Molecular motion and ordering in rubidium cyanide, studied with dielectric and Raman techniques

Rubidium cyanide, so far rarely investigated and apparently for the first time available in high-purity single-crystal form, has been studied with dielectric and optical techniques between 2 and 300 K. Starting from the pseudocubic orientationally disordered phase at room temperature, the crystals show under cooling—by a sharp drop in light transmission and dielectric constant—the transition into a multidomain structure of elastic dipole order at 132 K. Dielectric measurements give evidence for electric disorder in this phase, showing paraelectric ($T^{-1\mathrm{}}$) behavior down to 47 K, with Debye losses yielding an activation energy for reorientation of 0.107 eV and an effective dipole moment of $0.022e$Å per C${\mathrm{N}}^{-}$ dipole. While some indications for the onset of a gradual electric ordering become apparent below 47 K, the freezing-in dipole reorientation prevents its establishment under further cooling, leading most likely at lowest temperatures to an electrically disordered state in nonequilibrium. Raman spectra, taken in both the regimes of librational-translational and C${\mathrm{N}}^{-}$-stretching modes, are discussed and compared to the behavior in NaCN and KCN. Characteristic trends in the order-disorder phenomena and physical behavior with cation size become apparent among the alkalicyanide family.