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, ATh 2 Se 6 (A = K, Rb), adopt the orthorhombic space group Immm . ATh 2 Se 6 has a two-dimensional structure which is related to the ZrSe 3 -type structure with K + /Rb + cations stabilized between the layers. These compounds represent the intercalated form of ThSe 3 with 0.5 equiv of alkali metal ion. The stacking arrangement of the layers is slightly modified from that of ZrSe 3 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 4 a × 4 b superstructure. An atomic pair distribution function analysis and spectroscopy confirmed the presence of diselenide groups in the ZrSe 3 -type layer (invisible by the single-crystal structure analysis) and support the notion that these Se atoms in the [Th 2 Se 6 ] 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 Se 2 2 - and two Se 2 - species along both directions. Optical absorption, Raman spectroscopy, and atomic force microscopy as well as magnetic susceptibility measurements support these conclusions.