Far-infrared and millimeter wave spectroscopy of superionic copper conductors

Far-infrared and millimeter wave spectra of copper ion conducting crystal RbCu4Cl3+xI2−x, which has the same structure as the room temperature silver ion conductor RbAg4I5, were investigated. Broad absorption peaks observed around 40, 80, and 110-200 cm−1 at room temperature show doublet structures at low temperature; this may be attributed to the difference of local structure by chlorine and iodine ion. The 110-200 cm−1 bands seem to be symmetric breathing modes of CuX4 (X = Cl or I) tetrahedron and the frequency shift coincides with the square root of the mass ratio of conduction ions. The 80 cm−1 band seems to be Rb-X vibration in RbX6 octahedron. The 40 cm−1 band seems to be the attempt mode which is an outward motion of the mobile ion in halogen cage. The increase of the absorption intensity at the low energy side with temperature corresponds to an increase of the DC conductivity. Plasmon fitting in energy loss function spectra was attempted.