Inelastic neutron scattering and Raman spectroscopic investigation of l-alanine alaninium nitrate, a homologue of a ferroelectric material

The 2: 1 amino acid salt of l-alanine with nitric acid was crystallized and the vibrational spectrum measured at 25 K by incoherent inelastic neutron scattering (INS) spectroscopy. The spectrum was simulated using solid-state density functional theory based on a new 90 K structure determination. A feature observed at approximately 450 cm^-1 in the INS spectrum of l-alanine alaninium nitrate is noticeably absent in the calculation. Raman spectroscopy reveals spectral differences between the spectra at 77 and 293 K with a 450 cm^-1 feature appearing at low temperature. The nature of these spectral changes and the disagreement between the INS spectrum and its simulation are discussed in relation to an apparent structural change involving motion of a proton at low (<90 K) temperature.