Charge Density Wave Caused by Reducing ThSe₃ by One Electron. Superstructure and Short-Range Order in ATh₂Se₆ (A = K, Rb) Studied by X-ray Diffraction, Electron Diffraction, and Diffuse Scattering

The two isostructural compounds, ATh2Se6 (A = K, Rb), adopt the orthorhombic space group Immm. ATh2Se6 has a two-dimensional structure which is related to the ZrSe3-type structure with K+/Rb+ cations stabilized between the layers. These compounds represent the intercalated form of ThSe3 with 0.5 equiv of alkali metal ion. The stacking arrangement 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 (CDW), due to electron localization, resulting in 4a × 4b superstructure. An atomic pair distribution function analysis and spectroscopy confirmed the presence of diselenide groups in the ZrSe3-type layer (invisible by the single-crystal structure analysis) and support the notion that these Se atoms in the [Th2Se6] layers accept the extra electron from the alkali metal, and this results in breaking one out of four diselenide bonds. The superstructure is due to ordering of the three Se22- and two Se2- species along both directions. Optical absorption, Raman spectroscopy, and atomic force microscopy as well as magnetic susceptibility measurements support these conclusions.