ram an spectroscopy, and atomic force microscopy as well as magnetic susce ptibility measurements support these conclusions.